URD Quadruplex Wire for Underground AC Run

[mddorogi]

I've seen URD quadruplex wire and wanted to use it for underground AC runs. See attached below. There are a couple points I am confused about and having a tough time figuring it out. I have an AC run from PV AC combiner panel to 480/208 transformer at 230' long. The current is 180.55A, and including 1.25 factor that's 225.7A.

Using NEC table 310.15(B)(16), I'd use 300 mcm aluminum with ampacity of 230A. Voltage drop is less than 2%. There is no quadruplex wire using 300 mcm, but there is "slippery rock" using 350 mcm. It lists an direct bury ampacity of a whopping 385A. I called to ask where that number comes from and didn't get an answer. I have two questions:
- is URD wire NEC approved? Has anyone used it for underground AC runs in solar installations?
- how would they get a direct bury ampacity of 385A for 350 mcm aluminum wire?

 

[petersonra]

I don't think URD is a NEC recognized wiring method, but it might be dual rated as something that is.

In any case, you cannot use the manufacturers ampacity ratings. You have to use what is in the code.

 

[petersonra]

just curious. quadplex has four conductors. presumably 3 phases and a neutral. what are you planning to do about an EGC?

 

[mddorogi]

just curious. quadplex has four conductors. presumably 3 phases and a neutral. what are you planning to do about an EGC?

Add a run of USE wire for the EGC.

 

[letgomywago]

They sell mobile home feeder in some nice combinations

 

[pv_n00b]

I have not heard of it being used in projects under the NEC. From what I have read it's used in utility secondary distribution in residential developments. So good enough for the NESC but not the NEC I guess.
As for the ampacity, underground conductor ampacity is subject to a lot of variables that can increase or decrease the ampacity. I would not use their underground ampacity without knowing what variables they used, for instance, the RHO they used for the soil might not be the same as the RHO for the soil you have. Programs like AmpCalc and Etap can do underground thermal analysis to determine the ampacity of a conductor.

 

[electrofelon]

Around here URD is common jargon that refers to cable that is usually type USE-2/RHH-2. One supply house has URD that is ONLY listed as type USE-2. I have heard stories of URD that is not listed with any NEC wire type, and looks like what you have may be just that.

 

[mddorogi]

I find it bizarre that a wire can be good enough for utility companies and be used all over the place powering our houses, but not NEC approved and can't technically be used by anyone else.

 

[don_resqcapt19]

I find it bizarre that a wire can be good enough for utility companies and be used all over the place powering our houses, but not NEC approved and can't technically be used by anyone else.

It is a smoke and fire issue, just like communications utilities outside plant wire....cannot be used in a building.

 

[mddorogi]

It is a smoke and fire issue, just like communications utilities outside plant wire....cannot be used in a building.

I was referring to using URD wire in underground runs, direct bury. That's what utilities use it for, but it seems that technically we can't use it for underground solar AC wiring.

 

[jaggedben]

I find it bizarre that a wire can be good enough for utility companies and be used all over the place powering our houses, but not NEC approved and can't technically be used by anyone else.

Consider that utilities have organized training and more standard qualifications for employees, and most materials on their installations are actually specified by engineers. Whereas many NEC installations are done by DIYers, non-specialized contractors and handymen, and electricians-in-name-only. Given that, I think the wire requirements make more sense the way they are than the other way around.

 

[electrofelon]

I find it bizarre that a wire can be good enough for utility companies and be used all over the place powering our houses, but not NEC approved and can't technically be used by anyone else.

Well as a practical matter, you can get "NEC URD" from any number of places and I would say that most of the time it is NEC approved. In my experience, that national nassau cable place doesnt have particularly god prices plus you have shipping - maybe you were just using that as an example.

I do wonder why a manufacturer would sell such a wire without the additional USE markings. What exactly needs to be added to get it to meet USE type also? how much are they saving? Is it really worth excluding your product from that segment of the market and having another separate product and production run to save 1/3 of a cent per foot (complete guess) of cost?