What is in this article?:
- Sizing Circuit Protection and Conductors β Part 1
- SIDEBAR: Table and Column Selection
Understanding which NEC ampacity table to use when sizing circuit protection, and how to determine the minimum ampacity of your conductors
SIDEBAR: Table and Column Selection
Article 310 provides several ampacity tables: Table 310.15(B)(16) through Table 310.15(B)(21). On top of this, Annex B also provides several ampacity tables: Table 310.15(B)(2)(1) through Table 310.15(B)(2)(10).
Those Annex B tables provide more options for looking up your ampacity, but they aren’t required for that purpose. Always start with the Art. 310 tables, and carefully read the headings. If you don’t find an exact match, you might find it in Annex B.
If you don’t find an exact match in either place, then pick the closest match in Art. 310. You will need to modify it per 310.15(B)(1) through 310.15(B)(7). For most applications, the only modifications you’ll need to make will be the adjustment factors detailed in 310.15(B)(3) and the ambient temperature correction factors [310.15(B)(2)].
This ambient temperature correction requirement exists because you need a larger ampacity to allow for the higher temperature. Choose either the 86°F table [Table 310.5(b)(2)(a)] or the 104°F table [Table 310.5(b)(2)(b)] to find your temperature correction factor. Then, you can either multiply the table ampacity by the correction factor or divide your calculated amps by the correction factor prior to looking up the ampacity.
When you look at the ampacity table, you now have a choice of conductor temperature rating columns. Use the column that matches the rating of your lowest rated termination, device, or conductor [110.14(C)]. Terminations are normally rated for 60°C; thus, you normally use the 60°C column, regardless of the conductor’s temperature rating (unless you have special terminations rated for the higher temperature).
You might put a portion of a circuit in a higher temperature environment, and thus have occasion to use the higher temperature column for that portion if not including terminations in that space. Example D3(a) in Annex D provides an excellent illustration of such a situation.