Delta from a wye generator- ground the neutral or float?

[SteveFehr]

We just put in a 480/277V Y generator feeding a 480V Delta switchboard. During the inspection, we discovered the contractor had grounded the neutral in the generator with 3/0 (same size as the NEC-specced ground wire- looks comically small next to the line conductors). In speaking to the Caterpillar reps on-site, we found out the same N-G connection was also made internally. There is appx 25' of cabling between this generator and the first OCP in the switchboard.

Our senior technician feels strongly that both N-G jumpers should be removed and the delta left to float ungrounded. He's worried a L-G fault will fry the generator. I don't like that the line voltage could float all over the place during normal operation or that an L-G fault could potentially go unnoticed if one should occur. Our lead electrician agreed with me, which is why he wired it up that way.

What's best practice here? Any other pros/cons for either option?

 

[davidbeach]

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

Note that it specifically states "...does not establish a parallel path..."

Why are you saying that it permits a parallel path?

 

[DanDel]

SteveFehr, Art. 250.30(A)1, which you reference, states that the system bonding jumper connection "shall be made at any single point on the separately derived system"

Only Exception 2 allows two jumpers per my previous post, (See davidbeach's helpful post.

 

[jghrist]

The exception to 250.30(A)(1) does not allow bonding the grounded neutral conductor at both the service entrance and at a generator if the grounded neutral conductor is not switched. In this case, the generator is not a separately derived source. If the neutral is switched by an ATS, then the generator is a separately derived source and the grounded neutral conductor of that source is bonded at the generator.

 

[SteveFehr]

I was not referring to exception 2, I was referring to exception 1:

NEC 2005, 250.30(A)(1) Exception 1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

This exception excepts 250.30(A)1 and the single point.

 

[davidbeach]

No. Exception 1 wouldn't apply, and where it does apply it means that two sources don't each need their own bonding jumper but can use a single bonding jumper for both. So it is the opposite of what you are suggesting, it is permission to go from two bonding jumpers to one bonding jumper.