Motor Calc.

[Alwayslearningelec]

I’m not following how they came up with #1’s MI cable to feed the fire pump. This is 120/208v system and the SP-1 is 208v. 20HP motor @208v is 59.4 amps. The SS-2 disconnect switch is 400A. Not sure how they come up with #1’s to fed the motor.

 

[texie]

My quick calc shows this could be done with #4. Maybe voltage drop.

 

[SceneryDriver]

If it's a 400A switch, it may not accept conductors smaller than #1 on its lugs, and someone didn't want to deal with swapping lugs.


SceneryDriver

 

[infinity]

How did you get #1 AWG conductors from that diagram?

 

[Alwayslearningelec]

How did you get #1 AWG conductors from that diagram?

Feeder tag 100G referecnes a feeder schedule for #1's.

 

[infinity]

Got it. Could be upsized for VD as texie suggested. Fire pumps are one of the few places in the code where VD compensation is required. How long is the run?

 

[texie]

Fire pump circuits must be rated to handle the locked rotor current of the motor

Only the feeder OCPD, not the conductors.

 

[Jraef]

Only the feeder OCPD, not the conductors.

Yes, I saw that mistake and deleted it, but you caught it first.

 

[texie]

Yes, I saw that mistake and deleted it, but you caught it first.

You know how it is around here. Rough crowd.

 

[Alwayslearningelec]

Got it. Could be upsized for VD as texie suggested. Fire pumps are one of the few places in the code where VD compensation is required. How long is the run?

180'

 

[infinity]

180'

Then the conductor size is a little oversized for the application.

 

[topgone]

If it's a 400A switch, it may not accept conductors smaller than #1 on its lugs, and someone didn't want to deal with swapping lugs.


SceneryDriver

Could be. Looking at Hubbell, a 400A switch will accept #2 to 600MCM lugs! IDK how they did it with #1!