Enter the load current, ambient temperature, and number of current-carrying conductors — get the minimum AWG per NEC 310.16 (copper THHN 75°C column) with derating applied. Answers the "what size wire" question in one look.
Free PDF: full NEC 310.16 ampacity tables for copper and aluminum (60°C / 75°C / 90°C columns), derating factor lookup for temperature and conduit fill, breaker + wire sizing matrix for common loads (dryer / range / EV charger / heat pump), and the "before you pull wire" 6-check compliance checklist.
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The breaker protects the wire from short-circuit current. It doesn't protect the wire from being slowly cooked because you put 40A of load on a 40A-rated conductor that's actually derated to 32A because it shares conduit with five other circuits in a hot attic.
NEC 310.16 gives base ampacity. NEC 310.15 tells you how to reduce that ampacity based on real installation conditions. Skip the derating and you get insulation degradation, then eventual failure — sometimes 5 years later, sometimes 15, always as a callback to whoever installed it.
| Scenario | Combined factor |
|---|---|
| Indoor 20°C, single circuit in conduit | 1.00 — no derate, use base table |
| Attic 35°C, 3 circuits (6 conductors) | 0.91 × 0.80 = 0.73 — 12 AWG drops from 25A to 18A |
| Rooftop conduit 45°C, 4 circuits (8 conductors) | 0.82 × 0.70 = 0.57 — heavy derate, upsize aggressively |
| Rooftop 50°C+, 5+ circuits in single conduit | Under 0.50 combined — consider separate conduit runs or larger raceway to reduce fill |
Using the 90°C column with 75°C terminations. Most breakers, panels, and receptacles are rated 75°C at their terminals per NEC 110.14(C). Even if you install 90°C-rated THHN wire, you're limited to the 75°C ampacity at those terminals. Common error: sizing 40A circuit with 90°C-rated 8 AWG (rated 55A) and getting failed on inspection because 75°C 8 AWG is 50A — still works for 40A, but the reasoning was wrong.
Forgetting rooftop temperature adder. NEC 310.15(B)(2) requires a temperature adder for conduits exposed to sunlight on rooftops — 33°F (18°C) for conduit within 7/8" of roof surface at black asphalt in the sun. That "hot attic" 40°C becomes 58°C at the conduit. Ampacity drops from 82% to about 65%. Miss this and the wire cooks 3 summers in.
Counting current-carrying conductors wrong. Hot conductors always count. Grounded neutrals in most single-phase circuits count. Equipment grounds do NOT count. Neutrals in balanced 3-phase 4-wire circuits typically don't count UNLESS there's significant non-linear load (LED lighting, VFDs). When in doubt, count the neutral — an over-sized wire never got anyone in trouble; an under-sized one did.
Once the calculator gives you the required AWG, quote that size plus the labor to install it — don't downsize to save wire cost. A #6 vs #8 on a 100 ft run is maybe $60 in materials but saves you from a callback that costs $200 in truck roll and 3 hours of your time. And when insulacija eventually fails on the undersized wire, you're on the hook for the repair regardless of who inspected it.
If the derated ampacity forces you two or three sizes up from the "obvious" answer, that's the value of the calculator — you saw the problem before you priced the job, not after you installed it.
Bid panel upgrades and EV chargers with confidence, sell whole-house surge protection, and quote from a price book that keeps you off the truck at 8 PM writing estimates.