nsds 3 phase 3 wire gen set

[Sric]

i have a 350kw gen set. I will be fusing the generator brkr at 500a. I've been working with the local inspector and nothing seems black and white so I'm looking for code ref. to go along with all advice. the gen set is feeding a 3 pole ats. the ats will be feeding a distribution panel with 3 phase loads only. lieberts, pumps, dry coolers, etc.... it will also feed a ups which will feed a 480/120/208 transformer where we will be deriving out neutral for some 120v loads. The intent is not to run the neutral from the main service feed or from the gen set to the ats. this has been debated between me and the inspector weather or not i have to run the neutral. i dont believe i do but what then is the correct way to handle the xo termination at the gen set. i believe i can bond it because i'm not creating a parallel path for the neutral current because there is no neutral. also it has been debated if i can run parallel 4/0 to feed the distribution panel. 4/0 are good for 230a run parallel equals 460a. next size brkr is 500a. any thoughts and code ref. would be greatly appreciated.

 

[Smart $]

Before we get into code references, what I gather from your description is that the service is 480/277V 3? 4W grounded system. The gennie is to supply 480V 3? 3W. This is permitted, but no mention of grounding the gennie was mentioned. You either have to ground the gennie system or add ground fault protection. I recommend the former, since the service is grounded. This would make the gennie a 480/277V 3? 4W system, but you do not have to run the neutral to the ATS.

 

[Sric]

yes the service is 480/277 3 phase 4 wire. and yes since the gen is not large enough to require ground fault detection i believe i have to bond the gen. connecting the neutral to the frame. the problem i have with this in discussions is if i do this the inspector is telling me I have to run the neutral which i dont believe i do. i haven't been able to convince him otherwise yet. what i have to do is put this all on paper for him to examine further and i want to go into with as much spelled out for him as possible

 

[hillbilly1]

I have seen this on the prints and in the stores for the older Orange box stores for their emergency lighting, because they directly went to a stepdown transformer, they did not need the neutral.

 

[acrwc10]

If your Generator is a 3 phase 4 wire system and the neutral is bonded to the frame, you have an EGC that can be bonded to the building/utility EGC. As long as it is an addiquitly sized conductor for your EGC you do not need to run a neutral from the genset. Because you have no neutral load, what would be the point?
Now on the other hand if you do not have the neutral bonded to the (earth) frame at the genset you have a "Non grounded system" and would need ground fault detectors. Without them the system would not be able to know if only one phase shorted to ground, only phase to phase.
As far as running parallel 4/0 cu you should be fine as long as your load does not exceed 460 amps. you only need to run a full size EGC in each conduit. Full size meaning able to carry the full 460 amp. load.

 

[Sric]

i agree with all of the input so far my issue is convincing the inspector of not having to run the neutral and does it tell me somewhere I can bond the generator without running the neutral. he is quoting 250.24c. i don't think this applies for running the neutral to the ats/load only to the service disconnecting means which in this case is at the generator.

 

[Smart $]

i agree with all of the input so far my issue is convincing the inspector of not having to run the neutral and does it tell me somewhere I can bond the generator without running the neutral. he is quoting 250.24c. i don't think this applies for running the neutral to the ats/load only to the service disconnecting means which in this case is at the generator.

You are correct. 250.24 title is "Grounding Service-Supplied Alternating-Current Systems." 250.24(C) only applies to Services, i.e. utility supplied power.

Refer to 250.30 Grounding Separately Derived Alternating-Current Systems. Note 250.30(A)(8) Grounded Conductor starts with, "Where a grounded conductor is installed..." I take that to mean a grounded conductor is not required to be installed.