VFD output wiring - are multiple conductors per phase acceptable

[gjeffries]

I have an application where we are replacing a wye-delta starting circuit with a VFD. I would like to reuse the existing cable between the motor and the VFD, basically running parallel feeds, or multiple conductors per phase, however you want to say it. All 6 conductors are in one conduit and relatively close in length, so there shouldn't be a concern about inductive heating. This is a 480V application, 200HP motor, so the FLA on the motor is in the neighborhood of 250A. The conductors are about 100 feet long.

I know this "should" work, but I don't know if there are practical issues with running multiple conductors per phase downstream of a VFD (reflected wave is one concern). Does anyone know of any issue with this and can you cite any resources that describe why this is ok or not? Thanks in advance!

 

[petersonra]

I have an application where we are replacing a wye-delta starting circuit with a VFD. I would like to reuse the existing cable between the motor and the VFD, basically running parallel feeds, or multiple conductors per phase, however you want to say it. All 6 conductors are in one conduit and relatively close in length, so there shouldn't be a concern about inductive heating. This is a 480V application, 200HP motor, so the FLA on the motor is in the neighborhood of 250A. The conductors are about 100 feet long.

I know this "should" work, but I don't know if there are practical issues with running multiple conductors per phase downstream of a VFD (reflected wave is one concern). Does anyone know of any issue with this and can you cite any resources that describe why this is ok or not? Thanks in advance!

As far as the code is concerned, this is just a motor circuit. The tables tell us to use 240A as the FLC and that the ampacity of the conductors has to be 125% of that. So (2) 1/0 wires in parallel per phase is adequate, code wise.

At 100 feet in length, I would not get real excited about standing waves.

If the conductors are enclosed in a metallic raceway of some sort, I would not worry all that much about noise.

As long as the motor is an inverter duty motor, I would be unconcerned about this install.

If you wanted to make it perfect, you could run VFD cable instead of wires. I remain unconvinced that one gains all that much by doing so given the tens of millions of installations of VFDs on motors using just regular wires that have proven serviceable over many years. The other thing about a motor this size is that I don't think anyone makes a VFD cable large enough to have 300A of ampacity so multiple cables would be needed.