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Old March 20th, 2017, 02:41 PM   #1
DavidWalter84
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How to Properly Size VFD Cable

I need some help with properly sizing VFD cable for a AB Powerflex 755 drive at 65 amps output.

Am I supposed to size for 65 amps 100% or should I size it for 125 % of 65 amps?

Does it make any difference if it's interlocking armor type VFD cable?

What other considerations? It will be running in open air cable tray.
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Old March 20th, 2017, 03:11 PM   #2
Plastic
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Size the wire to serve the load

Quote:
Originally Posted by DavidWalter84 View Post
I need some help with properly sizing VFD cable for a AB Powerflex 755 drive at 65 amps output.

Am I supposed to size for 65 amps 100% or should I size it for 125 % of 65 amps?

Does it make any difference if it's interlocking armor type VFD cable?

What other considerations? It will be running in open air cable tray.
Do not size the wire based on drive capability, size the wire for the motor.
Motor FLA x 1.25.

Also note any application specific details.
Does the motor run with a light duty cycle: 10 seconds every 5 minutes
Or heavy duty cycle: Continuously at near FLA

Any above normal ambient air temperature in the motor cable environment?
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Old March 20th, 2017, 03:18 PM   #3
DavidWalter84
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The drive capability is much larger than the motor. almost 75% though and future proofing is why I want to size on VFD capability and use the VFD setting to protect the motor.
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Old March 20th, 2017, 03:24 PM   #4
Mark Snodgrass
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Article 430.122 of the NEC states you must use 125% for the conductor sizing, which would be 81A. This corresponds to #4AWG conductors.

What brand cable do you plan on using? I use Belden and they have a chart in their VFD cable catalog which gives the size, as well as part numbers, based on motor HP. I am guessing you are driving a 50HP motor, which would be #4 VFD cable. They do all the figuring for you based on the NEC, which makes the sizing very easy.
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Old March 20th, 2017, 03:35 PM   #5
jraef
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Quote:
Originally Posted by DavidWalter84 View Post
The drive capability is much larger than the motor. almost 75% though and future proofing is why I want to size on VFD capability and use the VFD setting to protect the motor.
If so, you must size the cable for the largest size motor you can hook up, because you don't know what the future guy will try to do. 65A drive would be 50HP ND (pump and fan duty), so NEC chart says cable must be capable of 81.25A so as was said, #4 cable minimum.

Side note: I hope you are not planning on using this in Vector mode of any sort. You don't want the drive to be more than 2x the motor size, otherwise the sensors are out of range for calculating the motor circuit. It's OK if you plan on running in V/Hz though because the sensors are only determining motor OL current, so inaccuracy at low speeds and low loads is irrelevant.
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Old March 20th, 2017, 04:46 PM   #6
DavidWalter84
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Quote:
Originally Posted by jraef View Post
If so, you must size the cable for the largest size motor you can hook up, because you don't know what the future guy will try to do. 65A drive would be 50HP ND (pump and fan duty), so NEC chart says cable must be capable of 81.25A so as was said, #4 cable minimum.

Side note: I hope you are not planning on using this in Vector mode of any sort. You don't want the drive to be more than 2x the motor size, otherwise the sensors are out of range for calculating the motor circuit. It's OK if you plan on running in V/Hz though because the sensors are only determining motor OL current, so inaccuracy at low speeds and low loads is irrelevant.
We will be using it in vector mode but the motor is 30 HP and the drive is 65 amp 50 HP ND so it should not be a problem? We were informed by Rockwell that the drive could be 50% larger than the motor so 60 HP ND.
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Old March 20th, 2017, 05:13 PM   #7
jraef
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Quote:
Originally Posted by DavidWalter84 View Post
We will be using it in vector mode but the motor is 30 HP and the drive is 65 amp 50 HP ND so it should not be a problem? We were informed by Rockwell that the drive could be 50% larger than the motor so 60 HP ND.
Ah, you said
Quote:
The drive capability is much larger than the motor. almost 75%...
I read that as 75% LARGER than the motor, as in your motor was 25% of the rating so 15HP or something like that. My bad; now that I think it through my way of seeing it doesn't even make sense...
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