How to calculate 2 motor in one machine?

[Huntxtrm]

I want to make sure, that I am doing this right. Please scold me, if not. I have a 480V 3 phase compressor. It has a 125 hp 3ph pump motor, and a 10 hp 3ph fan motor. I am taking 156A(NEC 430.250) for the main motor, and 14A(NEC 430.250) for the fan motor and combining them. For a total of 170A per NEC. I am multiplying this by 1.25 for continuous load to get 212.5A. I am using this in the 75deg column of 310.15b16 and coming up with 4/0 for my conductors, no temp derate. I am using the NEC total of 170A and multiplying by 2.5 for inverse time breakers, and getting 425A. Going up to a 450A OCPD. And using a #2 EGC per 250.122. does this all seem right? I will be using adjustable trip breakers, or should I just use fixed trip?

Thanks for any help. or Scoldings.

 

[Huntxtrm]

Anybody?

 

[augie47]

Have a potential problem.,,,,,
First, to size your conductor you would only need 1.25 X your largest motor FLA plus the FLA of the others.
This won't change your wire size.
The problem I perceive is having proper GFSC protection for you smaller motor. Per NEC 430.52 the 10 HO must have a maximum GFSC breaker protection of 35 amps. Is this addressed at the compressor ?

 

[gadfly56]

Skid Mount?

Skid Mount?

Is this skid mounted equipment? Sometimes the manufacturer will provide a nameplate that lists the FLA for the unit. This is what you would use for your calculations regardless of the individual motor nameplates. Still need to worry about the overload protection as augie47 points out.

 

[Huntxtrm]

Have a potential problem.,,,,,
First, to size your conductor you would only need 1.25 X your largest motor FLA plus the FLA of the others.
This won't change your wire size.
The problem I perceive is having proper GFSC protection for you smaller motor. Per NEC 430.52 the 10 HO must have a maximum GFSC breaker protection of 35 amps. Is this addressed at the compressor ?

Yes, the smaller motor is protected by the starter on the compressor unit. It is a skid mounted compressor unit.

 

[Huntxtrm]

Is this skid mounted equipment? Sometimes the manufacturer will provide a nameplate that lists the FLA for the unit. This is what you would use for your calculations regardless of the individual motor nameplates. Still need to worry about the overload protection as augie47 points out.

It is a skid mounted unit. Only Nameplate on the unit itself says 100hp/480v, no other info. The motors on the unit say 125hp and 10 hp. I was going off of the motor nameplates for sizing, since there was so little info on equipment name plate. Question, how could they call it a 100hp unit, if it actually has 135hp of motors on it?

 

[mjmike]

For the breaker, take the largest motor FLA X 2.5 assuming a thermal magnetic circuit breaker. If the resultant is not a common breaker size, pick the next size up. Now use that breaker size and add in all remaining motor FLA. Once you get that number, if it does not result in a standard size breaker, pick the next size down.

156A x 2.5 = 390A = 400A CB

400A + 14A = 414A = 400A CB