Please discuss voltage rating of grounded conductor

[electrofelon]

Clearly the insulation rating of a grounded conductor can be less than that of the phase conductors, in theory. Some examples are: Concentric neutral MV cable only has a thin jacket of the neutral strands, and the neutral/grounded conductor on a MGN distribution system are lower on the pole and have smaller insulators. I cant find any allowance for this in the NEC. In fact, I cant even find anything prohibiting exceeding the voltage rating of a conductor in general, other than 110.3 - did I miss it?

What about single conductor installations in conduit? What considerations would there be in selecting the grounded conductor voltage rating. During a fault, the voltage on the grounded conductor will of course rise, but that is the only condition I can thing of. The specific situation I am thinking of is a 2.4KV circuit with say a 600V grounded conductor.

Edit: also this system would be an outdoor MGN system.

 

[jaggedben]

Only NEC rule I can think of is that a service neutral is allowed to be bare.

 

[Dennis Alwon]

Not exactly sure what you are asking but I think this section could give you guidance although it is not exactly what you have.

300.3(C) Conductors of Different Systems.
(1) 1000 Volts, Nominal, or Less. Conductors of ac and dc
circuits, rated 1000 volts, nominal, or less, shall be permitted to
occupy the same equipment wiring enclosure, cable, or raceway.
All conductors shall have an insulation rating equal to at
least the maximum circuit voltage applied to any conductor
within the enclosure, cable, or raceway.

 

[kwired]

Clearly the insulation rating of a grounded conductor can be less than that of the phase conductors, in theory. Some examples are: Concentric neutral MV cable only has a thin jacket of the neutral strands, and the neutral/grounded conductor on a MGN distribution system are lower on the pole and have smaller insulators. I cant find any allowance for this in the NEC. In fact, I cant even find anything prohibiting exceeding the voltage rating of a conductor in general, other than 110.3 - did I miss it?

What about single conductor installations in conduit? What considerations would there be in selecting the grounded conductor voltage rating. During a fault, the voltage on the grounded conductor will of course rise, but that is the only condition I can thing of. The specific situation I am thinking of is a 2.4KV circuit with say a 600V grounded conductor.

Edit: also this system would be an outdoor MGN system.

With MGN, we kind of don't care if the neutral would fault to something grounded, as that neutral and earth are considered the same potential in such a system. Concentric neutral in such cables isn't "insulated" anyway. You can maybe call it "covered". Don't know if they make it anymore but have seen concentric neutral on old cable that is on outside of the cable and no overall outer covering, direct buried even.

 

[electrofelon]

Only NEC rule I can think of is that a service neutral is allowed to be bare.

Good point

Not exactly sure what you are asking but I think this section could give you guidance although it is not exactly what you have.

300.3(C) is interesting, but like you say it doesnt really apply.

Boiled down, the question is: 2.4 KV single phase grounded system in conduit with a 2.4Kv rated, say, MV-90 conductor, and a 600v rated grounded conductor.

1. Is there an NEC violation?
2. Disregarding NEC, is it reliable and safe?

Wait! I think I just found it: 250.184(A)(1) Insulation level shall be minimum 600 volts! Note 250.184(C)(3) and (4) requires a GE every 1300 feet.

 

[electrofelon]

With MGN, we kind of don't care if the neutral would fault to something grounded, as that neutral and earth are considered the same potential in such a system. Concentric neutral in such cables isn't "insulated" anyway. You can maybe call it "covered". Don't know if they make it anymore but have seen concentric neutral on old cable that is on outside of the cable and no overall outer covering, direct buried even.

Yeah I was curious what the insulation rating of the neutral is on concentric MV but I couldnt find any spec on it. Just a few months ago I saw some of that non covered concentric in Seattle when I was poking around in some hand holes. 25.5KV. Had never seen that before. IF it was installed when that building's service was put it, it was 1972.

 

[Sahib]

Only NEC rule I can think of is that a service neutral is allowed to be bare.

But there should not be any (spurious) grounding of neutral beyond service equipment.

 

[kwired]

Yeah I was curious what the insulation rating of the neutral is on concentric MV but I couldnt find any spec on it. Just a few months ago I saw some of that non covered concentric in Seattle when I was poking around in some hand holes. 25.5KV. Had never seen that before. IF it was installed when that building's service was put it, it was 1972.

I don't know a lot about MV cable, but recall rural POCO digging up/repairing a faulted line once that was using this non covered neutral. It was probably installed in mid to late 70's. Was on 12.5/7.2 kV system.

 

[electrofelon]

I don't know a lot about MV cable, but recall rural POCO digging up/repairing a faulted line once that was using this non covered neutral. It was probably installed in mid to late 70's. Was on 12.5/7.2 kV system.

I dont know a lot of MV either. One POCO here lets us pull primary and they make it up. Im ok with 15kv class, making up load break elbows on concentric neutral cable. Never seen MV-90 type cable or any concentric above 25.5KV in the wild. Im designing a 4000 foot step up step down system no due to POCO wanting over 100k, hence the question.

 

[MyCleveland]

I dont know a lot of MV either. One POCO here lets us pull primary and they make it up. Im ok with 15kv class, making up load break elbows on concentric neutral cable. Never seen MV-90 type cable or any concentric above 25.5KV in the wild. Im designing a 4000 foot step up step down system no due to POCO wanting over 100k, hence the question.

This question came up a short while back.
Try 250.184 (A) (1)