# Fire Pumps and CT cabinet



## jacksonceo65 (May 12, 2014)

We're working on a bid for which includes a 100hp fire pump.
Alliant Energy wants the service for the pump coming off the CT cabinet, not going through the main service.
The service for the building is 400amp and the pump will be 750amp
The question I have is, can I keep the CT cabinet at 800 amp, or does it have to be larger? And we'll keep the main service at 400amp.


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## ponyboy (Nov 18, 2012)

That pump doesn't pull 750 amps, it's probably more like 125 if it's 489 volt. The 750 is to cover the locked rotor current. Check article 695 in the NEC for more clarification, there's a lot of specialty clauses involved in fire pumps


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## micromind (Aug 11, 2007)

In a nutshell, the conductors to a fire pump need to be 125% of the full-load current of the motor. Any overcurrent device needs to be able to handle the locked-rotor current indefinitely. 

The idea here is that if the fire pump starts and for whatever reason the shaft won't turn, it's supposed to keep trying until the motor burns up. 

The motor will burn up before 125% wire will. Well, in theory anyway.......

Also, there can be no O/Ls anywhere in the circuit. 

So for a 100HP fire pump operating at 480, you'd need 2/0 CU, larger if it's a long distance from the source. The smallest breaker would be 800. Yes, 2/0s under an 800. 

The CTs won't matter, that's the POCOs call.


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