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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.

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).

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.

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.

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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:

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?

Solution:

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?

Solution:

• Voltage: 240V (Single Phase)

Circuit Current:

According to the ohm’s law,

• I = P ÷ V
• 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

Good to Know:

• 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.

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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.

Example 1:

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

Solution:

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
• I = 7.82A

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

Example 1:

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

Solution:

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

The minimum and recommended size of circuit breaker:

= 10.21A × 1.25

= 15.5 A

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.

Example 2:

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

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

Solution:

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

Example 1:

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

Solution:

• Current = Power / (Voltage × √3)
• I = 17000W / (415V × 1.732)
• I = 23.65A

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

• = (1.25 × 28A ) + (30A)
• = 75A

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Good to Know:

• 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.

• 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.

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.

 Load Type Maximum Circuit Breaker Size % of Current Resistive Loads, Heat, Stoves, Toasters, Water Heater etc. 125% Lighting Loads 125% 430-152 Motors Hermetically Sealed*, Air conditioners And heat pumps 175% Welders 200% MCP Breakers for Motors 125% or next larger size

* 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.

Electrical Technology

1. Tushar Kanti BiswasI says:

I went Drive calculating process

2. Serhii says:

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.)

3. Pragalathan Moodley says:

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

4. IKUOMOLA Agboola Olorunsoga says:

Love to always get updates. Thank you.

5. mutumwapavi binduki says:

6. Eshon Jonathan says:

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.

7. Francis Akowuah says:

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

8. Mukesh says:

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

9. Mukesh says:

CB used for protection of electrical equipment& machines.

10. Tom says:

There in no mention of operating time. Why?

11. Pankaj Giri says:

Please post on ckt breaker maintenance
Thank you

12. Joe Paul says:

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

13. Akudo Sharon says:

Good update

14. Akudo Sharon says:

15. Albert says:

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

16. Mr. Darrel Wolf says:

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 sales@prodesignersllc.com 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

17. DAVID HOUGH says:

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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