AmpSizerAmpacity derating explained
Derating multiplies the table ampacity by two factors: one for ambient temperature from Table 310.15(B)(1), and one for having more than three current-carrying conductors together from Table 310.15(C)(1). They multiply with each other — you do not pick the worse one.
12 AWG copper is 25 A at 75 °C. In a 104 °F attic that becomes 22 A. With six current-carrying conductors in the raceway as well, 0.88 × 0.80 gives 17.6 A.
The 90 °C column is where the arithmetic is allowed to start. THHN is a 90 °C conductor, so you may derate from 30 A rather than 25 — but 110.14(C) then limits the result to the 75 °C terminal rating, so it can never buy you more than the 75 °C figure.
The two tables
| Ambient | 60 °C wire | 75 °C wire | 90 °C wire |
|---|---|---|---|
| 50–77 F (10–25 C) | × 1.08 | × 1.05 | × 1.04 |
| 78–86 F (26–30 C) | × 1 | × 1 | × 1 |
| 87–95 F (31–35 C) | × 0.91 | × 0.94 | × 0.96 |
| 96–104 F (36–40 C) | × 0.82 | × 0.88 | × 0.91 |
| 105–113 F (41–45 C) | × 0.71 | × 0.82 | × 0.87 |
| 114–122 F (46–50 C) | × 0.58 | × 0.75 | × 0.82 |
| 123–131 F (51–55 C) | × 0.41 | × 0.67 | × 0.76 |
| 132–140 F (56–60 C) | not permitted | × 0.58 | × 0.71 |
| Current-carrying conductors | Multiply ampacity by |
|---|---|
| 1 to 3 | × 1 |
| 4 to 6 | × 0.8 |
| 7 to 9 | × 0.7 |
| 10 to 20 | × 0.5 |
| 21 to 30 | × 0.45 |
| 31 to 40 | × 0.4 |
| 41 and more | × 0.35 |
Worked examples
A garage circuit in a hot attic. 12 AWG THHN, 20 A load, attic at 113 °F. Starting from the 90 °C column at 30 A: the ambient factor at 113 °F for a 90 °C conductor is 0.87, giving 26.1 A. Then 110.14(C) caps it at the 75 °C value of 25 A. Result: 25 A, and the 20 A circuit is fine. Had we started from 75 °C, 25 × 0.82 would be 20.5 A — still fine, but with almost nothing left.
Two circuits in one conduit. Four current-carrying conductors, so the factor is 0.80. 12 AWG THHN from the 90 °C column: 30 × 0.80 = 24 A, capped at 25, so 24 A. Two 20 A circuits still work. Add a third circuit — six conductors, still 0.80 — and it is unchanged. Add a fourth, and at eight conductors 0.70 gives 21 A, which is the last stop before 20 A circuits stop fitting.
A panel feed through insulation. 3 AWG copper feeder, 100 A, bundled with four other cables through blown insulation at 104 °F. Ten conductors gives 0.50, ambient at 104 °F for a 90 °C conductor is 0.91: 115 × 0.91 × 0.50 = 52 A. The 100 A feeder is now less than half of what it needs. This is why panel feeds get their own raceway.
Where derating actually shows up
Rooftop conduit. Direct sun raises raceway temperature well above air temperature, and code editions have added specific adders for it over the years. Anything on a flat commercial roof deserves the calculation.
Attics above the insulation. Summer temperatures over a roof deck routinely reach 120 to 140 °F, which at 140 °F for a 90 °C conductor is a factor of 0.58.
Panel gutters and nipples. Conductors bundled together for more than 24 inches trigger the adjustment even without a raceway around them.
Retrofit circuits in existing pipe. The commonest one, and the one nobody recalculates. Adding a circuit to a conduit that already has two changes the ampacity of everything in it.
Not underground. Direct-buried and duct-bank runs are usually cooler than 86 °F, and the correction goes the other way — a rare case where derating gives something back.
Run your own numbers on the wire ampacity calculator, which applies both factors and shows the table value beside the corrected one.
Frequently asked questions
Do the two derating factors multiply or do I take the worst one?
They multiply. A hot space with a crowded conduit is the ambient factor times the adjustment factor, so 0.88 × 0.80 is 0.70 and 25 A becomes 17.6. Taking only the larger reduction is the most common derating mistake, and it is always in the unsafe direction.
When does the conductor count start to matter?
Above three current-carrying conductors in the same raceway or cable, or bundled together for more than 24 inches. Two circuits sharing a pipe is four conductors, so it starts sooner than most people expect: 80% at four to six, 70% at seven to nine, 50% at ten to twenty, 45% at twenty-one to thirty.
Does the neutral count as a current-carrying conductor?
The neutral of a normal two-wire circuit does. The neutral of a balanced three-wire multiwire branch circuit does not, under 310.15(E), because it only carries the difference between the two legs. A neutral serving nonlinear loads such as electronic ballasts and switch-mode supplies does count, because harmonic currents add rather than cancel in it.
What is the point of the 90 °C column if I cannot use it?
You can use it, just not for the final answer. 110.14(C) limits the circuit to the terminal rating, normally 75 °C, but the derating arithmetic is allowed to start from the conductor’s own 90 °C rating. On a crowded run that difference is what keeps a conductor legal — you derate from 30 A instead of 25 and are still capped at 25.
Does derating change the breaker?
No. The breaker is sized from the load and limited by the conductor’s ampacity — but the ampacity it is limited by is the derated one. So derating does not shrink the device, it grows the wire. The load has not changed and neither has the protection it needs.
How we calculate this
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