ampere trip formula

How To Calculate Circuit Breaker Trip Settings

Some brands of breakers are more sensitive than others and will trip when they near maximum capacity, even if your load does not exceed it. Try to leave some extra wattage available on each circuit so plugging in items like vacuum cleaners or coffee makers will not trip the breaker.

Each circuit breaker in your breaker panel is labeled with the maximum amperage (current) capacity for that circuit. This may differ among circuits, so always check each breaker's capacity individually when calculating the electrical load that will cause that breaker to trip. Standard household circuits in the U.S. are 120 volts, but some circuits have double that capacity for appliances such as stoves and air conditioners. These breakers have 240-volt capacities and will be approximately twice the size of 120-volt breakers. Wattage is the easiest measure of power load to calculate and monitor so you don't accidentally trip your breakers.

Look for the amperage notation on the breaker switch. This will generally be 15 or 20. Also look for the voltage notation, which may be on the breaker switch as well, and will be 120 or 240. If you cannot locate the voltage, assume that breakers that take up one panel slot are 120 volts and breakers that take up two slots are 240 volts.

Multiply the amps by the volts. In most circuits, this will be 20 x 120 = 2400 or 15 x 120 = 1800. The number resulting from this equation is the maximum wattage load you can place on the circuit before tripping the breaker.

Apply the same calculation to 240-volt circuits. For example, a 240-volt circuit with a 30-amp capacity would allow 7200 watts (30 x 240 = 7200).

Check the wattage for all electrical fixtures and appliances on the circuit. If the total wattage is over your maximum calculation, the breaker will trip.

Recommended

Sizing A Circuit Breaker: Breaker Size Calculator + Amp Chart

Sizing a circuit breaker is never easy. But it’s also not all that difficult . Everybody knows that we need an adequately sized circuit breaker that allows for sufficient electric current. If we undersize a breaker, the breaker will likely catch on flame. No pressure here.

How do you go about picking the correct circuit breaker size? Do you need a 10A, 15A, 20A, 30A, 40A, 50A breaker, etc?

Standard breaker sizes are 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 450A, 500A, 600A, 700A, 800A, 1000A, 1200A, 1600A, 2000A, 2500A, 3000A, 4000A, 5000A, and 6000A.

Sizing a circuit breaker is actually quite easy. You just need to know a couple of rules. These are:

• 80% NEC breaker rule. This is the most basic NEC (National Electric Code) rule that states that you can’t push the current over 80% of its specified ampacity. Example: If you have a 20 amp breaker, you can only allow for a 16A current. 16A is 80% of the max. the specified ampacity of the circuit breaker. This is a safety measure; you better have a bit of overhead to prevent the circuit from frying. You can read the full Article 240.4(B) in NEC 2014 on this here .

If you know how to calculate the amps and account for the 80% breaker rule, you can calculate the size of the breaker yourself.

To help everybody sizing these breakers out, we will explain how to determine the right size of a breaker. On top of that, we include a Circuit Breaker Size Calculator further on (just insert watts and volts, and you get the correct breaker size).

At the end, we also included the ‘just tell me the breaker size I need’ Breaker Size Chart that tells you what breaker size you need for devices with different wattages (from 50W units to big 20,000W devices).

Let’s look at how breaker size can be calculated manually (you can also use the calculator or/and chart below):

Table of Contents

How To Calculate Size Of A Circuit Breaker?

This is the easiest to explain with an example.

Let’s say that we have a simple 1,500-watt space heater running on a standard 120V circuit. What size amp breaker do you need for a 1,500-watt space heater?

First, you need to calculate how many amps does this heater draw like this:

Current (Amps) = Power (Watts) / Voltage (Volt)

In our situation this is:

Current = 1,500W / 120V = 12.5 Amps

Now we know that the 1,500W space heater draws 12.5 amps. We have to account for the 80% breaker rule. This means that these 12.5 amps should represent 80% of the breaker amps. To calculate the size of the circuit breaker needed, we have to multiply the amp draw by 1.25 factor like this:

Minimum Circuit Breaker Size = 12.5A × 1.25 = 15.63 Amps

We can’t use a 15A breaker because the breaker ampacity should be at least 15.63A. The next breaker size is 20 amps; that means we need to use a 20A breaker for a 1,500W space heater running on 120V standard circuit.

Here is the basic step-by-step procedure we did to determine the size of a circuit breaker:

• Calculate the amp draw. We use the basic electric power equation for this. If we know the wattage and voltage, we can quite easily calculate the amp draw.
• Multiply amp draw by 1.25 to account for the 80% breaker rule. The resulting amps are the minimum ampacity a correctly sized circuit breaker should have.
• Choose a circuit breaker size. We usually pick between 10A, 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A circuit breakers, and so on.

This is how breaker sizing is done manually. The easiest way is to use a dynamic calculator. You simply input that wattage and the voltage, and the calculator will tell you what is the minimum size of a circuit breaker you need. You can use this calculator here:

Circuit Breaker Size Calculator

Here is how this breaker sizing calculator works:

Let’s say you have a big 5,000W air conditioner (this is usually a 5-ton unit). It runs on a 220V circuit. What size circuit breaker do you need?

Just slide the wattage slider to ‘5000’ and voltage slides to ‘220’ and you get ‘28.41 Amps’. Therefore you need a circuit breaker with at least 28.41A ampacity. 25A breaker is too small; you need a 30A breaker .

You can do this for literally any device running on any voltage. You can also play around with numbers to see how the amps change.

If you want the ‘just tell me the circuit breaker I need’ you can consult this chart:

Breaker Size Chart (For 120V And 220V Circuits)

You just need to know the wattage of the device you need a circuit breaker for and you can check what size breaker you need if you run it on a standard 120V circuit or an upgraded 220V circuit:

As you can see, calculating what size breaker you need is not all that hard. Of course, with bigger amp draws, you can connect several 30A or 50A in parallel to increase the total breaker ampacity.

We hope this illustrates how everybody can figure out the size of circuit breaker they need. If you have any questions regarding this breaker sizing, you can use the comments below and we’ll try to help you out.

Related posts:

• 50 Amp Wire Size Details: Gauge, Breaker, 220/240V Example
• Thermostat Not Reaching Set Temperature: 7 Causes + Fixes
• Wire Gauge Wattage Charts For AWG Wires (4/0 AWG To 14 AWG)
• How Many Outlets On A 15 Amp, 20 Amp, 30 Amp Circuit? (NEC 210.21)
• Thermostat Wire Color Codes For 3-8 Wire Thermostats (Color-By-Color)

14 thoughts on “Sizing A Circuit Breaker: Breaker Size Calculator + Amp Chart”

Thank you, this is very help full.

very much help full

Excelente information. Thank you.

The most amazing easy method. very Helpfull.

Thank you, Imran, we try to simplify it as much as possible. It’s nice to see a bit of recognition.

Helpful. 1,500-watt space heater running on a standard 120V circuit. What size amp breaker do you need for a 1,500-watt space heater? how can I use 120 v circuit.

