Sizing a Ground Conductor

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dasarmin

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In this application an 800HP DC drive is being fed from a Switchboard, in the SB is a 1600 LSIG Circuit Breaker and in the drive enclosure is a 1600 Amp Mag Trip Circuit Breaker.
4 runs of 500 MCM in 3? EMT conduits will service the drive enclosure.
When sizing the ground conductor I generally use table 250.122; in this case a copper ground would be at least a 2 AWG corresponding Ampacity of the current carrying conductors in each 3? conduit.

An engineer here mentioned using table 250.66 to size the grounding conductor; which in this case, would be 1/0.

Is table 250.122 the correct table to use in the application above?
When would table 250.66 be used? Perhaps, if a Switchboard was feeding an I-Line Panelboard?
 
You need to size you EGC using 250.122 based on the ampacity entire circuit NOT on the size of conductors within each raceway.

I question the size of your feeders. Typically you can not protect 4 - 500's with a 1600 amp breaker. Since this is a motor circuit you may be ok. I don't know much about DC drives and what will ultimately protect the feeder conductors from overcurrent. One of the other members can provide some feedback.
 
Forgot to mention the 1600Amp switchboard CB will be dialed down to 1200Amps.

Looking at 250.122 (F), does this imply that the grounding conduit must meet the conductors in each raceway? Each of the four conduits will have a ground. If one ground conduit for all four conduits were planned then it would have to be sized for all four runs, therefore a 4/0 instead of #2.
 
dasarmin said:
Forgot to mention the 1600Amp switchboard CB will be dialed down to 1200Amps.

Looking at 250.122 (F), does this imply that the grounding conduit must meet the conductors in each raceway? Each of the four conduits will have a ground. If one ground conduit for all four conduits were planned then it would have to be sized for all four runs, therefore a 4/0 instead of #2.
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I think you need a 3/0 copper EGC in EACH raceway.

Your EGC is sized to table 250.122--which in your case will be 1200 amps
 
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