VFD feed to motor

[electro1205]

ok I have a motor that it FLA on the name plate is 230 volts is 68 amps and 480= 34 me and the super both did a load calc and can up with a 50 amp breaker but one of the engineer is telling us that the breaker had to be 70 amps with #8 wire is he correct or is he reading it wrong. The motor is hook up to a VFD that has a amp rating of 34 anp same as motor at 480 volts

 

[infinity]

The conductors to the motor are sized according to the ampacity in the tables in article 430 times 125%. Same with the drive conductors 125% times the drive nameplate ampacity. The OCPD is usually sized by the drive manufacturer.

 

[Ingenieur]

The conductors to the motor are sized according to the ampacity in the tables in article 430 times 125%. Same with the drive conductors 125% times the drive nameplate ampacity. The OCPD is usually sized by the drive manufacturer.

I thought the feeder would be sized for the drive input current
and the motor 1.25 x table fla

 

[ptonsparky]

I thought the feeder would be sized for the drive input current x 1.25
and the motor 1.25 x table fla

They happen to be the same today.

 

[ptonsparky]

Your 50 would most likely work but the fine manual may actually call for fusing. You have to read it.

 

[Ingenieur]

They happen to be the same today.

I knew you need the 1.25 for the feeder
how do you know the vfd input i = table fla?

150% x table fla if the drive is not marked???

 

[infinity]

You can look at the drive nameplate for the input current rating.

430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conduc-
tors supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an am-
pacity not less than 125 percent of the rated input current to
the power conversion equipment.

 

[Dzboyce]

That 125% of the drives INPUT amperage has tripped me up on inspection before. I’ve retrofitted drives to pumps and “assumed” the existing wiring from the previous Pump controller to the OCPD was sufficient. Usually a drive will have a higher input amperage than the motor’s fLA. This might result in needing larger conductors between the OCPD and the drive, than you need between the Drive and the motor.