Derating Main Feeders
- Thread starter colada13
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- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Are you saying that all conductors in your "paralleled 4/0 aluminum" are in the same conduit? And by "paralleled," do you mean that there is more than one conductor for each phase? If so, and if that adds up to more than three current-carrying conductors (which I believe it must), then yes, you have to derate.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I think I see a problem here. If the calculated load is ~380amps then you would need 600 MCM cable per Table 310.15(B)(6) for residential.
I see you have paralleled 4/0. 4/0 has a cir. mil of 211600. Multiply by 2 and you get 423200 cir mil. That's equivalent to a 400MCM cable --- you need 600mcm if you use aluminum. That would be paralleled 300MCM cables.
I may be wrong but I think I did it correctly. That's without derating.
You still must derate if the cable is in conduit with more than 3 CCC
I see you have paralleled 4/0. 4/0 has a cir. mil of 211600. Multiply by 2 and you get 423200 cir mil. That's equivalent to a 400MCM cable --- you need 600mcm if you use aluminum. That would be paralleled 300MCM cables.
I may be wrong but I think I did it correctly. That's without derating.
You still must derate if the cable is in conduit with more than 3 CCC
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I hadn?t realized you were talking about using that table as the basis for ampacity of a set of parallel conductors. You cannot do that. It says that you can use a 4/0 AL for a service of 200 amps. It does not say you can use a pair of parallel 4/0 AL for a service of 400 amps. If you intended configuration is not in the table, then you can?t do it. For a 400 amp service, you can use a 600 MCM AL, but not a pair of 4/0.colada13 said:
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Once again, I assert that you can't parallel conductors and use the values in Table 310.15(B)(6).Dennis Alwon said:
But further, I do not understand why you are talking about cross-sectional areas. We do not figure out the ampacity of parallel conductors by adding their areas. We take the ampacity of a single conductor and multiply it by the number of parallel sets. So I guess I missed your point here.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
you didn't miss my point I was mistaken. I was using cross sectional area to get the equivalent of a 600 MCM for 400 amp. I was saying 2 4/0 alum. wouldn't work. I should have realized that it wasn't dependant on the cross section.
- Location
- Illinois
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- retired electrician
Charlie,
CMP 6 does not agree. Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3." Note 3 in the 93 code is what is now Table 310.15(B)(6).
Don
CMP 6 does not agree. Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3." Note 3 in the 93 code is what is now Table 310.15(B)(6).
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
That may well have been the intention of CMP 6. But they did not write the article in such a way as to convey that intention. I do not dispute that you can place two sets of 4/0 AL in parallel, as described in 310.4. But the question is, what do you get when you do so?
If you look at the text above Table 310.16, it explicitly states that the values in the table represent the ampacity of conductors of various sizes and types. No such words appear in or near Table 310.15(B)(6). What does appear is the statement that you can use conductors as shown in the table for service and feeder conductors. That is all it says. The configuration of ?two parallel sets of 4/0 AL? is not in the table. So I submit that the code, as written, does not give us the freedom to use that configuration, and claim it suffices for a 400 amp service.
I believe that I have argued this point before on this Forum. My (admittedly vague) memory is that I did not convince the world to adopt my viewpoint.
If you look at the text above Table 310.16, it explicitly states that the values in the table represent the ampacity of conductors of various sizes and types. No such words appear in or near Table 310.15(B)(6). What does appear is the statement that you can use conductors as shown in the table for service and feeder conductors. That is all it says. The configuration of ?two parallel sets of 4/0 AL? is not in the table. So I submit that the code, as written, does not give us the freedom to use that configuration, and claim it suffices for a 400 amp service.
I believe that I have argued this point before on this Forum. My (admittedly vague) memory is that I did not convince the world to adopt my viewpoint.
I was on your side Charlie, but I just read that paragraph again, and now I think it probably should be interperted as applying to parallel conductors.
If you read it, it says "conductors as listed in table 310.15 B 6 shall be permitted as.....service entrance conductors, service lateral conductors, and feeder conductors...."
I think the language is there. They are saying conductors are permitted as listed. They didn't say feeders or services are permitted as listed.
But I don't think it matters for this case, since derating the 90 deg. column of Table 310.16 gives a higher ampacity than derating the values in table 310.15 B 2.
And I think we all agree, 2 sets of 4/0 are not enough wire for this load.
Steve
If you read it, it says "conductors as listed in table 310.15 B 6 shall be permitted as.....service entrance conductors, service lateral conductors, and feeder conductors...."
I think the language is there. They are saying conductors are permitted as listed. They didn't say feeders or services are permitted as listed.
But I don't think it matters for this case, since derating the 90 deg. column of Table 310.16 gives a higher ampacity than derating the values in table 310.15 B 2.
And I think we all agree, 2 sets of 4/0 are not enough wire for this load.
Steve
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