Ampacity Derating Calculator

Derate conductor ampacity for ambient temperature and bundling per NEC 310.15 — correction factor, adjustment factor and arithmetic shown.

What this does not check

It derates from the 75°C copper column of Table 310.16, and only that column. Aluminium conductors and the 60°C and 90°C columns are not covered. A 90°C conductor derates from its own ampacity column, then gets capped by the 75°C terminal limit — that is a different calculation, not this one with a factor swapped.
Terminal temperature ratings still govern separately under 110.14(C). A 90°C conductor landed on 75°C-rated terminals is limited by the terminals, whichever way this calculation comes out.
It does not apply the 240.4(D) small-conductor overcurrent caps, which limit the breaker on 14, 12 and 10 AWG regardless of the ampacity the conductor has left.
Raceway fill, voltage drop, and the 125% continuous-load factor are separate calculations and are not checked here.

How to use it

Pick the conductor size, enter the ambient temperature the conductor will actually see, and count the current-carrying conductors sharing the raceway or cable. The result is the ampacity that conductor still has after both derating factors apply.

Two inputs deserve care. Ambient is the temperature at the conductor, not outside the building — an attic in August and the conditioned space below it are not the same number. Conductor count is current-carrying conductors only: equipment grounding conductors never count, and a neutral counts only when it carries current, which it does on a circuit feeding non-linear loads and generally does not on the balanced legs of a multiwire branch circuit.

The method

Derating is two independent multipliers applied to the base ampacity:

derated = base ampacity × ambient correction × conductor adjustment

The base ampacity comes from Table 310.16 — this calculator uses the 75°C copper column. The ambient correction comes from Table 310.15(B)(1)(1), which is built on a 30°C ambient: below that the factor is greater than 1 and the conductor gains capacity, above it the factor falls away sharply. The conductor adjustment comes from Table 310.15(C)(1) and depends only on how many current-carrying conductors are bundled together.

Both apply. Taking only the more punishing of the two is a shortcut that produces a conductor smaller than the code allows.

Worked example

Ten AWG copper, in a raceway with six current-carrying conductors, in a 110°F attic.

The base ampacity is 35 A. 110°F is 43.3°C, which falls in the 45°C bracket, so the correction factor is 0.82. Six conductors put the circuit in the four-to-six band, so the adjustment factor is 0.80.

35 A × 0.82 × 0.80 = 23.0 A

That conductor has 23 A of usable ampacity — a 34% reduction, and well under the 30 A breaker that 10 AWG is often assumed to support. This is the case that catches people: nothing about the wire changed, only where it was run and what it was run with.

Where it bites in the field

The attic run above is the classic one. The others worth watching: a panel feeder sharing a raceway with several other circuits, where the conductor count alone takes 30% off; a rooftop raceway in direct sun, where the effective ambient is far higher than the shade temperature anyone measured; and long horizontal runs through an unconditioned space where the ambient changes along the run and the worst section governs.

When derating pushes a conductor below the load it has to carry, the answer is usually a larger conductor rather than a smaller breaker — the breaker protects the conductor, so shrinking it to fit protects nothing and just moves the problem.

Code references

NEC 310.16Base allowable ampacity, 75°C copper column2023 edition
NEC Table 310.15(B)(1)(1)Ambient temperature correction factors, 30°C base2023 edition
NEC Table 310.15(C)(1)Adjustment for more than three current-carrying conductors2023 edition

Section numbers and table values move between editions, and jurisdictions amend them. Confirm against the edition your authority having jurisdiction enforces.

Common questions

When do I have to derate for conductor count?: Once a raceway or cable carries more than three current-carrying conductors, Table 310.15(C)(1) applies — four to six take 80%, seven to nine take 70%, ten to twenty take 50%. On a balanced multiwire circuit the neutral usually does not count as current-carrying; on a circuit feeding non-linear loads it does.
Do the ambient and conductor-count factors multiply together?: Yes, and this is the most common place people go wrong. They are independent effects, so multiply the base ampacity by the ambient correction factor and then by the conductor adjustment factor. Applying only the worse of the two undersizes the conductor.
What ambient temperature should I use?: The temperature the conductor actually sits in, not the outdoor design temperature. A summer attic, a boiler room, or a rooftop raceway in direct sun can run far above the 30°C the table is based on.
Why does the calculator refuse some inputs instead of returning a number?: Because the table stops. Above 85°C ambient there is no published correction factor at all, and a 75°C conductor has none above 70°C. Returning a small number there would read as a heavily derated conductor rather than what it actually is — a condition the table does not cover.

For the method behind this — worked examples, the tables, and where it goes wrong — read the full guide.