Input Cable to the VFD

[nightfox1925]

Is it correct to consider the current rating of the VFD for cable sizing x 125% instead of using the VFD motor FLA? We have a 4160V system feeding a 4160V VFD connected to a 4kV motor. From there we calculate the voltage drop and S.C withstand, are there any other factors to re-consider for VFD input cable sizing

 

[dpc]

The VFD should have an input current rating of its own. That's what the incoming feeder should be sized for.

 

[nightfox1925]

Just as I thought...however, what if there are other feeders with their associated VFDs not yet defined or sized, is it still safe to consider the VFD motor FLA...or there will be some safety factors to be added to estimate the VFD input current?

 

[dpc]

I'd be conservative. The VFD chassis may have a higher current output rating than the motor it is connected to. Probably 125% of the motor FLA would be safe.

 

[LionelHutz]

I think it depends on the VFD but you would likely be OK. I know some have some fairly high horsepower blowers in them that you have to feed that would add to the current draw.

The input current is basically the motor current divided by the efficiency so if you're not sure then estimate the VFD current as something like 10% higher.