NEC in the Facility, May 18, 2010

How do you find the ampacity requirements that apply to your facility's power distribution wiring, and which of the many ampacity tables in Art. 310 is the right one?

To answer the first question, we can quickly cut Art. 310 down to size. About half of it is for 2,001V or more, and everything from 310.60 onward is for 2,001V or more. The parts that typically apply to facilities end at Section 310.15.

The requirements in 310.1 through 310.14 are general requirements. Then, you have 310.15 and the next section is 310.60 (2,001V and above). This means that, for purposes of calculating ampacity, your requirements are in 310.15 (unless you're working power distribution at greater than 2,000V).

The ampacity tables appear right after 310.15 and are named in sequence to it; thus, Table 310.16, 310.17, etc., through Table 310.21. But how do you know which of these six tables to use?

Read the subtitle of the table. For example, you'll notice in Table 310.16 "not more than three current-carrying conductors in raceway" among other items in that subtitle. If this matches your application, use that table. If not, look at the other tables.

However, what if you have more than three current-carrying conductors in raceway? There isn’t a table for that. The NEC answers this in Annex B. There, you'll find Table B.310.11, which has the adjustment factors you need to apply Table 310.16 for more than three current-carrying conductors. You'll also find a wealth of other information, including more tables and an explanation of how to calculate the ampacity yourself.