Trending

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

How to Calculate the Circuit Breaker Size? Breaker Size Calculator with Solved Examples

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.

Related Posts:

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:

Circuit Breaker Size Calculation for Single Phase Supply

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

The general rule of thumb is that circuit breaker size should be 125% of the ampacity of cable and wire or the circuit which has to be protected by the CB. Let see the following solved examples:

Example 1:

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

Solution:

Circuit Current: 16 A

Circuit Breaker Size: ?

CB size should be 125% of the circuit current.

= 125% x 16 A

= 1.25 x 16 A

Circuit Breaker Size = 20A

Related Posts:

Example 2:

What is the appropriate size of circuit breaker for 1920W, single phase 240V AC Supply?

Solution:

• Voltage: 120V (Single Phase)

Circuit Current:

According to the ohm’s law,

• I = P ÷ V
• I = 1920 W / 240V
• I = 8 A.

Circuit Breaker Size:

Simply, Multiply 1.25 (safety factor) to the load current.

1.25 x 8 A

Circuit Breaker Size = 10 A

Example 3:

What is the suitable size of circuit breaker for 230V, 2.760 kW load single phase circuit?

Solution:

• Current = Power / Voltage
• I = 2760 W / 230V
• I = 12 A

The minimum and recommended size of circuit breaker:

= 12A x 1.25

= 15 A

Circuit Breaker Size Calculation for Three Phase Supply

To find the breaker size for three phase supply voltage, we must know the exact kind of load as there are many factors affecting the load current. In other words, same rule won’t apply to the different types of loads i.e. light, motor, inductive or capacitive load as motor takes initially very high current during the starting process as well as power factor involvement. For residential use, we may follow the same formula as above for single phase with taking the √3 (1.732) due to three phase power formula.

Good to know: For the same load, the breaker size in three phase is less than the breaker size used in single phase AC circuits.

Lets find the correct size of circuit breaker for three phase circuits as follow.

Example 1: Which size circuit breaker is needed for 6.5kW, three phase 480V load?

Solution:

Power in Three Phase: P = V x I x √3

Current: P / V x √3

• I = 6.5kW / (480V x 1.732)   … (√3 = 1.732)
• I = 6.5kW / 831.36
• I = 7.82A

The recommended size of circuit breaker is

1.25 x 7.82A = 9.77A

The next closest standard of circuit breaker is 10A.

Example 2: Find the appropriate size of CB for 3-Phase 415V, 17kW load?

Solution:

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

Recommended Size of Circuit Breaker: 1.25 x 23.65A = 29.5A. The next closest value is 30A.

Circuit Breaker Size Calculation for Continuous & Non-contentious Load

As circuit breakers (CBs) and Overcurrent Protection Devices (OCPD) are designed for 100% rated current i.e. a 30A circuit breaker can safely carry the exact 30A current but NEC suggests 80% as a safe current limit as compared to the rated current of CBs. This is because all loads are not same i.e. some loads are simultaneous (continuous) while other are non-simultaneous (non-continuous).

In case of contentious loads for three or more hours, the load current should not exceed 80% of the rated current of circuit breaker and OCPD.

The 80% of a 30A circuit breaker is 24A. This way, a 30A circuit can be safely used for 24A circuit.

In other words, a load circuit having 24A, the appropriate size of breaker 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 x 28 A = 35A

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

• = (1.25 x 28A ) + (30A)
• = 75A

Related Post: Difference Between Relay and Circuit Breaker

Good to Know:

• An over sized breaker used for protection can damage the water heater or other connected appliances even leads to the fire due to overheat.
• An undersized breaker or same rating with load current breaker can trip and reset the circuit again and again. Use the correct size breaker.
• A single phase circuit breaker can’t be used for three phase supply voltage levels.
• A 3-Poles circuit breaker can be used on 3-Phase system using either 2 or 3 poles.
• A 3-Poles circuit breaker can be used on 1-Phase system only and only if indicated by the markings or instructed by user manual.
• 30A Breaker and 10 gauge wire can be used on 240V AC Supply.
• Breaker cannot be larger than ampacity of wire except for some loads like more loads.

In addition, A Circuit breaker rated for:

• 120V can only be used for 120V.
• 240V can be used for 120V, 240V but not for 277V (Commercial applications)
• 120-277 can be used for 120V, 240V and 277V.
• 120V can’t be used on 240V circuit and vice versa.
• 15A, 120V can’t be used on 20A, 120V circuit.

Circuit Breaker Size % and Amps Charts

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.

Related Posts:

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

Check Also
Close