Grounding for a VFD feeder circuit.

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Sora Komi

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Location
Charlotte, N.C.
Occupation
Facilities Controls Technician
Hello Everyone -

I have run into a situation that's new to me. We have an air handler that has eight blower motors. These motors are controlled by four drives, two motors per drive. (I have never done this, control more than one motor from a drive, but I know that I need to provide individual short-circuit and motor overload protection for each motor.) The motors have a FLA of 18.2 amps.

So, my main question is about the grounding. The drives are rated at having a continuous input current of 55 amps, and a continuous output current of 44 amps. They have internal factory provided input fuses which are rated at 80 amps. Now, these drives are being fed from a 300 amp circuit breaker that feeds the wires going to a wireway, and taps are going to be made in the wireway for each drive's input.

Normally, if fused individually from a circuit breaker or disconnect, you could use a #10 or #8 ground wire. But Table 250.122 calls for a #4 equipment ground for a 300 amp OCD. Does this mean that I need to make a connection in the wireway and run a #4 ground to each drive? And how is the motor ground wire sized in this type of installation? Thanks for all replies!
 
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OK, so a #4 EGC needs to be pulled in the conduit to the wireway, and then after the taps are made, the size of the ground wire to the VFDs isn't required to be larger that the tap conductors, got it. So that brings up another question. What size should the tap conductors be, or be based on? Since the drives have internal input fusing of 80 amps, do I need to make sure that the tap conductors are rated for 80 amps, or can the wire size be determined by the max input current rating (55 amps) of the drives?
 
ptonsparky, this is a one-of air handler installed in an hvac testing lab. there isn't really a label for it, and when I asked the HVAC technicians about the wiring of the motors, they said that they didn't know, they 'just let the local electricians figure it out'.
 
My take:
Feeder to Unit 300 A.
Tap to each VFD protected at 70 Amp CB (1.25*55=68.75)
Ignore the internal 80 amp fuses.
#4 to each VFD with #8 EG. Table 250.122

Use table 430.250 to determine ampacity of conductor to motor. 15 HP @ 480 is 21 amps (Guessing that is size of motor)
125% * 21 = 26.25 #10Thhn/thwn

Over current protection by FLA of motor nameplate.

The EG to a motor need not be larger than the conductors to it, so #10 from VFD to each motor. (Looking for that NEC article but maybe someone will chime in 1st)
 
Cool, thanks. Yeah, we had planned on #10 to each motor, wasn't sure about the grounding. Man, it is a little more stuff you have to consider when feeding two motors from one drive. i wish it had been designed one motor one drive.
 
ptonsparky, you said:

'
'Use table 430.250 to determine ampacity of conductor to motor. 15 HP @ 480 is 21 amps (Guessing that is size of motor)
125% * 21 = 26.25 #10Thhn/thwn'


Is this the way it needs to be done, by the code tables, or can you take the FLA from the motor nameplate, which was 18.2?
 
Sora Komi said:
ptonsparky, you said:

'
'Use table 430.250 to determine ampacity of conductor to motor. 15 HP @ 480 is 21 amps (Guessing that is size of motor)
125% * 21 = 26.25 #10Thhn/thwn'


Is this the way it needs to be done, by the code tables, or can you take the FLA from the motor nameplate, which was 18.2?
Click to expand...
Conductor size to motor is via the table.
Overcurrent protection is motor nameplate.
 
'Conductor size to motor is via the table.
Overcurrent protection is motor nameplate.'

Check, thank you. If I can ask one more question, since EGC does not have to be larger than the ungrounded wires, how much smaller can it be? Does the ground wire size need to be determined for these drives based on their 80 amp input fusing or 55 amp current draw, or #4 supply conductors?
 
Sora Komi said:
'Conductor size to motor is via the table.
Overcurrent protection is motor nameplate.'

Check, thank you. If I can ask one more question, since EGC does not have to be larger than the ungrounded wires, how much smaller can it be? Does the ground wire size need to be determined for these drives based on their 80 amp input fusing or 55 amp current draw, or #4 supply conductors?
Click to expand...
The internal 80 amp fusing is how the drives have been tested.

Look at the over current ahead of the conductors. Look at table 250.122. compare the values.
 
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