Hi Masum, alright, 1500W heater on 120V draws 1500W/120V = 12.5 amps. To apply the 80% rule, you have to multiply this current by 125% like this: 12.5A × 1.25 = 15.63 amps. So, a 15A breaker won’t cut it, but a 20A breaker will be great for a 1500-watt space heater on a 120V circuit. Hope this helps.

Hi I have a 1200 watt generator , the breaker has gone bad and there is no size printed on the breaker. Im not sure if you can calculate the size the same way you would for a AC unit. What size would you suggest ? Thanks

Hi Renald, the size of the breaker you need for a 1,200 watt generator really depends on the amps it will give out. You can calculate the amps if you know the voltage. Generators can have 12V DC, 24V DC, 110-120V AC, 240V AC voltages. Example: Let’s say your 1200W generator has a 24V DC voltage. You can calculate the amps like this: 1200W/24V = 50 Amps. This is the current in the wire. You can calculate the minimum required breaker size for this generator like this: 50 Amp × 1.25 = 62.5 Amps. In this case you can go for 70A breaker, or 3x30A breakers, since their capacity if north of 62.5 amps. Hope this helps.

This article is flawless thank you!

Great info! Much appreciated but I’m trying to determine what breaker or breakers I need for my bass boat. 3x 12V in series for my 36V trolling motor and 1x 12 V for my electronics and starting battery which also powers the various pumps and aerators. Any help would be greatly appreciated!

Hi Tom, the wattage you need is the key here. Electronics will have lower wattage but pumps and aerators… that should like high wattage stuff. Example: If you would put in 60A amp breaker on 36V voltage, and apply the 80% breaker rule, you will get 1,728W of power. It’s really hard to advise here without wattages but from the looks of it – 36V and energy-demanding pumps – the first presumption would be that you need big breakers, at least 60A. Hope this helps.

What about wire size??

Hi there, you can check the wire size calculator here .

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How to Determine the Load Capacity of Your Circuit Breaker

Understanding circuit breakers.

Circuit breakers in your electrical panel are considered "Safety buffers." Their job is to disconnect from power when they detect the passing current exceeds its amperage. When you don't measure your circuit breaker's load capacity, you run the risk of damaging your appliances, or worse…setting your building on fire! In this blog, we will go over the key essentials to understanding how much amperage your circuit can hold.

Key Essentials:

• Evaluating your electrical load capacity

Determining the number of electrical devices your breaker can handle

• Troubleshooting and inspecting your breaker box

When you’re considering installing a new heater , HVAC unit , thermostat , or any electrical appliance for that matter, it’s important to know exactly how much electricity your circuit breakers can handle before the circuit trips.

Every electrical appliance you use requires a specific level of electrical power for it to operate safely.  Commonly referred to as the “Power Rating,” this load level helps to determine how much power your device can handle without overheating (8).

Have you ever gone shopping for batteries, light bulbs, or even vacuum cleaners and noticed things like ‘9-volt batteries’ or ’12-watt light bulbs’ or ‘20 amps of power?'  Have you ever looked at these numbers and found yourself asking…

WATT does it all mean?

Well, before we jump into load capacity and all that other technical jargon, let’s learn a little bit about amps, watts, and volts.

What is an amp?

Amp is short for Ampere.  Amps measure the amount of electrical charge flowing past a given point in one second. In laymen’s terms, the number of amps indicates how much of an electrical current is being drawn through power cables (1).

What is a volt?

Voltage (volts, V) measures how strongly electricity is being pushed through a circuit. In other words, the number of volts tells you the amount of pressure being pushed (1).

What is a watt?

Wattage measures the amount of electrical power that a device uses.  Watts is the unit of measure that indicates the total amount of electrical current flowing through an electrical device (1). By measuring the amount of power a building consumes, the power company is able to determine your utility bill.

Still don’t get it? Let’s use a water hose as an example!

Like electricity running through a current, water flows through a hose. Amps would be the volume of water that flows through the hose, while the actual water pressure would be the voltage (1) . Watts on the other hand is directly related to the power that water could provide. For example, it could be powering a water wheel.

How to evaluate your electrical load capacity

Every circuit breaker has a specified amperage (amount of current). This rating is labeled on the breaker itself.  The standard for most household circuits are rated either 15 amps or 20 amps. An important note to remember is that circuit breakers can only handle about 80% of their overall amperage.  That means a 15-amp circuit breaker can handle around 12-amps and a 20-amp circuit breaker can handle about 16 amps.

• First, find the breaker that correlates to the electrical device you are using (This is normally either a 15-amp or 20-amp circuit).
• Multiply the amperage by 0.8. This is because a circuit breaker should never exceed 80% of its max amperage. Not doing this could leave room for calculation errors, or even worse – electrical fires!
• Calculate the amperage draw of ALL the devices you wish to plug into the circuit.

It’s very important to understand how much amperage your electrical device draws before installing them into your breaker box. Whether you’re looking to install a heater, AC unit, light switch, or GFCI outlet, there are a few steps you must take.

• Check the wattage (max power rating) on your device. This is usually labeled somewhere on the back of the device.
• Measure the voltage on the circuit you wish to install your electrical devices. Most household circuits are 120V and larger commercial spaces are 240V (5) . If you’re not sure, use a multimeter to test the voltage of your breaker (5).
• To calculate amperage, use the equation Amps = Watts/Volts.  For example, a 200W light bulb on a 120V circuit would draw about 1.67 amps.
• Calculate the TOTAL amperage rating of all devices. Make sure they DO NOT exceed 80% of the breaker’s total amperage.

Troubleshooting and inspecting your breaker panel

Your circuit breaker is an essential part in the safety of your home or building. It keeps your electrical wiring system from overheating. If you are experiencing frequent power outages, electrical trips, and other oddities, we’ve got some troubleshooting tips for you!

What are some common reasons for a circuit breaker to trip?

• Overloading the circuit with too many appliances that draw too much amperage
• Short in the electrical wiring, loose wiring, or wiring
• Circuit breaker is old, worn out, or damaged

These are just a few ways to troubleshoot a tripped breaker. Depending on the problem, some issues can be tackled at home while others will need the help of a qualified electrician.

Now that you have a little background knowledge on circuit breakers and how to troubleshoot trips, take your new skills and check out HomElectrical’s wide array of LED lights and HVAC units for your convenience.

1.  https://www.youtube.com/watch?v=9q31SzeVjP0

2.  https://www.bhg.com/home-improvement/electrical/how-to-check-your-homes-electrical-capacity/

3.  https://homeguides.sfgate.com/many-outlets-can-placed-20-amp-household-circuit-82633.html

4.  https://homeguides.sfgate.com/many-recessed-lights-15amp-breaker-84843.html

5.  https://www.wikihow.com/Determine-Amperage-of-Circuit-Breaker

6. https://www.wisegeek.com/what-is-a-power-rating.htm

Recommended Reading

Why Does my GFCI Outlet Keep Tripping?

Electrical trips can occur due to worn out insulation, conductive dust or debris, water, or electrical wiring deterioration. Discover more ways to troubleshoot your GFCI if it starts to trip.

