Wire size is one of those decisions where the cheap mistake and the expensive mistake look identical when the drywall goes back up. Undersized conductors run hot under load, degrade insulation, and end careers when an inspector pulls a panel cover. Oversized conductors waste copper budget and jam connector lugs. NEC Table 310.16 settles the argument.

The 75°C column is the answer for almost everyone

NEC Table 310.16 lists three temperature columns: 60°C, 75°C, and 90°C. The wire's insulation is rated higher (90°C for THHN, the most common building wire), but the termination at the breaker, lug, or device caps the usable ampacity at 75°C for any equipment rated for over 100 A and effectively for residential and commercial work generally.

Practical effect:

  • 14 AWG copper — 15 A, only on a 15 A breaker
  • 12 AWG copper — 20 A
  • 10 AWG copper — 30 A
  • 8 AWG copper — 50 A (75°C)
  • 6 AWG copper — 65 A
  • 4 AWG copper — 85 A
  • 2 AWG copper — 115 A
  • 1/0 AWG copper — 150 A
  • 2/0 AWG copper — 175 A
  • 3/0 AWG copper — 200 A

Aluminum is one to two sizes larger for equivalent ampacity. That is the whole story for normal terminations.

The 125% continuous-load multiplier

NEC 210.19(A)(1) and 215.2 say a circuit feeding a continuous load (anything operating 3+ hours: EV charger, electric heat, sign lighting, server room) must size both conductor and breaker at 125% of the continuous load. A 32 A continuous EV charger draws 32 A, but the breaker and wire size from a 40 A target — 8 AWG copper, 40 A breaker.

Skip this and a load that is at code on paper still trips under sustained draw, because thermal protection is calibrated for that 80% rule. Apply the 1.25 multiplier before entering amps in the wire gauge calculator.

Voltage drop is a separate test

Ampacity says the wire won't melt. Voltage drop says the load gets enough volts to actually work. NEC 210.19(A) IN 4 caps branch-circuit drop at 3% — typical breakpoint where LED lights flicker, motors hum, and electronics glitch.

Long runs blow past the ampacity sizing first. Examples:

  • 20 A on 12 AWG, 120 V, 100 ft. 4.0% drop — fails. Bump to 10 AWG or move to 240 V.
  • 40 A EV charger, 240 V, 75 ft. 8 AWG passes ampacity but drops 1.7% — fine.
  • 30 A subpanel, 240 V, 200 ft. 10 AWG is 2.6% — passes, but tight. 8 AWG is the safer call.

The calculator runs the drop check and bumps a size if it exceeds 3%.

Aluminum vs copper

Aluminum has 60% the conductivity of copper. The Al column in NEC 310.16 reflects that — a 100 A copper feeder is 3 AWG copper or 1 AWG aluminum, a 200 A copper feeder is 2/0 copper or 4/0 aluminum. Aluminum is materially cheaper at large gauges (service entrance, subpanel feeders), so the service drop into your house is almost always aluminum even though the branch circuits are copper.

Aluminum needs antioxidant compound (Noalox) at every termination and ratcheted lugs torqued to spec. Don't mix aluminum and copper at a lug — use AL/CU rated devices or a copper pigtail.

Common wire-size mistakes

Sizing for breaker, not for load. The breaker is the maximum, not the design point. A 20 A breaker with 8 A of LED lighting doesn't need 12 AWG up to that breaker rating — it needs 14 AWG and a 15 A breaker.

Forgetting derating in conduit. NEC 310.15(C)(1): more than 3 current-carrying conductors in a raceway derates ampacity. 4–6 conductors → 80%; 7–9 → 70%; 10–20 → 50%. Bundling many circuits in one EMT is the silent reason a properly-sized wire still cooks.

Ignoring temperature. Conductors run through an attic in Phoenix derate 12-15% for ambient. The 75°C column assumes 30°C ambient.

Estimate only. The wire gauge calculator uses NEC 310.16 75°C ampacity and a 3% voltage-drop check. Conduit bundling, ambient temperature, motor-circuit rules (NEC 430), and termination-rating exceptions are not modeled. Verify final wire size with a licensed electrician and the local AHJ before installation.