Generator Over Current Protection for Fire Pump

[bjenks]

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.

 

[MikeHalloran]

I'd want my fire pump to work even if the circuit and the generator are on fire.



Mike Halloran
Pembroke Pines, FL, USA

 

[DanDel]

Art. 695.4(B) refers to Art 430.62, which refers to Table 430.52, which, in your case using a magnetic(which is instantaneous trip)-only MCP, should be sized at 800% of the FLA. This means that your Inst (magnetic) setting should be at 800% of the FLA.
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).

 

[bjenks]

I think we are on the same page, but I believe 430.62(A) Exception 1 says for the purpose of the calculation, each instantaneous trip CB shall be assumed to have a rating not exceeding the max percentage of motor full-load current permitted by table 430.52 for the type of feeder protective device employed. The trip setting you mentioned is based on 430.52(3).

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.

 

[DanDel]

I don't believe that 430.52(C) applies to your installation, so I don't think you can use that exception (430.61(A) Exception 1).
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%.

 

[rob46]

1. Please make sure that ATS is listed for fire pump application.

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.

 

[bjenks]

I stand corrected... I got "short-circuit protection" and "instantaneouse trip CB" confused. You can only use Instanatneous trip CB in a listed combination motor controller because they are UL recognized and not UL listed. Kohler just told me that the only way to NOT loose the listing of the generator is to make sure they use Thermal-Magnetic CB rated at 175-250% of FLA. I think I got it straight now. I will specify Thermal-Magnetic Breaker rated at 250% of FLA for the Generator-ATS feeder.