Sizing Fire Pump Motors conductors

[MikeStewart]

I am trying to size the conductors for a three phase 460v fire pump motor. The fire pump motors manufacturer didn’t list the amps required for the motor on the name plate but they did list the max brake horsepower as 207.

Looking at Table 430.250 it doesn’t have a horsepower rating of 207 so I have to round up to 250 hp, wow. So now the table states I need to figure the motor is running at 302 amps. So I take the 302 and multiply it by 1.25 (25%) and I have a total of 377.5 amps. I’m assuming now divide that by 3 to be 126 amps per phase before I utilize table 310.16 and 310.15(B)(1)(1) (it’s an unconditioned pump house in the south).

My question Is, is there a formula I can use so I don’t have to take such a large hit rounding up from 207 to 250 on the horsepower? Can I add the 25% before if not? And probably the easiest question, when do I calculate the resistance in my wire length?

Thank you ahead of time.

 

[augie47]

From reading your post it is evident you need some input from an engineer or contractor familiar with fire pumps.
a. you don't divide the FLC numbers by 3 and (b) the deciding factor is often determined by VD as noted in 695.7
It is one area where professional input is vial.

 

[infinity]

Just curious what is the HP rating of the fire pump controller?

 

[topgone]

BHP= THD x GPM x SG/(3960 x PUMP EFFICIENCY). You'll need to know the total hydraulic head (feet) and the GPM output of your pump. Estimate the efficiency of your pump. That figure gives you an idea of the shaft rating you need for your pump (brake horsepower). You choose a motor that is greater than the required BHP of your pump.

 

[MikeStewart]

From reading your post it is evident you need some input from an engineer or contractor familiar with fire pumps.
a. you don't divide the FLC numbers by 3 and (b) the deciding factor is often determined by VD as noted in 695.7
It is one area where professional input is vial.

Maybe you could explain it to me so I’m not ignorant on the subject?

 

[augie47]

Maybe you could explain it to me so I’m not ignorant on the subject?

As noted by others, you need to find the specific details of the motor/controller (actual HP, LRA, etc)
Your conductor size would be dependent on the actual HP (for a 250HP motor it would be a 500 kcmil or parallel equivalent) but that number might not be relevant depending on the specifics of the type controller, the length of the feed, and the 695.7 requirement to limit the voltage drop.

 

[don_resqcapt19]

As noted by others, you need to find the specific details of the motor/controller (actual HP, LRA, etc)
Your conductor size would be dependent on the actual HP (for a 250HP motor it would be a 500 kcmil or parallel equivalent) but that number might not be relevant depending on the specifics of the type controller, the length of the feed, and the 695.7 requirement to limit the voltage drop.

And sometimes that requires the utility to replace the supplying transformer with a larger one.