Grounding Electrode Conductor size on SDS

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RollTideYall

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Birmingham, AL
I've got a customer that just purchased a trailer-mounted generator for use on a construction site to run some equipment. The generator is a 600KW, 3-phase, 4-wire, 480 VAC unit. The generator connections are wired to a circuit breaker (trip unit long-time set to 800A) mounted inside of the generator enclosure. The customer does not have any equipment that will require the neutral. We will be bonding the neutral to the ground in the generator enclosure and then running a grounding electrode conductor to a ground rod.

I know we should size the grounding electrode conductor based on 250.66 but this allows us to use a #6 copper for this purpose (assuming we use a rod electrode and it meets 250.52A(5)). Is that correct?

Also, with regards to the bonding jumper between the Neutral and the ground, it should be sized based on 250.66 as well, right? Since we will have 3 parallel conductors per phase (each is 350 kcmil), this would put the size for the bonding jumper at 2/0 copper. The two parallel conductors would be treated as one 1050 kcmil and thus you'd use that size for table 250.66, right? Thanks in advance.
 
RollTideYall said:
Also, with regards to the bonding jumper between the Neutral and the ground, it should be sized based on 250.66 as well, right? Since we will have 3 parallel conductors per phase (each is 350 kcmil), this would put the size for the bonding jumper at 2/0 copper. The two parallel conductors would be treated as one 1050 kcmil and thus you'd use that size for table 250.66, right? Thanks in advance.
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You're looking for the system bonding jumper size. Take a look at 250.102(C).
 
infinity said:
You're looking for the system bonding jumper size. Take a look at 250.102(C).
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Yea, so the size is correct, then. I was using a 2011 copy of the NEC without that table. Didn't notice til you mentioned it. Thanks. How about the grounding electrode conductor? Any thoughts there? #6 just seems really small in this instance. Thanks.
 
In your installation the ground rod will be doing next to nothing anyway so a #6 is sufficient.
 
ActionDave said:
Your answer is yes.
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So you're saying we do need to run a neutral with the phase conductors? Let me explain more of what's happening here. There is a lowboy trailer with this large generator on it. We're going to be mounting a 600A disconnect and two 100A disconnects on the trailer as well. So, the leads from the circuit breaker on the generator are going to be put in conduit to a distribution box and will be split off to the disconnects. Are you saying we should run a neutral to each of the disconnects? We will be running an EGC to each disconnect from the generator.

The neutral and ground are going to be bonded before they leave the generator so the neutral will have no purpose (all of the loads are 3-phase, 480vac) if we run it outside the generator because we can't ground it again (parallel paths). Am I wrong here? If so, why? Thanks.
 
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RollTideYall said:
So you're saying we do need to run a neutral with the phase conductors? Let me explain more of what's happening here. There is a lowboy trailer with this large generator on it. We're going to be mounting a 600A disconnect and two 100A disconnects on the trailer as well. So, the leads from the circuit breaker on the generator are going to be put in conduit to a distribution box and will be split off to the disconnects. Are you saying we should run a neutral to each of the disconnects? We will be running an EGC to each disconnect from the generator.

The neutral and ground are going to be bonded before they leave the generator so the neutral will have no purpose (all of the loads are 3-phase, 480vac) if we run it outside the generator because we can't ground it again (parallel paths). Am I wrong here? If so, why? Thanks.
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No, if the load doesn't require a neutral you don't need to run one. You still need an equipment grounding conductor, that is kind of what was said but in a somewhat confusing manner IMO.
 
kwired said:
No, if the load doesn't require a neutral you don't need to run one. You still need an equipment grounding conductor, that is kind of what was said but in a somewhat confusing manner IMO.
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Ok, that's kind of what I thought but I was just clearing the air for anyone that reads this in the future. Thanks.
 
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