Comparing GFCI Breakers and GFCI Receptacles

GFCI breakers and receptacles keep you safe from electric shocks and fires. Use a GFCI outlet in areas with a high chance of water coming into contact with the electrical current, such as a bathroom or kitchen.

20-amp GFCI outlets vs. 15-amp GFCI outlets

Circuits and outlets come in either 15-amp or 20-amp options, and the amperage of the outlet must never exceed the amperage of the circuit. Follow NEC requirements to make sure you have the correct voltage, and never overload your circuits.

What Is the Difference between a GFI and GFCI?

GFCI stands for Ground Fault Circuit Interrupter outlets, while GFI stands for Ground Fault Interrupter. Many electricians refer to GFCI instead of GFI, but neither word signifies a difference.

A true creative with a penchant for the spiritual and natural order. She loves the Earth, almost as much as she loves writing about it.

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How to Determine Amperage of Circuit Breaker

Last Updated: June 25, 2024 Fact Checked

This article was co-authored by Jesse Kuhlman . Jesse Kuhlman is a Master Electrician and the Owner of Kuhlman Electric based in Massachusetts. Jesse specializes in all aspects of home and residential wiring, troubleshooting, generator installation, and WiFi thermostats. Jesse is also the author of four eBooks on home wiring including "Residential Electrical Troubleshooting" which covers basic electrical troubleshooting in residential homes. There are 10 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 861,839 times.

Each circuit breaker has a specific rated amperage, or amount of current. When that amperage is exceeded, the circuit breaker shuts down the flow of current in that circuit to prevent damage to the wiring and appliances. Learn how to calculate the actual amperage of devices on the circuit and compare it to the rated amperage, so you can avoid unnecessary power interruptions and fire hazards.

How to Tell How Many Amps a Breaker Is

Look on the handle of the circuit breaker to find its amperage. This is the maximum amperage that the circuit can take before it trips. In the U.S., most standard household circuits are rated for 15 to 20 amps, although special appliances may have 30 or 50.

Finding the Amperage Rating of a Circuit Breaker

• In the United States, standard household circuits are rated for 15 or 20 amps. Specific appliances may need dedicated, high-load circuits for 30 or 50 amps.

• Breakers should be sized at 125% for a continuous load and 100% for a non-continuous load, which comes out the same when you multiply the breaker size by 0.8.

• For example, a two-pole breaker at 15 amps on each pole (breaker handle) would supply 240 volts to the appliance on that branch at up to 15 amps, not 30.

Finding the Amperage Draw in a Circuit

• Some devices will list the amperage directly, often labeled FLA, meaning "Full Load Amps". If it does, skip down to the next section to interpret that rating.

• Electrical circuits never operate perfectly. Some of the energy is lost to heat, and devices may draw more current to make up for this. Waste is low in most household circuits (below 10%), but it is still possible to trip the breaker if the total calculated amperage on paper is slightly below the breaker rating.

• Do not attempt this unless you have electrician's gloves and a basic understanding of electrical safety. These wires are live and removing the front panel of a breaker panel will expose you to dangerous voltages, so use extreme caution.

Reading a Device's Amperage Rating

• This section covers devices that list the amperage directly on the data plate, which should include all appliances with a motor. If your device only lists wattage (W), calculate the amperage from that value.
• This is not an appropriate method to determine safety features to protect the motor itself. [7] X Research source The circuit breaker protects the wiring of the electrical supply.
• High-power devices such as air conditioning units and ovens are best installed by a trained electrician.

• Most electrical codes allow a ± 5% tolerance for the voltage (or slightly more). [8] X Trustworthy Source Official UK government website Official website for the public sector of the UK government Go to source Do not run a device on a voltage supply outside this range.
• Household outlets in North America and some other countries are on a 120V standard. Most of the world uses 220–240V. [9] X Research source
• Many homes have 30-A or 50-A breakers dedicated to larger appliances such as water heaters, baseboard heaters, ovens, stoves, and heavy-duty power tools. The wiring and breakers for these circuits are required to be designed at 125% of the continuous load and 100% of a non-continuous load.

• This value may also be listed as full load amperage, running amps, rated amp, or just amps.
• Circuit breakers are rated to 100% of the listed amperage, meaning you can skip the 125% calculation. This information will be clearly listed on the circuit breaker electrical panel if you have this type of breaker.

• Do not confuse this with the RLA, a specially derived value of internal forces, listed on air conditioner units.
• You can oversize a breaker to 125% to help prevent your breaker from tripping if the circuit is designed strictly for motors.

• If your circuit breaker is rated at 100%, just add all of the amperages together.
• If your circuit breaker is rated for continuous loads at 80% or you do not know its rating, add the amperages of all devices running for more than three hours at a time and multiply by 1.25. Add to the result the amperage of all devices running for shorter periods of time.
• In either case, if the answer exceeds the circuit breaker amperage, move a device to another circuit.

• These values are often surprising if you do not have HVAC experience, and are further complicated by new technologies that allow for lower amperage than MOP implies. Consider hiring a professional if you are not experienced in the area.

Expert Q&A

• The total of all breakers in a panel may exceed the rating of the "main" breaker. This is normal in residential systems where it is unlikely that all loads on all circuits will be operating simultaneously. Thanks Helpful 0 Not Helpful 0
• There are consumer devices for easy measurement of the actual wattage or amps being used by a cord-connected appliance. They are sold as "plug-in watt meters". You plug it into a receptacle, then plug an appliance into the watt meter to directly read how much power it is using, as displayed on the meter. This may not provide a convenient branch-circuit total load, but it avoids the risk of trying to use a clamping ammeter on live wires inside a breaker panel. Thanks Helpful 0 Not Helpful 0
• The National Electrical Code® (NFPA 70®) is available online for free viewing (with minimal sign-up) or you may find a copy in a well-stocked library or bookstore. Thanks Helpful 0 Not Helpful 0

• Be sure you use a circuit breaker that is the same brand as the panel box you are installing, or it may not fit, and may violate the warranty. Thanks Helpful 2 Not Helpful 0
• The circuit breaker amperage is primarily limited by the gauge and material of the attached wiring. To prevent a hazardous setup, follow electrical safety codes (such as the NEC in the US). [13] X Research source The NEC (National Electrical Code) is published by the NFPA (National Fire Prevention Association) and that's not a coincidence! Thanks Helpful 0 Not Helpful 0
• If an equipment manual requires a specific circuit breaker or a "maximum overcurrent" number, that means the device can only be plugged into a circuit at that rating, for safety. For example, an 8-amp system may overheat and cause a fire hazard if it becomes overloaded while plugged into a 20-Amp circuit because the breaker does not immediately open the circuit during small overloads. Thanks Helpful 0 Not Helpful 0

