Understanding AWG (American Wire Gauge)

The AWG system is counterintuitive: the smaller the number, the larger the wire. A 2 AWG wire is much thicker than a 14 AWG wire. This is because AWG numbers originally referred to the number of drawing dies the wire was pulled through — more draws made thinner wire.

For electrical work, the most common sizes range from 14 AWG (for 15A circuits) up to 4/0 AWG (for large service conductors), then into kcmil (thousands of circular mils) for very large conductors.

Copper Wire Ampacity Table (60°C/75°C/90°C Rating)

NEC Table 310.16 lists ampacity for conductors in conduit, cable, or earth — up to three current-carrying conductors. Most residential and commercial wiring uses the 75°C column for terminations:

  • 14 AWG — 15A (60°C) / 20A (75°C) — typically used for 15A circuits
  • 12 AWG — 20A (60°C) / 25A (75°C) — standard 20A circuits
  • 10 AWG — 30A (60°C) / 35A (75°C) — dryers, A/C units
  • 8 AWG — 40A (60°C) / 50A (75°C) — ranges, EV chargers
  • 6 AWG — 55A (60°C) / 65A (75°C) — subpanels, large appliances
  • 4 AWG — 70A (60°C) / 85A (75°C) — large subpanels
  • 2 AWG — 95A (60°C) / 115A (75°C) — service feeders
  • 1/0 AWG — 125A (60°C) / 150A (75°C) — service entrance
  • 2/0 AWG — 145A (60°C) / 175A (75°C) — 200A service with derating
  • 4/0 AWG — 195A (60°C) / 230A (75°C) — 200A service conductors

Derating: When You Must Go Larger

The ampacity values above assume no more than 3 current-carrying conductors in a conduit and an ambient temperature of 30°C (86°F). When these conditions change, you must derate:

More than 3 conductors in a conduit (NEC 310.15(C)(1)):

  • 4–6 conductors: multiply by 0.80
  • 7–9 conductors: multiply by 0.70
  • 10–20 conductors: multiply by 0.50

High ambient temperature (NEC Table 310.15(B)(1)): Wiring in attics, boiler rooms, or near heat sources requires upward correction factors. For example, wiring at 40°C (104°F) ambient reduces 75°C-rated conductor ampacity by a factor of 0.88.

Voltage Drop and Wire Sizing

Ampacity isn't the only factor — long runs require larger wire to limit voltage drop. The NEC recommends (but doesn't always require) keeping voltage drop to 3% or less on branch circuits and 5% total for feeders plus branch circuits. For a 120V circuit, 3% = 3.6V drop maximum. This often means upsizing from 12 AWG to 10 AWG or larger on long runs.

Common Wire Sizing Mistakes

  • Using 14 AWG on a 20A circuit — only 12 AWG or larger is permitted on 20A breakers
  • Ignoring derating for conduit fill — common in panel feeders with many circuits
  • Forgetting that aluminum wire has lower ampacity than copper and requires anti-oxidant compound at connections
  • Not accounting for voltage drop on long outdoor or underground runs