Generator Over Current Protection for Fire Pump
Generator Over Current Protection for Fire Pump
(OP)
I am getting mixed feedback from different sources on how to size the overcurrent protection device for a generator feeding an ATS that is dedicated to the fire pump. When I read 695.4(B) it says overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized according to 430.62 to provide short-circuit protection only. 430.62(A) Exception No.1 says where one or more instantaneous trip circuit breakers are used the max is based on table 430.52. So when I sized the breaker I used a Magnetic CB that was sized at 250% of the FLA. About half the people I talk to say to size it the same as the primary source protection at RLA. NFPA 20-A.9.6.5 leads me to believe I am doing this right. Any other opinions? Please back up with code.






RE: Generator Over Current Protection for Fire Pump
Mike Halloran
Pembroke Pines, FL, USA
RE: Generator Over Current Protection for Fire Pump
The 250% sizing is for an Inverse-Time breaker, which usually has a magnetic trip also.
This is assuming that you are actually using an Inst-only device, usually called an MCP (Motor Circuit Protector).
RE: Generator Over Current Protection for Fire Pump
I wish there was a way to upload PDF files as I just received a Square D "Electrical Shortz" for "Sizing the Short Circuit Protective Device for a Fire Pump on a Generator." It actually confirms sizing it at 250% of FLA and then setting the trip as need.
RE: Generator Over Current Protection for Fire Pump
In any case, I don't see how the assumption that the inst-trip device has a rating not exceeding the max percentage of FLA allows you to use 250% for and Inst device setting.
If you indeed have an Inst-only device (MCP), the setting is 800% of FLA. If you have an inverse-time C/B, the setting is 250%.
RE: Generator Over Current Protection for Fire Pump
2. See thread 242-29536 re: MCP are not listed devices, they are recognized devices, used in combination starters...
3.Please note fire pump controller is not rated for indefinite time at locked currenr; per article 10.4.4.1(1)(a) of the 2007 NFPA 20 it shall open between 8 to 20 seconds at locked rotor current.
RE: Generator Over Current Protection for Fire Pump