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• ↑ Jesse Kuhlman. Master Electrician, Kuhlman Electric. Expert Interview. 29 April 2020.
• ↑ https://www.ecmweb.com/basics/sizing-circuit-breaker
• ↑ https://www.worldstandards.eu/electricity/plug-voltage-by-country/
• ↑ https://www.youtube.com/watch?v=ar7qkMHKw40
• ↑ http://ecmweb.com/nec/motor-calculations
• ↑ https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/293405/05-1410-electrical-supply-tolerances-and-appliance-safety.pdf
• ↑ http://ecmweb.com/basics/sizing-circuit-breaker?page=1
• ↑ https://electricalbaba.com/locked-rotor-starting-current/
• ↑ https://www.thespruce.com/why-circuit-breakers-trip-1824676
• ↑ http://ecmweb.com/code-basics/article-240-overcurrent-protection

About This Article

To quickly determine the amperage of a circuit breaker, find the amperage marked on the handle of the circuit breaker. Then, multiply the amperage by 0.8 to find 80% of the rated amperage. For double-pole breakers, don’t add the amperage of the two breakers together because they will still operate on the individual amperage labeled on the handles. For any circuit, operating above 80% for a long period of time can cause the breaker to trip. For tips on reading a device's amperage range and finding the amperage draw in a circuit, scroll down! Did this summary help you? Yes No

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How To Prepare Schedule of Loads

• This example emphasized the procedure rather than mimicking the actual loads of a residential unit.
• In this example the voltage drop and short circuit calculation is not included.
• The system voltage of this example is 220 VAC, 60 Hz.

Read:  How to Select Proper Type of Miniature Circuit Breakers MCBs

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36 comments:.

This comment has been removed by a blog administrator.

Nice blog… Thanks for sharing very useful information about electrical circuits. Learn Electronic Circuits

help: question 1: circuit 4; how did you compute circuit breaker: 20.45 amps? question 2: circuit 6-8; how did you compute circuit breaker: 5.45 amps?

Oops!? you multiplied the continous load by 100% and the noncontinous load by 125%.

in what chapter in the nec stated that 1 LO is rated 100VA?

This comment has been removed by the author.

..Hi, I'm just Wondering. Why did you always divide the AMPERAGE RESULT to 80% pf except Circuit 6-8 that you Multiply into 125%? thus this 80% pf different from 80% Demand Factor (DF)?

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Compare the wattage of your air conditioner and furnace. Since you only use one at a time, add only the larger wattage rating to your equation. Divide the resulting number of watts by volts (most homes use 220 volts) to get the number of amps, or the electrical load. URL: https://www.affordablelectricians.com.au/electrician-mitcham.html

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Home > Electrical Wiring > How to Find the Proper Size of Circuit Breaker? Breaker Calculator & Examples

How to Find the Proper Size of Circuit Breaker? Breaker Calculator & Examples

How to calculate the correct size of circuit breaker breaker size calculator with solved examples based on nec, iec ad ieee.

According to the NEC (National Electric Code), IEC (International Electrotechnical Commission), and IEEE (Institute of Electrical and Electronics Engineers), it is essential to have the proper size of a circuit breaker for all electrical circuits, whether in residential wiring or industrial and commercial installations. This is crucial to prevent electrocution, hazardous fires, and ensure the protection of connected electrical equipment and appliances.

For maximum safety and reliable operation of the electrical machines, it is recommended to use the correct and suitable size of circuit breaker according to the circuit’s current flowing through it.

“If we fail to use a correctly sized circuit breaker (whether oversized or lower than the rated current), the circuit, cables, wires, and connected devices may heat up or, in the case of a short circuit, start to smoke and burn. This is why a correctly sized circuit breaker is necessary for smooth operation.

• Related Post: How to Find The Suitable Size of Cable & Wire ? – Solved Examples

In this post, we will show how to choose the right size circuit breaker for electrical wiring installation and design, considering factors such as the related voltage level, wattage usage, and the difference in percentage to the circuit load and the current capacity of the circuit breaker (CB).

• Related Post: Difference Between MCB, MCCB, ELCB & RCB, RCD or RCCB Circuit Breakers

Table of Contents

What is a Circuit Breaker?

A Circuit breaker (CB) is a control and protection device which:

• Control (make or break) a circuit  manually or by remote control under normal and fault conditions.
• Break a circuit automatically under fault conditions (like over current,  short circuit , etc.).

A circuit breaker is used for switching mechanism and protection of the system

A circuit breaker is a switching as well as protection device used for ON/OFF operation of the circuit as well as prevent the electric shock. For accurate operation an d protection, even complex designs are used with circuit breaker like fuses , relays,  switches , earthing & grounding etc.

• Related Post:  Main Difference between Fuse and Circuit Breaker

How Does a Circuit Breaker Work?

In normal conditions when the circuit current rating is lower than the circuit breaker rating, the circuit operation is normal and it can be changed by manual operation. In case of fault or short circuit when the value of current exceeds the circuit breaker current, It will automatically trip i.e. break the circuit from the main supply.

For example, a 30 amp circuit breaker will trip at 30 amp no matter if is it continuous or non continuous load. That’s why we must select 20-25% higher size of current for circuit breaker than the flowing current in the cables and wires to the connected device.

If we use a 100A circuit breaker for 30A circuit, it wont protect the circuit from fault currents and may burn and damage the device as more than 30 amperes current won’t trip the circuit breaker. In short, we must use the proper size of circuit breaker according to the device i.e. CB current should not be lower nor highest but 125% of circuit’s current.

Related Posts:

• MCB (Miniature Circuit Breaker) – Types, Construction, Working & Applications
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Circuit Breaker Size Calculator

The following circuit breaker sizing calculator will show the difference in % to the load, voltage level in different countries and exact size of breaker in amperes.

Related Calculators:

• Electrical Cable & Wire Size Calculator for Copper & Aluminum
• Wire & Cable Size in AWG Calculator for 1 and 3-Phase Load

Sizing Circuit Breaker for 120V/240V, 1-Phase Circuits – NEC

To determine the appropriate size of circuit breaker for single phase supply, it depends on multiple factors like type of load, cable material and ambient temperature etc.

According to NEC 210.19(A), 210.20(A), 215.2, and 230.42(A), the general rule of thumb is that the circuit breaker size should be rated at 125% of the ampacity of the cable and wire for continuous loads (lasting for 3 or more hours continuously, such as a water heater) that need to be protected by the circuit breaker.

Good to Know: This calculation are based on the NEC guidelines. For more details, refer to NEC 210.19(A), 210.21, 210, 24, 215.2, 220.110, 220.14, 220.42, 220.45, 220.53, 220.55, 230.42(A), 310-14 and 517.22.

Let’s look at the following solved examples:

Example 1:- 120V – Single Phase Supply

Suppose a 12-gauge wire is to be used for a 16-ampere lighting circuit with a 120V single-phase supply. What is the appropriate size of a circuit breaker for this 16-amp circuit?

Circuit Current: 16 A

Circuit Breaker Size: ?

CB size should be rated at 125% of the circuit current.

