Table 310.15(B)(6)

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charlie b

Moderator
Staff member
This is just FYI, but is certainly open to discussion. I have already submitted this to the NFPA.
Recommended change: (Insert after the second sentence, which ends with, ?. . . associated with the dwelling unit.?) Use of paralleled conductors as a substitute for the conductor sizes shown in this table, or to supply larger service or feeder loads than those shown in this table, shall not be permitted. The feeder conductors to a dwelling unit shall not be required . . .
Substantiation: This change would clearly declare that Table 310.15(B)(6) cannot be adapted to suit the user?s design preferences.
Article 310.4 tells us that paralleling conductors is allowable, under certain conditions. But it does not tell us what changes take place, when we do so. It is commonly presumed that two parallel conductors (in separate raceways) will have twice the ampacity of either conductor alone. But Table 310.15(B)(6) is not about ampacity. The values for amperes shown in the left-hand column are not ampacity values, but rather are load values.

Table 310.15(B)(6) allows for the use of conductors in situations that exceed their ampacity values as given in Table 310.16. For example, a 2/0 copper conductor with 75C insulation has an ampacity of 175 amps, yet it can be used for a 200 amp service. That represents a 25 amp difference between the conductor?s ampacity and the load it will carry. That difference becomes 50 amps, when you use two in parallel. Absent any proof that the additional burden will not harm the conductor, and particularly noting that the user has no knowledge of the design basis that lies behind this Table, the user should not be allowed to presume that a pair of 2/0 conductors will be sufficient for a service of 400 amps.
 

winnie

Senior Member
It has long been my opinion that table 310.15(B)(6) is a very confusing beast, and I applaud your effort to clean it up.

I've given though to the idea that 310.15(B)(6) should be replaced with a table that essentially says that a residential service of size X could be served with conductors of ampacity Y, and then lets those conductors be selected using table 310.16. So rather than saying that a particular conductor size was suitable for a 200A service in _any_ conditions, the table would say that (for example) for a 200A residential service you need conductors of 175A ampacity, and then let you pick any 175A conductors from 310.16.

Maybe for '14 :)

-Jon
 

Lcdrwalker

Senior Member
Wouldn't the 220 calculations come into play here? Since the load isn't continuous would there be insulation degradation? If the load was continuous the max on a 200A service would be 160A. This would be well within the range of a 2/0 Cu. If the load calculations resulted in 200A continuous, wouldn't this require a minimum service of at least 250A and a conductor of at least a 4/0 Cu.? If everything here is doubled, I believe it would hold true for two conductors in parallel.
 

ryan_618

Senior Member
Charlie: I am eager to see what CMP-6 says about this. If I may offer a couple of suggestions to your substantiation:

1) Change the word "article" to "section" in the second sentence.
2) "But Table 310.15(B)(6) is not about ampacity"... Considering the fact that Section 310.15 is titled "ampacities", this can hardly be a true statement. In particular, refer to the charging language of 310.15(B).
 

charlie b

Moderator
Staff member
Thanks, Ryan, but as I said this is already submitted, and is being posted here just for information.
ryan_618 said:
2) "But Table 310.15(B)(6) is not about ampacity"... Considering the fact that Section 310.15 is titled "ampacities", this can hardly be a true statement.
The section may have that word in its title, but the table does not. Nor do the numbers shown in the left hand column represent ampacities. That was my point. I just hope the CMP understands what I tried to say.
ryan_618 said:
Charlie: I am eager to see what CMP-6 says about this.
So am I. It is one of several I submitted for which I do not care which way the CMP makes the call. All I want is for them to make a call, one way or the other, and make the results of that decision clear in the new wording.
 
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