Concentric Neutral and NEC

[Flapjack]

NEC recognizes that concentric neutral cables can be used as equipment grounding conductors, but does NEC recognize them as neutrals?
Okonite states on concentric neutral cable cutsheets that the cables can be UL listed as MV-90 for special orders and Southwire states that the concentric neutral cables can be special ordered to meet UL 1072, but I think this means that the concentric neutral can be used as a shield or EGC.
UL1072 states that the listing does not apply to concentric neutral cables.

Appreciate any help.

 

[jumper]

Gee whiz, can't you ever just ask something like a nice simple GFCI receptacle location question that I have even a small chance of answering?

Just teasing ya.

 

[Flapjack]

Haha, sorry. I had a discussion with a cable manufacturer and he had brought up this concern that NEC may not recognize it as a neutral and only as a shield to be used as an EGC per NEC 250.190(C)(2).

 

[jumper]

I am clueless, but I do not see anything in 250.184(A) that says you can use this shield thingy in a MV cable as a noodle. IDK.

 

[Flapjack]

Couple questions... NEC 300.5(I) Exception 2 states 'isolated phase... installations shall be permitted in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic sheath in close proximity where conductors are paralleled...'.

Does this only apply to paralleled installations?

I'm guessing this prohibits copper tape shields when it says nonmagnetic sheath? The use of the word 'or' throws me off. It makes it sound metallic sheathed cables are permitted if the raceway is nonmetallic. But NEC 300.3(B)(3) says where cables are run in different raceways, the conductors shall have a nonmetallic or nonmagnetic sheath.

Is it code compliant to split the neutral in 1/3 and route in each conduit (i.e. phase A and 1/3 neutral, phase B and 1/3 neutral, phase C and 1/3 neutral)?

 

[Ravenvalor]

http://forums.mikeholt.com/showthread.php?t=140034

 

[mwm1752]

I do not know of any code that prohibits a concentric grounded conductor but depending on the installation it is used in may cause noncompliance with the code -- service laterals would be the common use. -- not sure how you would use them as a feeder as they would be more of an ECG in that type of installation. I would say no to splitting it into 3rds as it is a current carrying conductor for many reasons -- why would you do this in the first place?

 

[Flapjack]

The client wants to reuse 3 utility owned conduits that currently have concentric neutral cables feeding a wye-wye transformer. But now the service point is moving and the 3 existing conduits will be owned by the client and fall under the NEC. They don't want to saw cut and install a new conduit.

If concentric neutral cables aren't NEC compliant to be used as a neutral, then I don't see any way for the conduits to be re-used.
I was thinking I could use type MV-90 cable and split the neutral in 1/3 in each conduit, similar to what the concentric neutral cables offer for three phase installs, but the conduit fill would be exceeded.

They could swap out the transformers to be delta-wye, but I am not sure whether "non-magnetic sheath" would apply to cable shields (copper tape, concentric neutral, etc.). If it does not apply, I could use the concentric neutral around the conductor as an equipment grounding conductor.

 

[don_resqcapt19]

Couple questions... NEC 300.5(I) Exception 2 states 'isolated phase... installations shall be permitted in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic sheath in close proximity where conductors are paralleled...'.

Does this only apply to paralleled installations?

The CMP says this exception and the one in 300.3(B)(1) only apply to underground paralleled conductors.

I disagree and believe that 300.3(B)(3) permits single conductor isolated phase installations, even above ground. Your AHJ will probably agree with the CMP.

I'm guessing this prohibits copper tape shields when it says nonmagnetic sheath? The use of the word 'or' throws me off. It makes it sound metallic sheathed cables are permitted if the raceway is nonmetallic. But NEC 300.3(B)(3) says where cables are run in different raceways, the conductors shall have a nonmetallic or nonmagnetic sheath.

Is it code compliant to split the neutral in 1/3 and route in each conduit (i.e. phase A and 1/3 neutral, phase B and 1/3 neutral, phase C and 1/3 neutral)?

You can't use the concentric as a neutral in an isolated phase installation. If you don't need a neutral you can use it as an EGC assuming it meets the sizing requirements in 250.122.

 

[mwm1752]

What was the voltage on the original cables feeding the transformer -- what will be the secondary voltages for the original cable to the disconnect? -- what is the cable type? -- how many conductors per cable excluding the concentric conductor?

 

[Flapjack]

The cable voltage is 25 kV. Each conduit has 1 conductor.

 

[ramsy]

The client wants to reuse 3 utility owned conduits that currently have concentric neutral cables feeding a wye-wye transformer. But now the service point is moving and the 3 existing conduits will be owned by the client and fall under the NEC. They don't want to saw cut and install a new conduit.

Perhaps its better to extend utility owned service as far along that old conduit as possible, if not keeping original transformer, & NEC feeders loadside.

 

[Flapjack]

Yes, but unfortunately this is not possible. New MV switchgear is being installed by the client.

The design shows a new conduit with type MV-90 cables from the 25 kV switchgear to the existing wye-wye transformer, but the client wants to use the existing conduits. The run is ~450 ft.

 

[ramsy]

If cost of maintaining MV cables can't be on utility, has Client considered age of MV cables and cost of replacing them now vs latter --after building is finished--