What ampacity really means, the five factors that change it, and why the safe number is almost always smaller than the table value.
Every wire has an ampacity: the maximum current it can carry safely. But that number moves with insulation type, ambient temperature, how many conductors share a conduit, and where the wire runs. The NEC ampacity tables are the starting point, not the answer. This guide explains what ampacity really means and the five factors that change it.
Safety note: educational guide, not an installation manual. Electrical work must meet NEC and local codes and be done by qualified persons.
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Ampacity is current-carrying capacity: how many amps a conductor can carry continuously without exceeding its insulation’s temperature rating. Exceed it and the insulation degrades — slowly at first, then catastrophically. The famous pairings (14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A) are the derated, code-limited versions for common branch circuits. The raw table values are higher — but you rarely get to use them.
THHN (90°C), THWN (75°C wet), XHHW (90°C) — the insulation’s rating sets the ceiling. But terminations (breaker lugs, receptacles) are usually rated 60°C or 75°C, and the lowest rating in the chain wins. That 90°C THHN still gets used at the 75°C column in most real installations.
Wire in a 40°C (104°F) attic carries less than the same wire in a 20°C basement — heat cannot escape as easily. The NEC provides correction factors: hot environments derate ampacity, sometimes dramatically.
More than three current-carrying conductors in one conduit means mutual heating — the NEC derates: 4–6 conductors to 80%, 7–9 to 70%, and it keeps dropping. This is why commercial conduit runs get upsized fast.
Aluminum needs a larger gauge for the same ampacity (12 AWG copper ≈ 10 AWG aluminum for 20A). Modern AA-8000 series aluminum is a legitimate, code-approved conductor for feeders — the 1970s branch-circuit problems were an alloy and termination issue, long solved.
Continuous loads (3+ hours) are limited to 80% of the conductor’s ampacity on top of the breaker’s 80% rule. EV chargers, lighting, and HVAC live here — size for the long haul, not the moment.
Start with the NEC table, apply temperature correction, apply conduit-fill adjustment, respect the termination rating, apply the 80% rule for continuous loads. Five steps, and the answer is almost always smaller than the headline number — which is exactly how safe installations are built.
The values electricians actually use, after the standard limitations (60/75°C terminations, noncontinuous loads, ≤3 conductors in conduit): 14 AWG → 15A. 12 AWG → 20A. 10 AWG → 30A. 8 AWG → 40A. 6 AWG → 55A. 4 AWG → 70A. 3 AWG → 85A. 2 AWG → 95A. 1 AWG → 110A. 1/0 → 125A. 2/0 → 145A. 3/0 → 165A. 4/0 → 195A. Aluminum runs one to two sizes larger for the same current. Memorize the first four and you can size most residential circuits; past 4 AWG, look it up and check your derating. And remember the rule behind the table: the breaker protects the wire, so these numbers are also breaker maximums — see our breaker sizing guide.
The maximum current a conductor can carry continuously without exceeding its insulation temperature rating. It varies with insulation type, ambient temperature, conduit fill, and termination ratings.
In standard residential branch circuits: 20 amps (copper). The raw NEC table value is higher, but termination ratings and code limits bring it to 20A in practice.
Yes — aluminum needs a larger gauge for the same current. Modern AA-8000 aluminum is code-approved and safe for feeders with proper terminations.
Conductors heat each other. More than three current-carrying conductors in a conduit triggers NEC derating: 80% for 4–6, 70% for 7–9, dropping further.
Typically 6 AWG copper (THHN/THWN) — but as a continuous load it needs the 80% rule applied, and local code plus an electrician should confirm. This is a common reason EV installs get 4 AWG.
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