Motor voltage drop calculations...

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bnbowen@uiuc.edu

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We have a 100 Hp, 480 volt, 3 phase motor (124 Amps) 200 feet away from a VFD. To calculate the voltage drop should I use the 124 amp motor running current or the 155 (124 x 1.25) amp wire sizing amps? Moving the VFD closer is not an option - darn. Allowing for 10% loss in the drive, the total load is then 124 x 1.1 x 1.25 or 170 amps. Should the breaker be 200 amps or 225? Am I over thinking this? It appears that I am walking the fence. Thanks for your help and have a good day/night!
 
VFD Cable Length

VFD Cable Length

bnbowen@uiuc.edu said:
We have a 100 Hp, 480 volt, 3 phase motor (124 Amps) 200 feet away from a VFD. To calculate the voltage drop should I use the 124 amp motor running current or the 155 (124 x 1.25) amp wire sizing amps? Moving the VFD closer is not an option - darn. Allowing for 10% loss in the drive, the total load is then 124 x 1.1 x 1.25 or 170 amps. Should the breaker be 200 amps or 225? Am I over thinking this? It appears that I am walking the fence. Thanks for your help and have a good day/night!
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What brand VFD are you using? I know for example that Danfoss specifies a maximum size cable from the drive to the motor. I think this because using a larger cable will cause distortion of the freq created by the drive. Danfoss specifies a max of 1000ft using unshielded cable. I would suggest you contact your drive mfg to determine the correct cable.
 
They want to use a Yaskawa Z1000 drive and Servicewire between the drive and the motor. Between the drive and the input breaker is just normal THHN. Happy Thursday and thanks. Brad
 
"We have a 100 Hp, 480 volt, 3 phase motor (124 Amps) 200 feet away from a VFD"

Normal operating voltage drop should be based on impedance of cable selected, nominal voltage, distance, and running amps.

"the total load is then 124 x 1.1 x 1.25 or 170 amps. Should the breaker be 200 amps or 225"

This would be the circuit providing power to the VFD, I assume. Why not 175A? This question also seems independent of the VD question.
 
Cable from the drive to the motor is sized per NEC FLA of a 100HP motor x 1.25, then adjusted for voltage drop based on circuit length and wire resistance. If you sized for voltage drop before the 125% factor, your wire will be smaller starting out, so your VD will be worse and you end up with a higher size anyway after derating.

Your incoming conductors must be sized per 430.122, 125% of the max amp rating of the VFD nameplate vs the motor. It will be higher in most cases.

The size of the breaker, or fuse, is determined by what the VFD mfr says you must use. In many cases, especially Asian drives, their UL listing is dependent upon having fast acting fuses ahead of them, often a specific brand as well. You must RTFM.
 
Jraef said:
Cable from the drive to the motor is sized per NEC FLA of a 100HP motor x 1.25, then adjusted for voltage drop based on circuit length and wire resistance. If you sized for voltage drop before the 125% factor, your wire will be smaller starting out, so your VD will be worse and you end up with a higher size anyway after derating.

Your incoming conductors must be sized per 430.122, 125% of the max amp rating of the VFD nameplate vs the motor. It will be higher in most cases.

The size of the breaker, or fuse, is determined by what the VFD mfr says you must use. In many cases, especially Asian drives, their UL listing is dependent upon having fast acting fuses ahead of them, often a specific brand as well. You must RTFM.
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Thanks a bunch guys!! You made my day. Brad
 
is this correct?

is this correct?

Jraef said:
Cable from the drive to the motor is sized per NEC FLA of a 100HP motor x 1.25, then adjusted for voltage drop based on circuit length and wire resistance. If you sized for voltage drop before the 125% factor, your wire will be smaller starting out, so your VD will be worse and you end up with a higher size anyway after derating.

Your incoming conductors must be sized per 430.122, 125% of the max amp rating of the VFD nameplate vs the motor. It will be higher in most cases.

The size of the breaker, or fuse, is determined by what the VFD mfr says you must use. In many cases, especially Asian drives, their UL listing is dependent upon having fast acting fuses ahead of them, often a specific brand as well. You must RTFM.
Click to expand...

Based on what you are saying, this would require 2/0 conductors. Is that correct? I just want to be sure I understand this. Thanks!!

Bob
 
bnbowen@uiuc.edu said:
We have a 100 Hp, 480 volt, 3 phase motor (124 Amps) 200 feet away from a VFD. To calculate the voltage drop should I use the 124 amp motor running current or the 155 (124 x 1.25) amp wire sizing amps? Moving the VFD closer is not an option - darn. Allowing for 10% loss in the drive, the total load is then 124 x 1.1 x 1.25 or 170 amps. Should the breaker be 200 amps or 225? Am I over thinking this? It appears that I am walking the fence. Thanks for your help and have a good day/night!
Click to expand...

It appears that sizing for the NEC current will result - 2/0 - in better than acceptable voltage drop, so you don't need to oversize your conductors for it.
 
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