Wiring a house where we will be running underground service conductors from a transformer w/ CT metering into a 400amp MDP. If we parallel the service, can we base our wire size on Table 310.15(B)7? It says that 2/0 is good for 200amps, so can we parallel 2/0's for 400amps. The problem is, the code says you can use this table when the SE conductors/feeders supply ALL loads of the house and obviously each set of paralled conductors will be sharing the load. Any thoughts? Thanks.
RESIDENTIAL SERVICE
- Thread starter SPierce
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- Logan, Utah
IMHO, 310.15(B)(7) can't be used for parallel service entrance conductors.
The left hand column is titled service or feeder rating which in your case would be 400 amps not 200 amps. So to use the table for a 400 amp feeder or service you could only use 400 Kcmil copper or 600 Kcmil aluminum.
Chris
The left hand column is titled service or feeder rating which in your case would be 400 amps not 200 amps. So to use the table for a 400 amp feeder or service you could only use 400 Kcmil copper or 600 Kcmil aluminum.
Chris
jumper
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I agree with the headbanger.
Plus 2/0 CU is good for 175 amps, so despite the actual load you can not use 240.4(B) because 350 is a standard size listed in 240.6.
Plus 2/0 CU is good for 175 amps, so despite the actual load you can not use 240.4(B) because 350 is a standard size listed in 240.6.
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
Agree. The conductor size is limited to a single set, e.g. not parallel conductors.
I would ask your AHJ. Our inspectors allow us to parallel 2/0 in residential 400A services. Think about it 2/0 feeding a panel where the load is 150 amps or 2/0 paralleled where each of the two panels load is 75 amps. As long as the MB is not more than 200A there really is no difference.
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- Lockport, IL
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For those who might be confused, table 310.15(B)(7) appears in the 2011 NEC. In the earlier versions, the table was 310.15(B)(6).
I agree that you can't get twice the ampacity by using two parallel conductors, because you don't know the ampacity of a single conductor. This is not an ampacity table. All you know is that we are allowed, under certain limited circumstances, to use conductors that would be too small, if we stuck with the ampacity values in table 310.16.
That said, I submitted a proposal for the 2011 NEC, adding words to 310.15(B)(6) that would explicitly forbid paralleling. The CMP disagreed with me, saying they intended us to be able to do what the OP suggests doing. They went further, saying the present wording allows this. I plan to challenge them again for the 2014 NEC. I still say that nothing in the NEC says we can use two or more sets of conductors that are listed in table 310.15(B)(7) for services and feeders of twice the tabulated ratings.
I agree that you can't get twice the ampacity by using two parallel conductors, because you don't know the ampacity of a single conductor. This is not an ampacity table. All you know is that we are allowed, under certain limited circumstances, to use conductors that would be too small, if we stuck with the ampacity values in table 310.16.
That said, I submitted a proposal for the 2011 NEC, adding words to 310.15(B)(6) that would explicitly forbid paralleling. The CMP disagreed with me, saying they intended us to be able to do what the OP suggests doing. They went further, saying the present wording allows this. I plan to challenge them again for the 2014 NEC. I still say that nothing in the NEC says we can use two or more sets of conductors that are listed in table 310.15(B)(7) for services and feeders of twice the tabulated ratings.
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