EGC for Variable Speed Drive

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augie47

augie47

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(e-mail question)
How does one size the equipment grounding conductor for the secondary side of a variable speed drive ? (The drive is protected on the line side by a 1200 amp breaker and the load side has (3) sets of 400 kcmil.. No details on the drive were provided.)
 
augie47 said:
(e-mail question)
How does one size the equipment grounding conductor for the secondary side of a variable speed drive ? (The drive is protected on the line side by a 1200 amp breaker and the load side has (3) sets of 400 kcmil.. No details on the drive were provided.)
Click to expand...

Very good question. If the drive has any kind of bypass, I'd say based on the 1200 amp overcurrent device for sure, no bypass, I'm not so sure.
 
Why would you size it for anything less than that required by the 1200 amp protection?

As far as an EG and the NEC, I see the VFD as no more than a motor starter. I know it is more but don't know of anything in the NEC that would change the EG size because you have a VFD.
 
ptonsparky said:
Why would you size it for anything less than that required by the 1200 amp protection?

As far as an EG and the NEC, I see the VFD as no more than a motor starter. I know it is more but don't know of anything in the NEC that would change the EG size because you have a VFD.
Click to expand...

The drive is power conversion equipment, the load side is separately derived and does have overcurrent protection built into the drive, and with most modern drives will never see the same fault current on the load side you might see on the line side.

You can't go wrong with sizing for the line side overcurrent protective device though.

How about situation where you have single phase input and three phase output?

Seen this one time where input was 240 volt single phase, output was 480 volt three phase - driving a 20 HP submersible pump.

Can't remember the input overcurrent device - maybe 125 or 150 amps. Output conductors only needed to be 10 AWG minimum, though they might have been increased for voltage drop. In that case I say at least no larger then the motor conductors, but "normal" EGC is not that much smaller on this circuit as compared to 1000-1200 amp circuits
 
augie47 said:
(e-mail question)
How does one size the equipment grounding conductor for the secondary side of a variable speed drive ? (The drive is protected on the line side by a 1200 amp breaker and the load side has (3) sets of 400 kcmil.. No details on the drive were provided.)
Click to expand...

I don't see how you get around sizing it according to table 250.122. OTOH, if you run it in rigid you don't need a wire type EGC at all.
 
Best practice is to have your output conductors triplexed and for each set of three, you use 3 separate EGCs, each sized for roughly 1/3 the rating of the CCCs, nested in with the CCCs. Here's what you want it to look like.
VFD cable.jpg
This diagram is from a discussion about using multi-conductor cables, but if you are doing your own, do it the same way. What you are after is as consistent of a geometry of the CCCs and EGCs as possible to reduce the effects of cable capacitance from the PWM output.
 
petersonra said:
.... OTOH, if you run it in rigid you don't need a wire type EGC at all.
Click to expand...
Per the NEC, true, but any VFD mfr will insist that you run a ground wire from the motor frame to the VFD ground terminal. And to that last point, always terminate the EGC directly to the VFD, then connect the VFD to the ground bus, not the other way around.
 
Jraef said:
Best practice is to have your output conductors triplexed and for each set of three, you use 3 separate EGCs, each sized for roughly 1/3 the rating of the CCCs, nested in with the CCCs. Here's what you want it to look like.
View attachment 17373
This diagram is from a discussion about using multi-conductor cables, but if you are doing your own, do it the same way. What you are after is as consistent of a geometry of the CCCs and EGCs as possible to reduce the effects of cable capacitance from the PWM output.
Click to expand...

How is that compliant with the code that requires a full sized EGC be run with each set of conductors. The OP indicated 3 sets of 400 KCM.

I think you are allowed to do what you are suggesting if it is a listed cable, but I don't think it is code compliant to make your own up like this unless you used (3) 3/0 EGCs with each set of (3) 400 KCM conductors.
 
Jraef said:
Per the NEC, true, but any VFD mfr will insist that you run a ground wire from the motor frame to the VFD ground terminal. And to that last point, always terminate the EGC directly to the VFD, then connect the VFD to the ground bus, not the other way around.
Click to expand...

You could certainly add a bonding bushing to the conduit at both ends and a short EGC jumper to the motor/VFD ground terminals.
 
Jraef said:
Per the NEC, true, but any VFD mfr will insist that you run a ground wire from the motor frame to the VFD ground terminal. And to that last point, always terminate the EGC directly to the VFD, then connect the VFD to the ground bus, not the other way around.
Click to expand...

Makes less noise that way as well.
 
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