How this calculation works
Start with the table, then apply what the installation does to it.
The three reductions
Ambient temperature. Table 310.16 is built for a 30°C (86°F) ambient. An attic in July, a boiler room, a rooftop in direct sun — all of those run hotter, and the correction factors in 310.15(B) cut the ampacity accordingly. Rooftop conduit exposed to sunlight used to carry a large adder; the current rule in 310.15(B)(2) treats it more simply, but the ambient is still whatever the space actually is, not what you wish it was.
Conductor bundling. More than three current-carrying conductors in the same raceway or cable and 310.15(C)(1) applies. Four to six is 80%. Seven to nine is 70%. Ten to twenty is 50%. Grounds never count. Neutrals count only when they carry more than unbalanced current.
Terminal temperature. This is the one that catches people. 110.14(C) says the circuit ampacity cannot exceed the temperature rating of the terminals, and almost all equipment is rated 75°C. Some residential gear at 100 amps and below is 60°C. So even though you buy 90°C THHN, you generally cannot use the 90°C column as your final answer.
Why you still buy 90°C wire
You start derating from the 90°C column, then cap the result at the terminal rating. THHN in a hot attic with six conductors gives you more usable ampacity than a 75°C conductor would, even though neither one can be loaded past the 75°C terminal number. That is the whole reason 90°C insulation is worth buying.
The small conductor rule
240.4(D) caps overcurrent protection on the small stuff regardless of what the ampacity math says: 15 amps on 14 AWG copper, 20 on 12 AWG copper, 30 on 10 AWG copper. There are exceptions for motor circuits and a few other cases, but for general branch-circuit work those three numbers are hard limits.
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Frequently asked questions
Can I use the 90 degree column?
For derating, yes. For the final answer, usually no. You start from the 90°C column, apply your correction and adjustment factors, then cap the result at the terminal rating — which is 75°C on most equipment and 60°C on some residential gear. That is what 110.14(C) requires.
Do grounds count toward the conductor adjustment?
No. Equipment grounding conductors are never counted as current-carrying, because they only carry current during a fault. Neutrals are counted only when they carry more than the unbalanced current — which happens on circuits feeding nonlinear loads.
What ambient temperature should I use for an attic?
Whatever it actually reaches, not the outdoor design temperature. Unventilated attics in hot climates routinely hit 50 to 60°C, and the correction factor at 50°C takes a 90°C conductor down to 82% of its table value. If you are not sure, measure it on a hot afternoon.
Does upsizing for voltage drop change my ampacity?
No, but it does change your ground. Increasing the ungrounded conductors for voltage drop requires a proportional increase in the equipment grounding conductor under 250.122(B). The ampacity of the larger conductor is whatever the table says for that size.
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