You can only use 310.15(B)(6) if this is the feeder to the entire dwelling load, like the feeder from an outside main disconnect to the main panel inside the house. A sub panel off the main is sized with 310.16. It has to do with load diversity.
Re: Is this a Feeder?#55546 09/01/0504:59 PM09/01/0504:59 PM
Table 310.15(B)(6) is one of the areas of the code that can cause a fistfight to break out. While I am in complete agreement with the post from Gfretwell, there are many parts of the country where this is not enforced this way and you would be able to use the table for your feeder. If your work is subject to inspection, it is best (as always) to consult with the AHJ prior to commencing work.
Few things are harder to put up with than the annoyance of a good example.
Re: Is this a Feeder?#55547 09/01/0505:21 PM09/01/0505:21 PM
Given what copper prices are doing, every little bit helps.
On the other hand, reduced resistance in feeders means more efficient deliver of power to the load, and lower lifetime costs. If you take the 'Copper Development Association' at their word, you'd be upsizing from 310.16 by a size or two anyway.
On the left foot, I'd rather do away with 310.15, and instead change the calculations for service size to give a _separate_ result for conductor ampacity and required OCPD, in much the same way that oversized OCPD can be used with a given conductor ampacity for specific loads such as motors or welders.