# Short circuit calculations

1. Aug 1, 2009

### Josh111

I have a 24 KW 3 phase 120/208 volt synchronous generator I bought from China and I am having trouble finding the available short circuit amps or the %Z rating. Is there a chart anyone is aware of for common short circuit capacities?

2. Aug 2, 2009

### MATLABdude

Hint: $p=iv$

EDIT: This usually just gives you the maximum current you're able to draw at roughly the nominal output voltage. How to figure out the short-circuit draw, I have no idea. But you could start by finding the coil resistance (if this is possible--it's probably too low to measure with your standard household multimeter). If you're worried what might happen when you have a short, you should probably get a small breaker panel, or an appropriate in-line fuse.

Last edited: Aug 2, 2009
3. Aug 2, 2009

### Josh111

I am aware of this but what I need is the short circuit amps amps

I need to know the amperage that will flow in the event of a short circuit. Every power source has a impedance. This and the applied voltage determines the maximum current flow when there is a short.( this is not the regular amperage rating of the power source but much greater)

The reason this is important to know is because when a short circuit occurs, the maximum current (much greater than rated current of power source or the breaker trip rating) flows for a short moment before the breaker trips. (Usually about 3/60 second)

The danger is that if the breaker is not rated to withstand the short curcuit current of the power source, it may explode!!

Considering the above it is necessary for breakers on a system to be able to withstand the short circuit current. The maximum short circuit current a breaker may withstand is called the "interrupting rating" in the USA. A common interrupting rating in USA for breakers in smaller systems is 10000 amps. Many breakers say:" interrupting rating 10000 amps" or "suitable for use on a circuit capable of delivering not more than 10000" amps. Larger systems require higher ratings which are commonly available. This should not be confused with the breaker's trip amps.

4. Aug 2, 2009

### Josh111

It is not enough to know the resistance because the source is also inductive. With something stationary like a transformer it may be possible to find the inductance in henrys but with a motor inductance decreases when RPM increase( thus a motor draws more current at startup than at rated speed.

5. Aug 2, 2009

### MATLABdude

That's true; however, you know that the inductance will only increase (usually slightly) the coil impedance. 24 kW is a building-suitable backup generator, and unfortunately, this is far outside my experience. However, since this sort of things *is* done quite frequently, you can probably contact Siemens, Square-D or whomever is your favourite fuse / breaker manufacturer and ask them what an appropriate interrupter would be.

Although you're probably feeding with 3/0 or 4/0 cabling, ampacity (with any given temperature-rated insulation) is only for continuous current, and I have no idea whether it'd hold up with 10 or 15 kA spikes (these looks like standardish household interrupting capacity). I have a feeling that whatever is causing the short--or the generator windings themselves--will vapourize in such a case.

Good luck; hopefully someone else more knowledgeable can weigh in on this.