Motor branch circuits on VFDs

[Isaiah]

Does a 460V, VFD driven motor mitigate the need to add 125% to the motor FLA?


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[Dale001289]

Does a 460V, VFD driven motor mitigate the need to add 125% to the motor FLA?


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NEC 430 says you need to add the 125%


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[ptonsparky]

Check 430.122

 

[ptonsparky]

You may need larger conductors to the line side of the VFD than load side to the motor.

 

[Isaiah]

You may need larger conductors to the line side of the VFD than load side to the motor.

Does the cable need to be shielded to the motor ?


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[petersonra]

You may need larger conductors to the line side of the VFD than load side to the motor.

Why? The current is less on the input side.

 

[petersonra]

Does the cable need to be shielded to the motor ?


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Only if you believe the people selling such cables.

 

[David Castor]

The feeder to the VFD needs to be sized based on the VFD maximum nameplate amps.. The feeder from VFD to motor still needs to be 125% of NEC motor FLA.

The cable from the VFD to the motor is a potent source of RFI. If it's in metal conduit, it probably is OK. If it's in PVC, then I recommend shielded cable. Okonite CLX is a good option as well. The NEC does not require shielded cable.

 

[ptonsparky]

Why? The current is less on the input side.

10 HP = 14 Amp via NEC chart
AD VFD
10 hp output =17 A
Input CT 18.7 A
VT 22.6 A

 

[petersonra]

10 HP = 14 Amp via NEC chart
AD VFD
10 hp output =17 A
Input CT 18.7 A
VT 22.6 A

That's likely a foreign made vfd. They often have weird numbers like that. No idea where they come from.

 

[Isaiah]

The feeder to the VFD needs to be sized based on the VFD maximum nameplate amps.. The feeder from VFD to motor still needs to be 125% of NEC motor FLA.

The cable from the VFD to the motor is a potent source of RFI. If it's in metal conduit, it probably is OK. If it's in PVC, then I recommend shielded cable. Okonite CLX is a good option as well. The NEC does not require shielded cable.

Is RFI the same as EMI…electromagnetic interference?


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[ptonsparky]

That's likely a foreign made vfd. They often have weird numbers like that. No idea where they come from.

Won't argue that point.

OP still needs to consider the input amps of the VFD.

 

[ptonsparky]

That's likely a foreign made vfd. They often have weird numbers like that. No idea where they come from.

FWIW a WEG 10 HP VFD was in line that both line & load conductors could be the same size. Output current was still lower than the input.

 

[don_resqcapt19]

Why? The current is less on the input side.

Because the code requires that the conductors to the line side of the VFD have an ampacity that is at least 125% of the rated input current of the VFD. Often the horse power rating of the VFD exceeds the horse power of the motor being supplied by the VFD.

 

[BarryO]

Does the cable need to be shielded to the motor ?

The NEC doesn't care. But those lines are potent sources of electromagnetic interference and nearby equipment may need them to ensure proper operation; e.g. cables in CNC's are usually shielded.

 

[don_resqcapt19]

Does the cable need to be shielded to the motor ?


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The instructions for the VFDs pretty much specify the use of shielded cable. However if you run single conductors in ferrous metal conduit, I don't see a need for shielded cables. I do specify the use of XHHW conductors as that is the type of conductors that is normally used in the VFD shielded cables. It is more resistant to the spikes on the output of the VFDs.

 

[Eddie702]

Conductors on VFD input are sized at 125% of VFD rating plate. Wire between the VFD (or a motor starter) to the motor can be downsize to the rating of the overload protection in the VFD or motor starter

 

[ptonsparky]

Conductors on VFD input are sized at 125% of VFD rating plate. Wire between the VFD (or a motor starter) to the motor can be downsize to the rating of the overload protection in the VFD or motor starter

Wire to motor needs to be 1.25 of chart FLA.

 

[David Castor]

Is RFI the same as EMI…electromagnetic interference?

Radio frequency interference. A type of EMI.

 

[masterinbama]

The feeder to the VFD needs to be sized based on the VFD maximum nameplate amps.. The feeder from VFD to motor still needs to be 125% of NEC motor FLA.

The cable from the VFD to the motor is a potent source of RFI. If it's in metal conduit, it probably is OK. If it's in PVC, then I recommend shielded cable. Okonite CLX is a good option as well. The NEC does not require shielded cable.

Also multiple cables in a common conduit or wireway can become problematic if not shielded