fire pump conductors

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brentp

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Why are my engineered drawings showing service conductors to the fire pump controller that seem to be way oversized? I have seen this on the last 3 jobs. My current project has parallel 500's landing on a 250A breaker. 100' max length.

The lugs on the controller breaker seem to never accept the conductors (1-350 lug per phase in this instance)on the drawings. Is this common?

How do I figure service conductor size to the controller?

This is a great place to learn, so I thought I'd start here with my research.

Brent
 
Re: fire pump conductors

See 695.6(C). Watch out for the voltage drop as required in 695.7. Minimum conductor size is 125% of full load, but many people make the mistake of sizing the conductors at 600%.
 
Re: fire pump conductors

Brent
The OC device must be able to handle the Lock Rotor Amps of the pump and other associated equipment. We generally use 600% as a figure.
However as ga77 says the conductor is sized at 125%. It is better to run the pump until destruction rather that trip the breaker during a fire.
 
Re: fire pump conductors

NFPA 20 (last time I read it) requires a fire pump controller to perform an across the line start even though the controller may have other normal starting provisions. There are other details to this that escape me at this time. It has to do with the voltage drop as mentioned previously. I think there is also something of a time factor related to full pump pressure, but it's been a while. NEC 695 is a somewhat of a parallel of NFPA 20. NFPA 20 includes more performance requirements.

That is maybe why the engineer oversised the conductors.

If you look at a FP controller you will see a switch handle that will close and start the pump and have to manually pinned in the closed position. This is for the firefighter use.
 
Re: fire pump conductors

I have been confused on this issue before also. The OC device has to be oversized to handle the locked rotor current (say 600% for example).

What code article tells me I don't have to size the conductors for increased breaker size?

Or do I have to size the wire for the breaker?

STeve
 
Re: fire pump conductors

Steve, to make this simple, size the overload protection for locked rotor (usually 600% of the FLA and, for that matter, it is not even required). The rest of the motor circuit is the same as any other circuit (125% of the FLA for conductor sizing, 115% for switch sizing, etc.) The motor branch-circuit short-circuit and ground-fault protective device is to be set for at least locked rotor and the overload protection on the listed fire pump controller is to be left alone. The controller overload device will open up after about 20 seconds of locked rotor current.

The reason the fire pump motor circuits get so complicated is that everyone attempts to apply their own common sense to the circuit. This usually results in either oversized motor circuit conductors, undersized overload protection, or undersized motor branch-circuit short-circuit and ground-fault protective device. :D
 
Re: fire pump conductors

I have wired fire pumps in the past that have been Wye-start Delta-run motors which has reduced the conductors from the controller even further.

Roger
 
Re: fire pump conductors

Thanks Charlie, that clears it up. And I think I found the code references that allow the wire to be protected at less than its ampacity. 695.6(C) covers sizing the wire. And table 240.3 lists fire pumps as an "other article" for exceptions to 240.4.

Steve
 
Re: fire pump conductors

Thanks for the help guys. I can now talk intelligently with my engineer. :)
 
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