How to Properly Size VFD Cable

DavidWalter84

Member
Join Date
Mar 2016
Location
Memphis, Tennessee
Posts
17
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.
 
Size the wire to serve the load

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?
 
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.
 
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.
 
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.
 
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
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...
 

Similar Topics

I want to improve a semi custom product I am delivering to a customer with implementation of safety relays. The system is fairly simple with just...
Replies
5
Views
543
I've been developing a tester using a Micro850 in Connected Components Workbench and part of that tester requires me to scan a barcode every test...
Replies
11
Views
2,126
I have a UDT with multiple elements that all show on the display perfectly using direct reference. For example I have the following instance on a...
Replies
2
Views
1,393
Hello: Today I have been testing a number of OPCUA servers with Logix symbolic client functionality. I discovered that when you create a user...
Replies
20
Views
6,668
Hello, We have two identical SLC 500 PLCs, one for each package. Part of the programs of one of them is not working properly. Both projects are...
Replies
7
Views
2,057
Back
Top Bottom