As I read the table in 310.15 I believe they list 4 being good for 100A as a panel feeder. I see it quoted here that the 6 60 combination is acceptable as a feeder combination. Its not listed, but due to the next size rule is this acceptable?

 

Table 310.15(B)(6) cannot be used to size the feeder for a subpanel, unless that 'subpanel' is the _main_ panel for your home (eg. a 'subpanel' fed from a combination meter/main disconnect where all of the power in your home goes through that subpanel).

For subpanels, you generally need to use table 310.16. Usually you use the 60degree column, though sometimes you can use the 75degree column. #6 copper wire has an ampacity of 55A. It is commonly used for a subpanel feeder, and the 'round-up' rule is applied.

Note: the round up rule does _not_ say that you can use #6 wire to carry 60A. What it says is that you are permitted to _protect_ #6 wire with a 60A breaker. The load on the #6 wire is still required to be less than 55A (presuming 60C limitations). This is a point where the code is generally applied a bit loosely; people install a '60A' subpanel in a garage/workshop, with a #6 feeder, and don't bother actually doing the load calculation to make sure that the actual calculated load is less than 55A; they simply presume that it will be...and they are usually correct.

-Jon