Fire Pump & Utility Transformer Size

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faresos

Senior Member
hello everyone,

I'm working a project where we have a 300KVA, 208Y/120V utility transformer and 25HP fire pump tapped from the transformer secondary. I was asked to confirm that if the service transformer does not need to be sized for the building load plus the locked rotor current of the fire pump and jockey pump per section 695.3(A). I have done this calculation before when we had a dedicated transformer for the fire pump (separate service) but not sure what is exactly we need to do when we have the building load as well. The estimated demand load is at 240KVA. Any thoughts? Thanks,
 
If it's a utility-owned transformer, all you can do is tell them what you are connecting to it. The size is their responsibility and not yours.

You would never use the locked rotor current to size the transformer. That only enters the math when you are calculating OCPD sizes. LRA also may come into the picture if you are calculating the motor starting voltage drop for compliance with 695.7, but the type of starter matters here too (wye-delta, SSRV etc.) Minimum transformer size is under 695.5 and it doesn't use LRA.
 
Thank you for your response and that was my thoughts as well but I thought I might have been missing something all this years!
 
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