How to Derate Conductor Ampacity in Arizona Heat

1. Start in the 90°C column — that's what it's for

THHN, THWN-2, and XHHW-2 are 90°C insulations, and 310.15(B) lets you apply every correction factor against the 90°C ampacity even though your terminations are only rated 75°C. That headroom is the whole game: 10 AWG copper is 35 A at 75°C but 40 A at 90°C, and it's the 40 that the correction factors multiply.

The 90°C value is a calculation starting point only — the final answer can never exceed what the termination rating allows. We'll close that loop in step 5. But skip the 90°C start and every derated run comes out a size fatter than it needs to be.

2. Pick an honest ambient for each part of the run

The correction table works in bands, so precision matters less than honesty. Tucson's summer outdoor design temperature sits around 41–45°C (roughly 106–113°F) — use that band for shaded outdoor raceways. Interior conditioned space stays at the 30°C base with no correction. Attics and sunlit surfaces run well past outdoor ambient on a summer afternoon; treat an unventilated attic run as at least the 46–50°C band unless you have measurements saying otherwise.

A single circuit takes the worst ambient it passes through, not an average. Twenty feet of 50°C attic in a 200 ft run derates the whole conductor — heat doesn't care that the rest of the run is comfortable. If a short hot segment is killing the calculation, reroute the segment; that's usually cheaper than upsizing 200 ft of copper.

3. Apply the ambient correction — Table 310.15(B)(1)

Read the factor from the row for your ambient band and the column for your insulation temperature. For 90°C conductors: 36–40°C is 0.91, 41–45°C is 0.87, 46–50°C is 0.82, 51–55°C is 0.76, and it falls off a cliff from there — 61–65°C is 0.65, 71–75°C is 0.50, 76–80°C is 0.41.

Notice how the desert eats margin: the same 10 AWG THHN that carries 40 A in the book carries 40 × 0.87 = 34.8 A in a Tucson summer before you've accounted for anything else.

4. The rooftop rule: 7/8 in. is the line — 310.15(B)(2)

Raceways and cables installed in sunlight on or above a rooftop, with the bottom less than 7/8 in. above the roof surface, must add 33°C (60°F) to the outdoor ambient. That's not a typo. A 43°C afternoon becomes a 76°C design ambient, and the 76–80°C row cuts a 90°C conductor to 41% of its table ampacity — 12 AWG THHN drops from 30 A to about 12 A.

The fix costs almost nothing at install time: get the raceway 7/8 in. or more off the deck on strut or listed stands and the adder disappears entirely — you correct for the true outdoor ambient only. This is why we spec strut-mounted rooftop runs on every Tucson job and why a conduit lying on sleepers cut from scrap is never the bargain it looks like.

5. Count current-carrying conductors and apply Table 310.15(C)(1)

More than three current-carrying conductors in one raceway or bundle can't shed heat like three can, so the table stacks a second factor: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20. The factors multiply with the ambient correction — they don't replace it.

Count carefully before you panic: equipment grounding conductors never count, and a neutral that carries only the unbalanced current of a system doesn't either. But the neutral of a circuit run from a 4-wire wye that serves mostly nonlinear load — LED drivers, VFDs, computers — does count, per 310.15(E). Nine circuits of line-to-neutral LED lighting in one raceway is 18 current-carrying conductors, and that's the 50% row.

6. Close the loop at the terminations — 110.14(C)

The derated 90°C number has to land at a lug, and the lug wins. Circuits 100 A and under (or conductors 14 through 1 AWG) are evaluated at 60°C unless the equipment is listed and marked for 75°C — which nearly all modern commercial gear is; over 100 A, 75°C applies. The conductor's usable ampacity is the lower of the corrected 90°C value and the 75°C table value for that size.

Example: 8 AWG THHN corrected to 44 A is fine on a 75°C termination (8 AWG is 50 A at 75°C), so 44 A stands. But an uncorrected 90°C value can never be used just because the wire survives it — 10 AWG THHN at 40 A into a 75°C lug is still a 35 A conductor on its best day.