= 125% × 16 A

= 1.25 × 16 A

Required Circuit Breaker Size = 20A

Good to Know:

• NEC 210.19 for continuous load circuits (Article 100) suggests that a 20-amp breaker should be used at 80% of its rated load for continuous circuits. This means a 20A breaker should be used for 16A load circuits, based on a 125% factor or 80% load current, especially for continuous load circuits.
• According to NEC Table 310.15(B)(16), the correct wire size is  #12 AWG copper  or  #10 AWG aluminum for a 20A circuit breaker and outlet .
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Example 2:- 240V – Single Phase Supply

What is the appropriate size of circuit breaker for 2.8kW, single phase 240V AC Supply?

• Load: 2,800W
• Voltage: 240V (Single Phase)

Circuit Current:

According to the ohm’s law ,

• I = 2,800 W / 240V
• I = 11.66 A.

Circuit Breaker Size:

Simply, Multiply 1.25 (safety factor based on 125% factor) to the load current.

1.25 × 11.66 A = 14.58A

The next nearest standard size of breaker is 15A, which is the suitable option

Required Circuit Breaker Size = 15 A

• NEC 210.19 for continuous load circuits (Article 100) suggests that a 15-amp breaker should be used at 80% of its rated load for continuous circuits. This means a 15A breaker should be used for 12A load circuits, based on a 125% factor or 80% load current, especially for continuous load circuits.
• According to NEC Table 310.15(B)(16), the correct wire size is  #14 AWG copper  or  #12 AWG aluminum for a 15A circuit breaker and outlet .
• What is the Right Wire Size for 15A Breaker and Outlet?
• What is the Correct Wire Size for 25A Breaker and Load?

Sizing Circuit Breaker for 3-Phase Supply – NEC

To determine the breaker size for a three-phase supply, it’s important to know the exact type of load, as various factors influence the load current. In other words, the same rule won’t apply to different types of loads, such as lighting, motors, or inductive/capacitive loads. For example, motors draw significantly higher current during startup and involve power factor considerations. For residential use, we can apply the same formula as for single-phase circuits, with the addition of the √3 (1.732) factor, as required by the three-phase power formula.

Good to know: For the same load, the breaker size in a three-phase system is typically smaller than that in a single-phase AC circuit.

Let’s calculate the correct circuit breaker size for three-phase circuits as follows.

Which size circuit breaker is needed for 6.5kW, three phase 480V load?

Power in a three phase circuit: P = V × I × √3

Current: P / V × √3

• I = 6.5kW / (480V × 1.732)   … (√3 = 1.732)
• I = 6.5kW / 831.36

The recommended size of circuit breaker is

1.25 × 7.82A = 9.77A

The next closest available standard circuit breaker in size is 10A .

Good to Know: The breaker and wire size calculations are valid for resistive loads. For inductive loads (e.g., motors and HVAC systems), refer to NEC Article 440, especially sections 440.22 and 440.32.

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Sizing Circuit Breaker for 230V, 1-Phase Circuits – IEC

What is the suitable size of circuit breaker for 230V, 2.85kW load single phase circuit?

• Current = Power / Voltage
• I = 2,850 W / 230V
• I = 12.40 A

The minimum and recommended size of circuit breaker :

= 10.21A × 1.25

Next Standard Size Breaker = 16 A

The next standard size available is 16A to use with 2,850 watts of load in 230V single phase supply system.

• Related Post:  Smart WiFi Circuit Breaker – Construction, Installation and Working

What is the recommended circuit breaker size (rating in I n ) to protect a 22A load circuit at 60°C?

For the 22A load current, the next available breaker is 25A. As the temperature involved, the rating of the 25A breaker would derate according to the rating factor table 3 in IEC charts given below.

The derating factor in table 3 for 25A breaker at 60°C is 24.1A.

Applying the 0.8 factor = 24.1A × 0.8 = 19.28A.

The 19.28A is not suitable for 22A load, so we will select the next available breaker which is 32A.

The derating factor in table 3 for 32A breaker at 60°C is 30.6A.

Applying the 0.8 factor = 30.6A × 0.8 = 24.48A.

Suitable Breaker Size: 32A

A 32 A circuit breaker would therefore be selected for 22A load current.

Good to Know : Examples 1 and 2 are based on IEC 60364, 60898-1, 60947-2 (International Standards), IET Wiring Regulations, BS 7671 (British Standards).

Sizing Circuit Breaker for 3-Phase Supply – IEC

Find the appropriate size of CB for 3-Phase 415V, 17kW load?

• Current = Power / (Voltage × √3)
• I = 17000W / (415V × 1.732)

Recommended Size of Circuit Breaker: 1.25 × 23.65A = 29.5A .

The next closest value of circuit breaker is 30A .

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Circuit Breaker Size Calculation for Continuous & Non-contentious Load

Circuit breakers (CBs) and Overcurrent Protection Devices (OCPDs) are designed to handle 100% of their rated current, meaning a 30A circuit breaker can safely carry 30A of current. However, NEC 210.19 recommends using only 80% of the rated current as a safe limit, especially for continuous loads. This is because not all loads are the same; some are continuous, while others are non-continuous.

For continuous loads that last three or more hours, the load current should not exceed 80% of the breaker or OCPD’s rated capacity.

For example, 80% of a 30A circuit breaker is 24A. Therefore, a 30A circuit breaker can safely be used for a circuit with a 24A load.

In other words, the appropriate size of the breaker for a 24A load would be:

24A ÷ 0.8 = 30A.

Example 1: CB Size for 30A Non-contentious Load

• An exact 100% rated for 30A circuit breaker can be used for 30A non-continuous load.

Example 2: CB Size for 28A Contentious Load

• In case of continuous load, rate of %125 is applicable.
• 1.25 × 28 A = 35A

Example 3: CB Size for 30A Non-contentious Load & 28A contentious Load

• = 125% Continuous Load + 100% Non-continuous load
• = (1.25 × 28A ) + (30A)
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• Using an oversized breaker for protection can damage the water heater or other connected appliances and may even cause a fire due to overheating.
• An undersized breaker or one with the same rating as the load current will trip repeatedly, causing the circuit to reset frequently. It’s essential to use the correct size breaker.
• A single-phase circuit breaker cannot be used for three-phase supply voltage levels.
• A 3-pole circuit breaker can be used in a 3-phase system with either 2 or 3 poles. However, a 3-pole circuit breaker can only be used in a single-phase system if specified by the markings or user manual.
• A 30A breaker with 10-gauge wire can be used on a 240V AC supply. The breaker cannot exceed the wire’s ampacity, except for specific loads.

Additionally, circuit breakers have voltage ratings that must be adhered to:

• A 120V breaker can only be used for 120V circuits.
• A 240V breaker can be used for both 120V and 240V circuits, but not for 277V (common in commercial applications).
• A 120-277V breaker can be used for 120V, 240V, and 277V circuits.
• A 120V breaker cannot be used on a 240V circuit, and vice versa.
• A 15A, 120V breaker cannot be used on a 20A, 120V circuit.

Related Post:  How to Find Voltage & Ampere Rating of Switch, Plug, Outlet & Receptacle

Circuit Breaker Size % and Amps Charts

NEC Charts and Tables

The maximum safe current limit is 80% of the rated size of breaker except some motors. Keep in mind that the breaker size should not increase the max amperage rating of cable and wire. Below is the given chart showing the % of max current rating of breaker size for different types of load currents.

