# Sizing OCD and Feeder of VFD



## Tom Solanto (Mar 11, 2011)

I am curious how you guys size up Branch Circuit Short Circuit protection for the motor that is being supplied by a VFD. Let's use a 30 HP 3 phase motor at 460 volts for an example.

Do you guys size it up based on the VFD or the Motor?


----------



## Cow (Jan 16, 2008)

I'd recommended using the size from the drive manual.


----------



## micromind (Aug 11, 2007)

430.122(A) states that conductors that feed VFds need to be sized at 125% of the input rating of the VFD. 

It doesn't matter what the rating of the motor is, just the VFD.


----------



## McClary’s Electrical (Feb 21, 2009)

I was always taught to size them by the motor, so the drive could be removed and replaced by a contactor and the wires would be large enough.


----------



## oliquir (Jan 13, 2011)

mcclary's electrical said:


> I was always taught to size them by the motor, so the drive could be removed and replaced by a contactor and the wires would be large enough.


the vfd current is higher than the motor fla most of the time, so your wires are probably too small.


----------



## McClary’s Electrical (Feb 21, 2009)

oliquir said:


> the vfd current is higher than the motor fla most of the time, so your wires are probably too small.


Nope not true. In fact the opposite is true. The input of a 480 volt 10 hp drive is always (fun what I've seen) a few amps less than fla of a 10 hp motor starting across the line.


----------



## Cow (Jan 16, 2008)

mcclary's electrical said:


> Nope not true. In fact the opposite is true. The input of a 480 volt 10 hp drive is always (fun what I've seen) a few amps less than fla of a 10 hp motor starting across the line.


McClary he is saying a drive consumes some power too, in addition to the motor current. The NEC tells us we have to size our wire to the drive INPUT rating not the motor current rating.

Here's one example from an old drive that I needed to remember the power requirements for:


----------



## minichopper6hp (Apr 19, 2014)

Yes but most of the power factor is in rush which is significantly less on a vfd.


----------



## oliquir (Jan 13, 2011)

i have one in my shop now, it is a 7.5 hp at 600v. fla of vfd is 10.4A
and fla of motor from cec is 9A


----------