7. Worked example: the office home-run bundle

Six 20 A office circuits share a rooftop EMT run on strut (no adder), Tucson summer, 41–45°C band. The neutrals serve line-to-neutral LED and receptacle loads from a 208Y/120V panel — 310.15(E) says count them, so that's 12 current-carrying conductors: the 50% row. 12 AWG THHN: 30 A × 0.87 × 0.50 = 13.05 A. That will not support a 20 A circuit.

Upsizing alone doesn't rescue the single raceway — 10 AWG at the same factors is 40 × 0.87 × 0.50 = 17.4 A, still short. The working fix is splitting into two raceways of three circuits each: 6 current-carrying conductors is the 80% row, and 12 AWG comes back to 30 × 0.87 × 0.80 = 20.9 A — passes, with 240.4(D) still capping the breaker at 20 A. Bundle count, not wire size, is usually the lever.

8. Worked example: the feeder that crossed the roof

A 200 A, 208V feeder in 3/0 THHN copper (225 A at 90°C, 200 A at 75°C) crosses 40 ft of sunlit roof. On strut at 4 in.: 225 × 0.87 = 195.8 A — just under 200, so the feeder needs 4/0 (260 × 0.87 = 226 A) unless the run can be shaded or rerouted. Strapped to the deck under 7/8 in.: ambient becomes 76–80°C and 3/0 collapses to 225 × 0.41 = 92 A — less than half the feeder rating.

Same wire, same roof, three answers depending on installation height and honesty about ambient. Do the math at design time; the change order after the thermal camera finds it is always worse.

Why do electricians start ampacity calculations in the 90°C column?

Because NEC 310.15(B) permits correction and adjustment factors to be applied to the ampacity for the conductor's insulation rating. THHN and XHHW-2 are 90°C insulations, so the corrections multiply a bigger starting number. The final ampacity still can't exceed the termination rating — usually 75°C — but the 90°C start preserves real headroom on derated runs.

What ambient temperature should I use for design in Tucson?

For shaded outdoor raceways in summer, the 41–45°C correction band is the honest choice. Interior conditioned space uses the 30°C base with no correction. Unventilated attics and sunlit rooftop segments run hotter — at least the 46–50°C band, and a sunlit rooftop raceway under 7/8 in. above the deck adds a further 33°C by rule.

What is the 7/8-inch rooftop rule?

NEC 310.15(B)(2): where raceways or cables are installed in sunlight on or above a rooftop and the bottom is less than 7/8 in. above the roof surface, you must add 33°C (60°F) to the outdoor ambient before applying the correction factor. Mount the raceway 7/8 in. or higher — ordinary strut does it — and only the true outdoor ambient applies.

Do neutrals count as current-carrying conductors?

A neutral that carries only unbalanced current from a multiwire circuit doesn't count. But per NEC 310.15(E), the neutral of a two-wire circuit does, and so does a wye-system neutral where the major portion of the load is nonlinear — LED drivers, VFDs, and computer power supplies. In a modern office building, count them.

Can I still put a 30 A breaker on 10 AWG if the derated ampacity is only 24 A?

No. The overcurrent device has to protect the conductor at its corrected ampacity — 24.4 A in our rooftop example — so the circuit is limited to a 25 A device at most under the next-size-up rule of 240.4(B), and the load has to fit under the corrected ampacity too. Derating changes the conductor's real capacity, not just paperwork.

Does conductor derating apply to short raceway sections?

Mostly yes, with one narrow break: NEC 310.15(C)(1) exempts conductors in raceway nipples not exceeding 24 in. from the bundling adjustment. There is no similar general exemption from ambient correction — a circuit is derated for the worst thermal segment it passes through.

Is XHHW-2 better than THHN for desert work?

Both are 90°C insulations with the same derating math. XHHW-2's thicker cross-linked insulation tolerates wet locations and rough pulls well and holds up in sustained heat, which is why many desert spec writers prefer it for feeders and rooftop runs; THHN/THWN-2 remains the workhorse for branch wiring. Either way, the correction factors — not the insulation brand — decide the size.