Replacing a 40HP Motor with a 50HP Motor

[SparkOmatix]

I have an Allen Bradley Size 3 Starter with a maximum HP rating of 50 HP, an overload block adjustable to 75 Amps, a 3P 100 Amp Breaker, and (3) #4 THHN Wires. The Compressor motor is approximately 200 linear feet from the MCC panel. This is the current configuation on a 40HP motor. I am looking to replace the 40HP motor with a 50HP motor. My question is this...

Do I look at the cable capacity from a 75C (85A) standpoint or a 90C (95A) standpoint.

Some values I calculated are as follows:

HP = 50
kVA = 53.98
Voltage = 480
FLA = 65
Demand Factor = .8
Diversified Amps = 52
Diversified KVA = 43.18
Breaker Starter Size = 100
Over Load Current = 75.75
Max Motor AMPS = 93.44

If I go with the 75C approach, I would have to upsize the cable to a #3, yes? However, the 90C view says I'm ok with a #4, no?

Does the voltage drop need to be no greater than 3% or is 5% acceptable?

Any insite is appreciated.

 

[dpc]

Assuming you must comply with NEC:

1. You have to use 75 deg C ampacity for conductors to determine minimum size. This is due (mainly) to the ratings of the terminations. If you were derating the conductor for high ambient, etc, you can derate starting from the 90 deg C rating (if it has one), but for sizing the feeder, the 75 deg C should be used.

2. 50 hp motor - NEC full load amps = 65 125% of 65 = 81 A. Conductor must be rated at least for 81 A. That will require #3 AWG (copper), minimum. For motor feeder sizing, you must use the FLA values in the NEC, unless your actual motor has higher FLA than the table.

The motor is happy with 460 A at the motor terminals and will run OK at plus or minus 10% of 460. 5% drop at full load amps is a little more drop than I would design for but it will likely work OK.






David Castor

http://www.cvoes.com

 

[SparkOmatix]

You state "Conductor must be rated at least for 81 A. That will require #3 AWG (copper), minimum."

Looking in the 2011 Code Book, Article 310, Table 310.15(B)(16) I see that a #4 at 75C is good for 85A; therefore, replacing the #4 with #3 would not be required, no?

Also, is it 125% of FLA (65A)or 125% of the OverLoad Current (75.75A)...as shown in my initial post as "Max Motor AMPS"?

 

[SparkOmatix]

DPC,

By the way...thanks for taking the time offer up your thoughts and share your knowledge.

 

[rbulsara]

Also refer to NEC 110.14 (C)(1). One can argue, and even win at times, but I would use 60 dec C column ampacity for sizing conductors for 100A or less or sizes #1 AWG and smaller.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Yes, my mistake. I looked at my personal "cheat sheet" for motor sizing without referring back to the NEC. As Rafiq states, for ampacities up to and including 100A, I generally stick with the 60 C rating, due to the termination issues I mentioned earlier.

So, if all terminations at both ends are rated for 75 deg C, I agree that #4 AWG copper would be adequate. For a new design/installation, I'd go with the #3, but since the #4 is already installed, I understand why you want to stick with it.

Sorry for the confusion and thanks for the clarification, Rafiq.

(It is 125% of the motor FLA. I don't know what other term even means. It could be referring to the service factor amps.)





David Castor

http://www.cvoes.com