fire pump w/ transformer

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bwyllie

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MA
I am powering a fire pump from a 480:208 transformer, the pump is 208V, 3-phase, 88Amps. My plan is to use a 75kva (88x.208x1.73x1.25=39.6 kva,). The service OCPD on the primary of the transfomrer will 250A((88x.208x6)/.48)=228.8Amps). My question how do I size the primary conductors to the transformer, would it simply be 48Amps (39.6/.48/1.73) and use #8s?

Thanks,
 
Not sure of the code cycle that you are on, but here's a paste from the 2002 Handbook:

695.5 Transformers.
Where the service or system voltage is different from the utilization voltage of the fire pump motor, transformer(s) protected by disconnecting means and overcurrent protective devices shall be permitted to be installed between the system supply and the fire pump controller in accordance with 695.5(A) and (B), or (C). Only transformers covered in 695.5(C) shall be permitted to supply loads not directly associated with the fire pump system.
(A) Size. Where a transformer supplies an electric motor-driven fire pump, it shall be rated at a minimum of 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump(s) motor loads, and 100 percent of the associated fire pump accessory equipment supplied by the transformer.
(B) Overcurrent Protection. The primary overcurrent protective device(s) shall be selected or set to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motor(s) and the full-load current of the associated fire pump accessory equipment when connected to this power supply. Secondary overcurrent protection shall not be permitted.

I don't think that the 2005 wording is substantially different.

Good luck
 
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