Fire Pump Overcurrent Protection on the Normal and Emergency Side

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jojo

Member
Location
Philippines
Occupation
Electrical Engineer
Hello guys, let me take your stand on this. I noticed in some designs that OCPD rating for a fire pump on the normal side is different on the the generator/emergency supply side. As an example, a 250Hp fire pump, 460V. Its LRA per NEC Table 430.251(B) is 1825 Amperes; its FLA is 302 Amperes per NEC Table 430.250.

To size its conductor ampacity, we calculate it as 1.25 times the fire pump FLA (302A x 1.25= 377A) ~ 400A ampacity.
To size its OCPD, we size it based on the locked-rotor current of the fire pump per NEC 695.4 (B)(1). So for a 250Hp fire pump motor, its OCPD rating would be 2000A.

What I notice is the OCPD rating on the fire pump's emergency/generator side; which is sized at 2.5 times fire pump's LFA (302A x 2.5 = 755A ~ 800A OCPD). Any idea the emergency side is size that way? Is it because there's a limitation on the generator side during motor starting? I'm confuse. Thanks in advance guys.
 

jojo

Member
Location
Philippines
Occupation
Electrical Engineer
The fire pump's conductor size on the normal and on the emergency side is the same. It is sized at 1.25 times the fire pump's full load amps (302A x 1.25 = 377A); 400A conductor minimum. Thanks again..
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The NEC requires the disconnect on the utility side to be sized for the locked rotor current.

My understanding is that the same requirement does NOT apply to the emergency side. And I don't believe there is any other requirement for the emergency generator supply to have a disconnect or OCP sized for the locked rotor current.
 

jojo

Member
Location
Philippines
Occupation
Electrical Engineer
Thanks steve66. Does it mean that the OCP on the normal side is sized based on the locked rotor current because of the disconnect?
 
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