* Motors except Hermetically Sealed 00-250% NEC

The following two charts shows the suitable circuit breaker sizes with wire gauge and different level of voltages.

IEC Charts and Tables

Table 1: The following table shows the typical appliances having various wattage rating and suggested rating of MCB and types of circuit breakers . for 230V, single-phase AC circuits for IEC following countries.

Table 2: The following table illustrates the size of circuit breaker and related conductor selection for domestic circuits in 230V, single phase circuits for IEC following countries.

Table 3: The following table shows the derated/uprated current values for different ambient temprature while selecting the breaker size for 230V, single phase circuits for IEC following countries.

Table 4: The following table illustrates the typical current carrying capacity of cables with suggested cable size, current rating in amps and recommended circuit breaker rating in amperes for 230V, single-phase AC circuits for IEC following countries.

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17 comments.

I went Drive calculating process

NEC 240 first sections have requirements for max OCPD based on wire gauge up to #12. 20A max unless special cases in NEC (motor loads, HVAC etc.)

Please forward me all this information if it is free. Thanks

Love to always get updates. Thank you.

may i have a link to download this books

i have really enjoyed the lesson concerning the correct size of circuit breaker to be used for controlling electrical equipments for both domestic and industrial purposes… i will be happy if you could send me copy of the lessons through my email address provided below. Thank You.

The solved examples clear the confusing concept of sizing a circuit breaker. Thanks for explanation.

Very nice engineering field for polytechnic student read about electrical engineering& technology

CB used for protection of electrical equipment& machines.

There in no mention of operating time. Why?

Please post on ckt breaker maintenance Thank you

Certain aspects on this needs review

It is not correct that a 30A circuit breaker will trip at 30A.

The specifications clearly state that it shall carry 100%of the rated current

Further, hydraulic magnetic circuit breakers carry 100% of rated current continously and comply with UL specifications

Good update

Keep me updated please,really good!

Thanks very much for the Lesson…can you send to me through my emails, please?

Dear Owner, My name is Darrel Wolf with Pro Interior Designers LLC. I’m interested in your company ordering some circuit breakers for a electrical project am working on for my client. I want you to get back to me how much it will cost to order the 3-Pole Circuit Breaker, 200A, T3 Type for Symmetra PX250/500kW PD3P200AT3B. I will appreciate if you can send me an estimate to my company email address [email protected] and the the lead time on these order and i will gladly call you back to pay everything in advance for you to order them in to be collected from your store.Thank you and i will look forward to your reply soon.Please feel free to text me on 817-900-2949 if you have any question.

Sincerely, Mr. Darrel Wolf

HELLO, CAN ANYONE HELP ME OUT. I HAVE A CRABTREE (36 YEARS OLD) MCBs BOX WITH 11 WAYS ALL USED. FITTED WITH VARIOUSE SIZED TYPE 1 MCBs. THIS PROBLEM HAS ONLY JUST ARROS, WHEN PLUGGING IN MY 240 VOLT STICK WELDER THE 32 AMP MCB CLICKED OUT, I THEN PUSHED IT BACK IN, BUT HAVE NO POWER AT MY SOCKET, THE FUSE IS OK. IF THE MCB IS DAMAGED WHAT TYPE SIZE MCB SHOULD I INSTALL IE B C 32 OR 40 AMP PLEASE HELP IF YOU CAN, MANY THANKS

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Calculating Electrical Load Capacity for a Home

Timothy Thiele has an associate degree in electronics and is an IBEW Local #176 Union Electrician with over 30 years of experience in residential, commercial, and industrial wiring.

The Spruce / Kevin Norris

• Electrical Capacity
• Capacity vs. Load
• Calculating Capacity
• Calculating Load
• Finding Appliance Wattage

Electrical capacity isn't something you need to think about on a day-to-day basis, but it becomes important when planning upgrades, looking for new appliances, and installing kitchen or laundry essentials such as dishwashers and dryers.

Calculating how much power your home needs involves an electrical load calculation, knowing your home's electrical capacity, and making sure the load is using no more than 80 percent of it.

What Is Electrical Capacity?

Electrical capacity is the maximum amount of electricity a circuit, panel, or generator can handle before it short circuits, trips—or in the worst-case scenario, causes an electrical fire. It's measured in amperage (or amps).

You can estimate your home's electrical capacity based on its age.

• Homes built before 1950 with knob-and-tube wiring and screw-in fuses typically have a capacity of 30 amps.
• Homes built between the 1950s and 1960s tend to have a capacity of 60 amps .
• Homes built from 1960-1980 (or upgraded older homes) will likely have a capacity of 100 amps.
• Homes from 1980 onwards have a minimum capacity of 200 amps .
• Newer homes have a capacity of 400 amps , with larger homes of 10,000 square feet and above seeing a higher capacity of 800 amps.

The main source of power in your home is an electrical panel: a metal box typically found in the basement, utility room, or garage. The panel brings in electricity from external wires outside and distributes it to the branch circuits in the outlets throughout your home. It also contains a circuit breaker, which is a switch that can be turned on or off manually, and will automatically shut the system down in the event of an overload.

Plugging in everyday phone and laptop chargers, table lamps, small appliances, and fans won't make a big impact on your home's electrical capacity, but knowing the limit becomes important when installing or upgrading larger appliances such as laundry machines or heaters.

Electrical Capacity vs. Load

Electrical capacity is the amount of electricity your home can support, while electrical load is the amount actually being used.

Calculating how much power your home needs involves looking at the amperage load all of your appliances and fixtures consume, and making sure that load never exceeds 80 percent of the electrical capacity. If the load is regularly above 80 percent of the capacity, it's time to consider upgrading your electrical panel.

How to Calculate Electrical Capacity

The first step to figuring out electrical capacity is to calculate the conversions between watts, volts, and amps—since you'll see all three of these terms used to describe electricity used. They can be converted in a couple of different ways:

• Volts x Amps = Watts
• Amps = Watts/Volts

Using these formulas will help you to understand all of the electricity in your home on the same measuring system, since appliances list their load in watts, while electrical capacity is often listed in amps, and outlets in the typical US home are 120 volts; while the overall system is 240 volts.

For example, a 20-amp, 120-volt branch circuit has a total capacity of 2,400 watts (20 amps x 120 volts), and with the 80 percent rule in mind, can support light fixtures and plug-in appliances adding up to 1,920 watts of power without the risk of circuit overload .

The same formula can be used to determine the capacity of the house's overall electrical service. Because a home's main service is 240 volts, the math looks like this:

• 240 volts x 100 amps = 24,000 watts
• 80 percent of 24,000 watts = 19,200 watts

In other words, a home with a 100-amp system can support up to 19,200 watts of power load at any given time.

How to Calculate Electrical Load

After you know the capacity of individual circuits and of the home's full electrical service, you can then compare this with the load, which you can calculate simply by adding up the wattage ratings of all the various fixtures and appliances that will be drawing power at the same time.

