Short Circuit Calculation For Circuit Breakers

If I understand this correctly you want to calculate the size of each breaker you will need? Then you aren't calculating the short circuit current, you want to understand the load you are placing on each circuit.

For example, if you have a 480 volt supply to a device that requires 100kw of power then you will need 100,000W / 480v = 208A. Add about 15% headspace so that the breaker won't trip for surge current during start up and the size breaker you want is 239A, which is probably going to be a 250A breaker because that's what is available.

This procedure iz wrong.........................how can you calculate current capacity of motor..............u directly divided load by volatge........wt abt PF,starting current of motor, u should divide by 1.732 if u r in 3 phase.................if u know then tell 2 others r better 2 ask

Renato,

There are two different short-circuit values to consider. 1) The withstand rating, which is what the breaker will accept from the line without becoming inoperable (or even exploding). 2) The let-thru rating which will tell you how much the down-stream equipment must be able to withstand.

When you are applying equipment, both are necessary. All breakers available in the USA (and probably most elsewhere also) are rated for at least 10,000AIC (amps interrupting capacity, the line side withstand rating). You can buy them with typical ratings of 22,000AIC, 65,000AIC and up to 200,000AIC.

An old but very usable source for calculations was in the technical manuals published by Cooper/Bussman Manufacturing (a major fuse company) a number of years ago. The circuit breaker manufacturers will have data for selecting breakers and series ratings for coordinating protection between upstream and downstream breakers.

One source, which gets pretty technical is: http://ecmweb.com/mag/electric_shortcircuit_calculation_methods

I believe the Bussman information can be downloaded from: http://www.bussmann.com/library/docs/EPR_Booklet.pdf. It is a very good teaching guide for the topic.

Other sources can be found where I found this, searching on "short circuit calculation".

Work Safe! --JMM

Bottom line:

You can calculate the available short circuit in a given system so that you can select the proper breaker rating, but you cannot "calculate the rating" of a breaker. That must be done by testing.

dear sir ,

we need the basic formula of desigine the circuit breakers of new load .

kindly send the formula thanks