Multi-Motor Load Feeder Calculation

[StephenSDH]

Just hope for someone to look over my numbers and see if I am on the right page.

Control Panel with
(1) 50 HP Motor 65A FLA @ 480
(8) 1 HP Motors 2.1A FLA @ 480

Feeder size Based on NEC 430.24
(65A * 1.25) + (2.1 * 8) = 98A Minimum (Select #3 THHN)

Breaker Overcurrent Protection Based on NEC 430.62
(65A * 2.5) + (2.1 * 8) = 179A Maximum (Select 150A Breaker)

Ground based on 150A Breaker and NEC 250.122
#6

I just want to clarify that this is not considered a tap and the NEC doesn't have any distance limitations on the feeder length.

Thanks Ahead of Time,

Steve

 

[david luchini]

Just hope for someone to look over my numbers and see if I am on the right page.

Control Panel with
(1) 50 HP Motor 65A FLA @ 480
(8) 1 HP Motors 2.1A FLA @ 480

Feeder size Based on NEC 430.24
(65A * 1.25) + (2.1 * 8) = 98A Minimum (Select #3 THHN)

I'd say this is correct.

Breaker Overcurrent Protection Based on NEC 430.62
(65A * 2.5) + (2.1 * 8) = 179A Maximum (Select 150A Breaker)

150A is acceptable, but it is not the maximum permitted per 430.62, 175A is the maximum permitted.

The calculation would actually be 65A * 2.5 = 162.5 => 175A (next size up per 430.52.)

175 + (2.1 * 8) = 191.8A Maximum, which would be a 175A c/b.

Ground based on 150A Breaker and NEC 250.122
#6

I just want to clarify that this is not considered a tap and the NEC doesn't have any distance limitations on the feeder length.

Thanks Ahead of Time,

Steve

#6 ground would also be OK for a 175A c/b. This is not a tap and there is not length limitation on the feeder. Each of the motors would need a branch circuit OCPD in accordancee with 430.52.

(The "control panel" may actually full under Article 409, but I think the conductor and OCPD sizing would be the same.)

 

[StephenSDH]

I see you can use a 175A breaker. I leaned down to 150A because on my square d slide chart they show a using a 100A (FAL34100) breaker for a 50HP motor which is only FLA x 1.5. I guess it has the do the the instantanious trip of that breaker that they don't use the full 2.5. I don't believe all the motors will be be running at the same time, so I didn't want to oversize too much more then I need. I am not installing the feeder or selecting the breaker so I don't know what breaker type they will choose.

I do have individual overcurrent protection for each motor.

Thanks, Steve