Sizing Circuit Breakers

Dear,

What kind of circuit breakers are you talking about ?

Is it for main power source? Voltage ?

Is it for lighting applications ? residential - hotels ?

Hello engrwali,

As you can see from response, there are many considerations depending on the type of load and application. There are no simple formulas for every situation. If you are not ready to be more specific, you may want to look for a copy of your local electrical code. There you will find a lot of information that schools do not cover. And that is just the beginning!

After you recover from that "very dry" reading material and find yourself a little concerned about how much the schools skipped, come back to this forum and ask your questions. There are a lot of very good people here that love to help. And congratulations on your entry into the world of engineering!

The circuit breaker must be adequate to protect the downstream circuitry.

• There would be no point in fitting a 32A breaker to a lighting circuit, where the cable in the circuit was rated at 10A maximum - the cable would disintegrate before the breaker tripped!

Hello Mr. Wali

I dont know exactly the place which circuit breaker is used is it for high voltage or low voltage but I see you are asking as if it is for a distribution board. I think there is no specific formula as you mentioned but you are going to calculate the load easily by power(either in Kw or in KVA) one by one and also there is one important point the total power is not the load of your building or place because not all equipments start at the same time you must multiply by load factor then it is your load according to that load you will find the amperage of your circuit breaker. You can search and use internet there are many technical data..

Good Luck

When you select circuit breaker, keep in mind that the circuit breaker should protect both the electrical loads as well as the cable. You can follow the steps such as

1. Calculate load current (example, 100A)

2. Select cable size based on load current, ambient temperature, cable type, raceway etc. considering the local code. Generally, the cable has to be selected with the de-rated ampacity not less than 125% of continuous load (example, 125A)

3. Select the circuit breaker with rating not more than the cable ampacity (example, the available one between 125A and 100A, close to 125A is better selection to avoid nuisance tripping).

The circuit breaker rating also should not be greater than the one used in upstream circuit for maintaining the coordination.

In addition, other factors like short circuit rated current, voltage range, AC/DC etc. must also should be considered.

Hope it helps.

- MS

Dear all,

If this is LV circuit, and if you can put up with my awful hand writing, you can try

http://www.akihito-shigeno.com/TERAKOYA/TERAKOYA.html

Regards

Sorry wrong http. the correct one is;

http://www.akihito-shigeno.com/files/EES_H001_008_R1.zip

Your calculations are good and very helpful.

-MS

GA from me, good job.

thanks

Dear sir,

Very good and very useful materials. Thank you for the same.

Regards,

Prashanth

India

thanks mr akihito for your concern your calculations helped me alot.

i would like to ask you and all other fellow members on forum and guests that is. Is there any thumb rule to size CB for example for LV Db's submain DB's, Main DB's etc

thanks and regards

First look for the data plate on the device you're connecting or maybe a wire. Find FLA or LRA, the full load in amps. The load will be shown in watts or kilowatts. Find out the current amp wise. Now divide the wattage by the voltage of the circuit. Now install the right circuit breakers for the math outcome.

By one and also there is one important point the total power is not the load of your building or place because not all equipments start at the same time you must multiply by load factor.