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Input cables for VFDs - sizing the EGC

Input cables for VFDs - sizing the EGC

(OP)
Hello,
I am working with an Eaton SPX 300 Hp drive. The drive manual recommends that for incoming power cables, phase conductors should be 2 @ 300 kcmil, and the ground conductor should be 1 @ 300 kcmil. The question I have is about the ground. The upstream breaker is 500A. According to NEC, for this size breaker I would need a #2 ground. The Eaton manual recommends a 300 kcmil, which has an ampacity of 250% more than what is required. I've asked Eaton tech support why they recommend such an over-sized ground, and they did not know.

Can anyone here explain why I should use the 300 ground?

Thanks
EE

RE: Input cables for VFDs - sizing the EGC

The NEC is a set of minimum standards, not a design guide. The NEC is only concerned about safety grounding. The recommended practice of using a full size ground wire for feeding inverters is because of known issues with wanting common mode noise created by the PWM inverter to have the lowest impedance path to the grounding electrode as is reasonably possible.

Read chapter 3 in this document.
http://literature.rockwellautomation.com/idc/group...

Actually, read the hole thing...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Input cables for VFDs - sizing the EGC

I agree with Jraef.

RE: Input cables for VFDs - sizing the EGC

(OP)
Thank you

RE: Input cables for VFDs - sizing the EGC

(OP)
Ok. Please help me to understand this. The EGC is for clearing faults. It's not a current carrying conductor. Nor is it a grounded conductor. Your reply implies that there will be current flowing from the VFD chassis back to the MCC ground bus? I understand the PWM issues, and harmonics, etc. But I don't see how there is going to be ground currents on the EGC from the feeder to the VFD, or on the EGC to the motor.

RE: Input cables for VFDs - sizing the EGC

The high frequency switching of the VFD causes current to pass through the conductor to ground capacitance into the ground.

Ideally, you would have both a properly terminated full braided shield on the cable between the VFD and the motor, and the VFD would have internal capacitors from the positive and negative bus to ground. That way, these high frequency currents get returned directly to the VFD bus where they came from. Otherwise, they go back to the power source transformer and get back onto the phase conductors via the bonded neutral before finally getting back to the DC bus via the VFD input rectifier.

RE: Input cables for VFDs - sizing the EGC

And to that point, because these are high frequencies, they travel on the surface of the conductors. Larger conductors = more surface area.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Input cables for VFDs - sizing the EGC

(OP)
I agree in concept with these premises. But we are talking about sizing the ground conductor to carry 285A on a drive rated at 400ish A. Skin effect included, this doesn't quite add up. Or I just don't understand it.

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