Cable Ampacity Derating Explained

Ampacity is the current a conductor can carry continuously without exceeding its insulation temperature rating. The NEC's ampacity tables, most often Table 310.16, give a starting value that assumes no more than three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C. Where either assumption fails, the table value has to be reduced.

Step 1: Start from the right column

Table 310.16 has columns for 60°C, 75°C and 90°C insulation. A copper 4/0 conductor, for example, is listed at 195 A, 230 A and 260 A respectively. For a 90°C-rated conductor such as THHN or XHHW-2, derating can start from the 90°C column. That's the practical value of 90°C insulation: it absorbs derating that would otherwise force a bigger conductor.

Step 2: Correct for ambient temperature

Above 30°C, the conductor has less headroom before it reaches its insulation limit. NEC 310.15(B) gives the correction factors:

Raceways and cables exposed to sunlight on or just above a rooftop run much hotter than the surrounding air, and the NEC adds a temperature adder for the case where they sit very close to the roof surface. Hot aisles, plenums and equipment yards deserve the same scrutiny.

Step 3: Adjust for more than three current-carrying conductors

Conductors bundled together can't shed heat as well. NEC 310.15(C)(1) reduces ampacity when a raceway or cable holds more than three current-carrying conductors:

Equipment grounding conductors don't count. The neutral of a balanced three-phase circuit usually doesn't either, but it does count when most of the load is non-linear, such as server power supplies or drives, because it carries triplen harmonic current. The harmonics guide explains why. Short raceway nipples of 24 inches or less are exempt from adjustment.

Step 4: Cap at the terminal rating

A conductor is only as good as what it's connected to. Under NEC 110.14(C), the final ampacity can't exceed the value in the column matching the lowest-rated termination, which is usually 75°C for equipment rated over 100 A, and often 60°C for smaller equipment unless it's marked otherwise. Derating is done from the 90°C column, but the result is then compared against the 75°C (or 60°C) value, and the lower number wins.

Worked examples

Nine 10 AWG copper THHN conductors in one conduit, 40°C ambient, 75°C terminals.

• 90°C table value: 40 A
• Derated: 40 × 0.91 (ambient) × 0.70 (nine conductors) = 25.5 A
• Terminal cap: the 75°C column gives 35 A, which is higher, so 25.5 A governs.

A 4/0 copper feeder, four current-carrying conductors (the neutral counts, for non-linear load), 40°C ambient.

• 90°C table value: 260 A
• Derated: 260 × 0.91 × 0.80 = 189 A
• Terminal cap: 230 A at 75°C, so 189 A governs.

Without the correction and adjustment, the same 4/0 feeder would be sized as a 230 A conductor, 41 A more than it can actually carry. The same derating also interacts with the continuous-load rules, which size conductors at 125% of continuous load before derating is compared.

Common mistakes

• Using the 90°C column as the final answer when the terminations are only rated 75°C.
• Not counting the neutral on circuits feeding non-linear load.
• Checking conduit fill and forgetting the adjustment that the same conductor count triggers.
• Using the room temperature for a raceway that runs through a hot aisle, plenum or rooftop.
• Applying the factors in the wrong column, for example a 75°C correction factor to a 90°C table value.

Frequently asked questions