HV conductor shielding as return conductor?

J

Julio Bro

Member
This use will be applied in new projects at a city.
Allegedly the shielding specification has been "tested" able to carry 1/3 of the conductor's amp capacity. As such, in a 3 phase circuit, three conductor's shielding adds up to the capacity of a fourth conductor, where needed.

I feel this would bring many problems and I haven't found info online about this use. So, I don't know if this has been applied successfully for power distribution at a city or town levels.

Any experience, example, or research source?
 
ron said:
Jacketed Concentric Neutral (JCN) Cable is different than a shielded cable in NEC 250.190(C)(2). Concentric neutrals are more conductor than a shield and can carry fault current.
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They are also rated for neutral current. Around here that is all they use for single phase direct burial.

Mark
 
Julio Bro said:
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I feel this would bring many problems and I haven't found info online about this use. So, I don't know if this has been applied successfully for power distribution at a city or town levels.

Any experience, example, or research source?
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Pretty much every power company in the United States uses 1/3 neutral MV cables. There's probably 23.7 bajillion miles of it installed.
 
electrofelon said:
Pretty much every power company in the United States uses 1/3 neutral MV cables. There's probably 23.7 bajillion miles of it installed.
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And some of the older installations did not have a jacket over the concentric neutral and as the neutral corroded away, the neutral to earth voltage went up.
 
electrofelon said:
Pretty much every power company in the United States uses 1/3 neutral MV cables. There's probably 23.7 bajillion miles of it installed.
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How often do they need the full ampacity of the center conductor? Particularly for a feed to an individual or even limited number of single phase transformers? A 167 KVA transformer, which is typically the largest single phase unit you will find here anyway, would only draw about 23 amps if fully loaded on a 7200 volt primary.

If you are distributing three phase wye with three of these cables and combine the three neutrals you effectively have same size neutral as ungrounded conductors though you probably really don't need that large of a neutral most of the time anyway.
 
kwired said:
How often do they need the full ampacity of the center conductor? Particularly for a feed to an individual or even limited number of single phase transformers? A 167 KVA transformer, which is typically the largest single phase unit you will find here anyway, would only draw about 23 amps if fully loaded on a 7200 volt primary.

If you are distributing three phase wye with three of these cables and combine the three neutrals you effectively have same size neutral as ungrounded conductors though you probably really don't need that large of a neutral most of the time anyway.
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Typically I see full neutral for single phase. Also, IIRC, 1/3 neutral cables start a little bigger, 1/0 I think, where full neutral goes down to # 2. I have used #2 full neutral for some step up/step Downs.
 
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