How do you size the conductors upstream of a VFD?

FreeAtLast

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F
Join Date
Mar 2019
Location
Nashville
Posts
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I have a branch circuit with the following motor and drive:

Motor
1.1hp
480VAC

VFD
1.5kW Normal Duty
2.0hp

NEC 430 seems to suggest that I should use 430.250 to find the tabulated Full Load Current of the motor. But is this (3Amps) only used for the conductors between the motor and VFD? Does anyone know what codes apply to the rest of the branch circuit? How would I size the conductors upstream of a VFD?

Thanks for any help.
 
The smallest conductor generally permitted for branch circuits for residential, commercial, and industrial locations is 14 AWG copper, which is overkill for your application.
 
If it is 1/4hp motors I'll usually downsize to 14AWG otherwise its 12AWG for everything else power wiring in the panel unless current requirements are higher.
 
Ken Moore said:
Not sure what the code is for small motors. At most industrial locations, the minimum for a drive supply is 12AWG, just in case the need something larger in the future.
Click to expand...



Yes, it's been my experience that industrial sites typically specify 12AWG as a minimum wire size for any power circuit.
 
FreeAtLast said:
I have a branch circuit with the following motor and drive:

Motor
1.1hp
480VAC

VFD
1.5kW Normal Duty
2.0hp

NEC 430 seems to suggest that I should use 430.250 to find the tabulated Full Load Current of the motor. But is this (3Amps) only used for the conductors between the motor and VFD? Does anyone know what codes apply to the rest of the branch circuit? How would I size the conductors upstream of a VFD?

Thanks for any help.
Click to expand...



As for upstream vs. downstream, I'd always size my line side wire based upon the drive's maximum capability, even if the drive is oversized for the application.
 
These guys are right that a lot of specs will require 12 gauge min. However, you need to check the manual on the drive you spec to see what gauge range the terminals will handle.

The problem with a 12 gauge min wire spec is that a lot of low amp devices won’t take a 12 gauge wire, without a hack modification.

There is nothing wrong with using 14 gauge where applicable.
 
Article 430 in the NEC is the definitive place to look. On the first or second page it has a nice diagram showing exactly where to go.
 
Your VFD manual will guide you. It should give you a size for overcurrent protection. You can size to that. My practice has been to use a min. of #14 for all motor circuits. (I work in an industrial environment) And always,always, always wire to the maximum capacity of the VFD, not just the motor.
 
ValeoBill said:
Your VFD manual will guide you. It should give you a size for overcurrent protection. You can size to that. My practice has been to use a min. of #14 for all motor circuits. (I work in an industrial environment) And always,always, always wire to the maximum capacity of the VFD, not just the motor.
Click to expand...
This is the correct answer. VFD manual specifies the overcurrent size, and your job is to size the line side wiring to match or exceed that. Anything else is wrong.

Motor wiring will be sized to minimum 125% of FLC from the NEC motor FLC tables.
 
430.122 dictates that for feeding a VFD, the line side conductors must be sixed for at least 125% of the VFD rated input current. For most VFDs, the input current is actually less than the output current.
 
Last edited:
jraef said:
430.122 dictates that for feeding a VFD, the line side conductors must be sixed for at least 125% of the VFD rated input current. For most VFDs, the input current is actually less than the output current.
Click to expand...

JREF,
Thanks for the follow up.
 

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