You can calculate the overall amperage needed to power your home with these steps:

• Add together the wattage capacities and ratings of all lighting branch circuits, outlets, and permanent appliances.
• Subtract 10,000.
• Multiply by 0.4.
• Add 10,000.
• Divide the total by 240.

A typical 1,500 square-foot home has an electrical panel with a 200-amp capacity, or 38,400 watts (calculated by multiplying 200 amps x 240 volts, and multiplying the total by 0.8 for the safety margin).

Example Electrical Loads

While it's always best to be extra-cautious with electrical loads, remember you also don't run every electrical appliance and fixture in your house at the same time.

For example, you wouldn't always be running your washing machine, dryer, microwave, and dishwasher, and would only ever use the heater or air conditioner , not both. When using the table and adding up your total electrical load, you only need to factor in one of those appliances at a time.

Where to Find the Wattage of Your Appliances

The electrical load each of your appliances needs to run should always be clearly marked in watts on the back or bottom of the appliance, or on its metal nameplate. If the wattage isn't listed, the amps and volts might be instead, and you can multiply the amps by the volts to calculate the watts.

When to Call a Professional

While an electrical load calculation is something that can be done yourself using the different formulas, it's best to call a professional if you're unsure of your home's age or electrical capacity. They will be able to examine your electrical panel and share the amperage capacity for you to keep in mind moving forward.

If you're considering upgrading your home's electrical panel or oversizing the electrical service to prepare for future electric car purchases, high-powered hobbies such as pottery or welding, or a tankless water heater installation, you'll need to bring in the pros, too.

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Electrical Engineering Centre

Sharing The Knowledge about Electric Motor,Circuit Diagram,Cable,Wire,Formulas,Theory,Motor control,HVAC,Video,Earthing & Many More

• Electrical Formulas
• Search Results
• Miniature Circuit Breaker Sizing

When we design or build a electrical installation,we must know how to choose the suitable circuit breaker for the any application that we design.For electrician,this is compulsory knowledge.So here i want share my little knowledge about circuit breaker sizing method.

This calculation for sizing the miniature circuit breaker .For other circuit breaker likes MCCB,ACB,VCB,OCB and etc,it have a additional factor and calculation that we must consider.

For that why i will discuss it in my next post…so this time i want explain the simple way how to sizing the miniature circuit breaker for our application.

 How to sizing a miniature circuit breaker?

Why it so important? Ok..it have many reason as below :

• For electrical safety purpose
• To not over cost to purchase circuit breaker when we over sizing it. or not suitable
• To prevent the equipment from damage before it can use it.
• To easy when we want purchase the mcb,because we have all detail  information about it.
• To make the design matter more effective and faster to complete.

How to select a suitable  MCB for the application?

we must focus in 3 main points to choose a right MCB for electrical circuit :-

1) Ampere rating ( In )

2) Short circuit breaking capacity ( If )

3) Application type

Calculation for Miniature circuit breaker ( MCB ) sizing

Example : We want install 1 unit single phase motor.full load ampere is 20 ampere. Power supply 240VAC,50 Hz

1) Ampere Rating ( In )

Formula : In = Iz + (0.25 x Iz)

In = ampere rating      Iz = Load ampere

Calculation :

In = 20 ampere + (0.25 x 20 ampere)

In = 25 ampere@ 32 ampere

* in market we cannot found 25 ampere of MCB,so we can used 32 ampere for our application.

Refer the ampere rated in technical data for MCB from manufacture

From the technical data,we selected the ampere rated for MCB is  : 32 ampere

Formula : If = V / 0.276 (R cable)

If = short circuit breaking capacity

V = Voltage

R cable = resistance cable ( 0.276 )

Calculation :-

If = 240 / 0.276

If = 870 ampere

From the answer,we can decide to select rating for one ( 1 ) kA

3) Application type.

From application,it state the single phase motor,so we can choose TYPE C for MCB.It for general application and for small motor.Please refer the table below for MCB type for the application.

* For more detail,please refer the manufacture Technical data / user manual or contact the supplier.

ABB MCB Technical data

MCB Type,Instant trip and Application table

—————————————————————————-

1                                  2.7 – 4 In                   Domestic and commercial

B                                  3 – 5 In

—————————————————————————

2                                  4 – 7 In

C                                  5 – 10 In                     General use,small motor

3                                 7 – 10 In

D                                10 – 20 In                    High in rush equipment

4                                10 – 50 In

K                                                                       Frequent short duration

Z                                                                       Semiconductor devices

So…the conclusion from the calculation,we get the specification for MCB to install on our application :-

1) Rated ampere is : 32 ampere

2) Short circuit breaking capacity is : 1 kA

3) Type of MCB is : Type C

For more detail,please contact the electrical engineer for their suggestion.This is a simple way how miniature circuit breaker sizing .Please refer the IEE manual for electrical guideline.

Please give an example for selection of MCB(type C or D)for a 3phase induction motor,say 10HP,15Amp- if started using an across the line(DOL) as well as wye-delta starter.

why multiply with 0.25 every time with the full load current of the component?

mcb current capacity is 125% of rated capacity. that why we r adding 25% of rated

Please advice me the book reference for the study of cable sizing calculation

Hi, Please read my last post about cable sizing –> http://www.electricneutron.com/wiring-techniques/electrical-cablewire-sizing/

from your example :- 10 hp;15 ampere

the calculation is :

15 ampere x 0.25 = 3.75 + 15 ampere = 18.75 ampere So we can choose the MCB ampere rating = 20 ampere For induction motor with DOL stater,the suitable type is C or D

As the starting current is not the same for DOL and star-delta starters can we use smaller MCB for star-delta starting?. Is it correct if I say 20A for star-delta and 25A for DOL starting

U can use a smaller MCB but make sure it not below from the full load ampere rating.One more important thing is the TYPES of MCB must suitable with your application.Example MCB type C or D for motor stater.For more safety about overload protection,we still have the overload relay :D

For frequent start/stop or DOL starting, OL relay won’t protect. MCB will trip for inrush current.So we used MCB type C rated 2xFLA for DOL and 1.5xFLA for star delta starting. Another important aspect is the rating and type of MCB should match the duty of motor -AC1,2,3 etc. For frequent start/stop like pressure switch operated water boosting pumps should be use type C duty AC3 or type D AC1?. Therefore the formula 1.25xFLA is for which duty class and will the tripping and contact wear problem will be solved for some class of duty by using class D MCB?. Please elaborate.Manufacturer’s literature will be a useful guide.

Please advice me. if i have 5 unit 10 Amp MCB type and 1 incoming fuse isolator. what would be the rating of fuse inside isolator..example:total all MCB = 50A?

