Using 600V cable on neutral on 4160V system

[mmt019]

I work at a University where I have a 4160V, 3 phase run originating out of a distribution lineup (solidly grounded). Sorry for the lengthy description so bear with me.

It runs from this lineup to various manholes (which has a ground-rod) to pad-switches (which also has a ground rod); and from a tap on a pad-switch into the building HVL disconnect (which has a ground rod).

Typically, if the run consists of buildings with three-phase transformers, (3) 5kV rated cables w/concentric neutral (33% rated/cable) does the trick. No grounding conductor is ran with this. The concentric neutral is eimply brought to each ground point all the way back to the distribution lineup. We utilize the multi-grounded system over 1kV as indicated in NEC 250-184 I think.

Here's Question 1: If you have single phase transformers for some buildings, your now utilizing the concentric neutral as a return path. Does the code require an additional grounding conductor now? The NEC states that if you are utilizing a single point ground system, then you have eo run a grounding conductor along the run. Since this is so, does it mean its not required in a multi-grounded system?

Question 2: Since I have a solidly grounded system, can I just use 600V rated cable for the neutral so I can use a more readily available 5kV shielded cable? The code appears to allow this.

 

[davidbeach]

Question 2 is easy, yes you can use 600V insulation on grounded conductors.

Question 1 is much more difficult and will probably need to by answered by the AHJ. Concentric neutral is not addressed in the NEC, and there are no concentric neutral cables available list as a two conductor assembly. So depending on how your AHJ looks upon the need for listing, concentric neutral may or may not be usable. Utility practices and NEC practices are vastly different when it comes to MV distribution.

I did one project for a National Park (who is also the AHJ in that situation) where we specifically referenced only the NESC and omitted the NEC to get around any possible questions regarding the use of concentric neutral. (We also wanted to get around the 360 degree bend limit, but that is another story.) The project was in a state (which didn't have any jurisdiction, but just to be perfectly clear to all involved) that has a strong insistence on listed products only if there are any listed products, and since there are listed MV conductors, there would be no acceptable use of concentric neutral.