Which temperature column should I use?

Use the column matching your conductor insulation: 60 C for TW/UF, 75 C for THW/THWN/XHHW, and 90 C for THHN/THWN-2/XHHW-2. The 90 C column is mainly used as a starting point for derating, not as the final terminal rating.

What is the 110.14(C) termination rule?

Equipment terminals are rated 60 C or 75 C, and NEC 110.14(C) requires you to size the conductor from the column matching the lowest-rated termination in the circuit. You may still derate from the 90 C column, but the final ampacity cannot exceed the termination column value.

How is the ambient correction applied?

Table 310.16 is based on a 30 C ambient. NEC Table 310.15(B)(1) gives a multiplier for other ambients, lower than one above 30 C and higher below it. This tool multiplies the base ampacity by that factor for the selected column.

Does this handle four or more conductors in a raceway?

No, the base table assumes three or fewer current-carrying conductors. With four or more you must apply an additional NEC 310.15(C)(1) adjustment factor, such as 80 percent for four to six conductors, on top of the ambient correction.

Why do copper and aluminum differ so much?

Aluminum has higher resistance per area than copper, so it carries less current at the same size and heats up faster. A given aluminum conductor is typically rated one to two sizes lower in ampacity than the same copper size.

Wire Ampacity Calculator (NEC 310.16)

NEC Table 310.16 is the foundational ampacity table for conductors in raceways and cables, and reading the wrong column or skipping the ambient correction is a common sizing error. This tool returns the exact published value and the temperature-corrected ampacity.

How it works

The tool looks up the published Table 310.16 ampacity for your material, size, and insulation column, then applies the Table 310.15(B)(1) ambient factor:

base      = Table 310.16[material][size][temp column]
factor    = Table 310.15(B)(1)[temp column][ambient band]
corrected = base × factor

The base table assumes a 30 C ambient and not more than three current-carrying conductors in a raceway. The temperature column must match the conductor’s insulation type.

Notes and example

A 12 AWG THHN copper conductor reads 30 A in the 90 C column, but NEC 110.14(C) caps a 75 C termination at the 75 C value of 25 A, and most 12 AWG breakers are limited to 20 A by 240.4(D). At a 36-40 C ambient the 90 C factor of 0.91 brings the 30 A starting point down to about 27.3 A, which still leaves room before the termination cap. Always check terminations and conductor-count adjustment before trusting the table number alone.