You can used isolator switch 40 ampere or 62 Ampere..it ok if ampere size is bigger.It spare for future additional of mcb

15 ampere x 0.25 = 3.75 + 15 ampere = 18.75 ampere

Dear sir , in above example to calculate ampere rating why you addaed 15 , plz explain

Dear sid… first we multiply the full load ampere value with 25%.After that we add back with total ampere value to get total ampere for MCB rating.

sir plz tel me 63 amprs load i have 4.a/c 1`5 ten 1000wts light

Hi guys Please someone can confirm if I can use 3 phases breaker on a 1 phase load. It may sounds weird, but I do have this spare and dont wont to buy one new 1 phase breaker. Believe is ok, but nor sure. Thanks

can you update me about this post: The calculation for sizing the miniature circuit breaker.For other circuit breaker likes MCCB,ACB,VCB,OCB and etc,it have a additional factor and calculation that we must consider.

can you plz tell me why you are multiplying total load current (Iz) with 125%?

i have been trying this but i was multiplying the load current with 120% and getting the right ans for the ratings which are given in watts but couldn’t got the ans for the ratings which are given in hp.

can you plz solve this for me??

mcb rating for 0.5 hp. my sol. is— total load current will be Iz = 0.5*746/220 = 1.69A

In = 1.69(120\100) = 2.03A

according to this we can use mcb of rating 2.5A.

but the ans is 6A.

can you solve this??

Your calculation 100% right. Pls ….mcb class selection Ex= class B for Light Class C for soficticated Class L for ( B+C)

hello sir, I am having problem with 40hp 3phase motor 63amps MCB trips after running for an hour,when i bypass the MCB and run for an hour it blows off the incoming 100A fuse.checked motor meggar,good more than 20Momhs,checked phase to phase coil winding-good.phase to phase voltage also good-415v.What could be the reason for the above problem thanks.pls help.

Please check your phase either is completed 3 phase or not.

you need 125 MCCB/Fuse 40 x 746 = 29840 watt FLA = 29840/611 FLA = 49 Amps Assume your are using star-delta starter: 49 x 2.5 = 123 Amps So the nearest MCCB available in market is 125 Amps

1) If This is VFD what is the breaker size? 2) If this is soft Starter What is the Breaker size? 3) Assume if this is DOL starter what is the breaker size? 4) For selecting of cable we have to use the rated current or breaker selected current.

Why 49×2.5 using sir…

How do we know R Cable ? or 0.276 is a default value ?

Those fumula are very useful for electrician thanks

I have 15 amps motor but am using mcb 63 amps this is not suitable how to calculate mcb amps I dnt no guide me

15 amp motor In=Iz+(0.25×Iz) Amp rating =15 +(0.25×15) =15+3.75 =18.75 amp. Because…market avaleble MCB is 20amp =20 amp, c class.

why we the 0.25 factor.. and i am using the fan motor rating is 3.8 amps can u tell me appropriate MCB

Your motor watt is =1000 Because mcb rating is =4.75amp Your appropriate MCB is 6amp C class

Great…nice sharing

why are used the constant values to rated current and what is the relation between resistance cable and the MCP circuit breaker ….. and your equation i can u used it in all times and for all difference value of rated current …… ABB more price and less clear compare with Schneider…..

Hi! This is my first visit to your blog! We are a team of volunteers and starting a new initiative in a community in the same niche. Your blog provided us beneficial information to work on. https://www.localunitedservices.com/electrician-san-diego You have done a wonderful job!

your above calculation of the mcb sizing is wrong how do u arrive at 25@32ampere,the working is confusing.

Hi sir, In our machine we are using 4 servo motor and 2 induction motor, servo motor ratings are total 2300w, induction motor loads are 2000w, how i can select the main MCB for this machine.

Hi, Please calculate total full load ampere for your system ( 2300W + 2000w).Use this formula to get FLA –> P = I x V x 1.732. After you get the FLA than you can sizing the MCB.

The incoming neutral of two sources of supply should not be mixed up ? Why?

For three phase input and three phase and single phase output , TPN MCB is preffered over FP MCB .Why?

as per the above example,i came understood that in an small non ac house each and every equipment (right from small water pump to geyser or water heater) of different amp rating should have different,different mcb’s rather than traditional 16amp or 20amp single mcb for the whole small house, Is it right sir,replay me soon.

how many sizes start mcb and and last size

Hi,You can refer Manufacturing Catalong

How we can calculate mccb for 350hp motor

Hi, Please get the full load ampere for 350hp motor,and used formula to get suitable breaker size

350 Hp =350×746 =261100 watt Amp =593.40 Rating =741.75 amp Appropriate MCCB is= 800 amp,4 pole ISLCX0028 . (standardelectrical.net/pdf/mccb.pdf)

Dear Sir what is . Selection method for MCCB,

How much rating of circuit breaker for 1500kva?plz answer with formula. How can i calculate PFI rating?plz answer with formula and example.

I am starting a fitness centre. I am taking a supply of 40 kW to cater to all my requirement. Supply is 3 phase. What should be the mcb capacity at my distribution box. What type of mcb, how many poles and current capacity. Builder has installed a 63 amp mcb. Should I change it.

Lights 100w fan 50 w + one 5a socket What is the range of mcb

Hi Sir i have 6 stright light one have pole having 6 bulb one bulb for 2000watts and i am using each pole one breaker separat separate 32 amps breaker how should be main breaker size

excuse me sir… Why you chooses resistance cable with 0.276 it depend on table or calculate? can you saw me plz

Please help:

I will start a project in U.S and my incoming source is 480V/3P/60Hz/100A. What size mccb should i use that is good. How about 690V/3P/50-60Hz/100Amp/?KA?? Need help to size up the KA!!

I want to use 13A socket 6 nos for onl circuit so what size of breaker can I use? how can I calculate the breaker size?

Hi , I have fitted 6 emergency lights with 8 watts each light, now Iam not sure what right rate of mcb I should used in consumer unit, please can anyone help me calculate the mcb amps !? Thank you all

Sir pls tell which ampere rating mcb should we used for different different load ??

I have 3 phase 10 hp hydraulic pump motor and 2 nos. 15 kw single phase heater in my panel.

Please help me to get the rating of main incomer MCB and power cable for this. And pls tell me the type of mcb also

Thanks in advance.

How to test mcb for its tripping without much resource

I have 63 kV transformer and I need to run 100 HP motor shall I run in the same transformer or have to go for 100 kV transformer.pls reply

And also if I use 100 HP motor with 1440 rpm than can I use 125 amps capacity MCB and star Delta fully automatic relay and contactors. Pls reply

Dear sir, Let me know, which circuit breaker can be use for fixing 2 nos 3 phase 12kW heaters( 2 nos)

I want a 3 phase mcb which can trip whenever any phase is absent… Please tell me which mcb is suitable….

Must use phase failure device

Please use Phase protection relay

A 6A type B mcb is used as a combined overload,short-circuit and earth fault protective device in circuit. (a)what is the maximum operating current as, (1)An overload protection device? (2)a short-circuit protection device? (b)what is the maximum allowable earth fault loop impedance of the circuit?

Dear Sir, Im pleased u that… where did you get this formula In=Iz+(0.25*Iz)

Sir what is the formula for mcb for om service entrance

I have need to run (3 Phase) 20 HP Load (Motar and light ) How do use MCB Sub Meter Capacity pls reply

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