Submittal Reviews: Concentric Neutral and URD/UD

[charlie b]

I received a submittal from the EC on the topic of medium voltage cables. The specs call for a 1/3 concentric neutral; the EC submitted full concentric neutral. The specs call for type URD. The EC submitted UD. On the surface, it would seem we are getting more than we asked for, and that should make the owner happy. But this owner has been hard to get along with, and I want to make sure. Here are some things I don't know about these terms:

1. What is the purpose of a concentric neutral?
2. Since each ungrounded, single conductor has a concentric neutral, what do we do with the concentric neutral wires (i.e., do we find a way to ultimately connect them all to the neutral bus)?
3. If each of the A, B, and C has one of these things, and if we do connect them to the neutral bar, then does that mean we don't need to run a separate neutral wire (this I tend to doubt)?
4. Other than the obvious "more is better," what would make a full concentric better (or perhaps not as effective as) a 1/3 concentric neutral?
5. I don't know why the specs would have called for URD on a office building project, given that the "R" is residential, would use of UD be better?
6. Was it likely an error on the part of the spec writer to have called for URD instead of UD?

TIA.

 

[masterinbama]

As for the 100% VS 1/3 neutral you are getting basically a larger neutral conductor.


All I can find defining the difference between URD and UD Is Okonite lists URD as Underground Residential Distribution cable and Southwire lists UD as Underground Distribution cable.

 

[don_resqcapt19]

Charlie,
Is this a installation that is covered by the NEC? I don't think URD is an NEC compliant wiring method.

 

[bob]

I received a submittal from the EC on the topic of medium voltage cables. The specs call for a 1/3 concentric neutral; the EC submitted full concentric neutral. The specs call for type URD. The EC submitted UD. On the surface, it would seem we are getting more than we asked for, and that should make the owner happy. But this owner has been hard to get along with, and I want to make sure. Here are some things I don't know about these terms:

1. What is the purpose of a concentric neutral? It acts as the system neutral. Utilities will install 1, 2, or 3 cables but usually there is no 3 phase service available.
2. Since each ungrounded, single conductor has a concentric neutral, what do we do with the concentric neutral wires (i.e., do we find a way to ultimately connect them all to the neutral bus)? Yes but remember the transformers are usually single phase
3. If each of the A, B, and C has one of these things, and if we do connect them to the neutral bar, then does that mean we don't need to run a separate neutral wire (this I tend to doubt)? I am assuming you have a 3 phase primary so the concentric neutrals are bonded together.
4. Other than the obvious "more is better," what would make a full concentric better (or perhaps not as effective as) a 1/3 concentric neutral?
5. I don't know why the specs would have called for URD on a office building project, given that the "R" is residential, would use of UD be better? Okonite describes the URD cable as URO cable
6. Was it likely an error on the part of the spec writer to have called for URD instead of UD? Or maybe he did not know.

http://okonite.com/Product_Catalog/section2/index.html Okonite Product Catalog

http://okonite.com/Product_Catalog/section2/section2-pdfs/2-10.pdf commercial grade cable. I would prefer this.

http://okonite.com/Product_Catalog/section2/section2-pdfs/2-34.pdf 15 kv URO(URD) with no cover over neutral

http://okonite.com/Product_Catalog/section2/section2-pdfs/2-36.pdf 15kv URO(RED) with cover over neutral. If you choose the URD, this would be
my choice.

 

[Hv&Lv]

I received a submittal from the EC on the topic of medium voltage cables. The specs call for a 1/3 concentric neutral; the EC submitted full concentric neutral. The specs call for type URD. The EC submitted UD. On the surface, it would seem we are getting more than we asked for, and that should make the owner happy. But this owner has been hard to get along with, and I want to make sure. Here are some things I don't know about these terms:

1. What is the purpose of a concentric neutral?
2. Since each ungrounded, single conductor has a concentric neutral, what do we do with the concentric neutral wires (i.e., do we find a way to ultimately connect them all to the neutral bus)?
3. If each of the A, B, and C has one of these things, and if we do connect them to the neutral bar, then does that mean we don't need to run a separate neutral wire (this I tend to doubt)?
4. Other than the obvious "more is better," what would make a full concentric better (or perhaps not as effective as) a 1/3 concentric neutral?
5. I don't know why the specs would have called for URD on a office building project, given that the "R" is residential, would use of UD be better?
6. Was it likely an error on the part of the spec writer to have called for URD instead of UD?

TIA.

1) to serve as a neutral conductor on a Wye system versus a shielding on shielded cable, which is to protect from corona discharge at the contact points.
2)Connect them as you would any other neutral
3) Correct, you do not need to run a seperate neutral conductor on concentric neutral cable. You would have to for shielded cable if it were used on a Wye system
4) 1/3 neutral is for three phase applications. (1/3*3=1) Full neutral is used for single phase or V phase applications. It can be used for three phase also, it just costs more.
5) UD=URD
6) see #5

 

[Hv&Lv]

http://okonite.com/Product_Catalog/section2/section2-pdfs/2-10.pdf commercial grade cable. I would prefer this..

That is ok I guess if you are on a delta system. Needs a grounded conductor pulled in with it.

[http://okonite.com/Product_Catalog/section2/section2-pdfs/2-34.pdf 15 kv URO(URD) with no cover over neutral..

Not much good anymore. Old technology in my opinion. I have seen too many strands become brittle with the acid soils, and the strands deteriorate to the point of conducting against the carbon semiconductor, eating at the insulation to the point of failure.

[[http://okonite.com/Product_Catalog/section2/section2-pdfs/2-36.pdf 15kv URO(RED) with cover over neutral. If you choose the URD, this would be
my choice.

Excellent choice

 

[augie47]

As Don mentioned in post 3, if this is a NEC job you might want to note the product can be UL or CSA listed on "special orders".

 

[charlie b]

Thanks for the replies, everybody.