NEC Article 310 — Conductor Ampacity, Temperature Ratings, and the 75 °C / 90 °C Rule

NEC Article 310 defines conductor ampacity — how much current a wire can safely carry. The most-referenced piece is Table 310.16, the ampacity table.

Common copper conductor ampacities at 75 °C (the limit for most residential terminations):

• 14 AWG: 20 A (but limited to 15 A by 240.4(D))
• 12 AWG: 25 A (but limited to 20 A by 240.4(D))
• 10 AWG: 35 A (but limited to 30 A by 240.4(D))
• 8 AWG: 50 A
• 6 AWG: 65 A
• 4 AWG: 85 A
• 3 AWG: 100 A
• 2 AWG: 115 A
• 1 AWG: 130 A
• 1/0 AWG: 150 A
• 2/0 AWG: 175 A
• 3/0 AWG: 200 A
• 4/0 AWG: 230 A

The temperature-rating rule (110.14(C)): the conductor's allowable ampacity is the LOWEST of:

1. The conductor's insulation temperature rating
2. The connected equipment's termination temperature rating

In residential, almost ALL terminations (breakers, panel lugs, receptacles) are rated 75 °C. So even though THHN insulation is rated 90 °C with a higher ampacity column, you're still limited to the 75 °C value because of the terminations. This is THE most-misapplied rule in residential.

Common conductor types in NJ residential:

• NM-B (Romex) — 90 °C insulation but limited to 60 °C ampacity per 334.80. Used inside dry living spaces only.
• THHN/THWN-2 — 90 °C dry / 75 °C wet. Used in conduit. Standard for service entrance, sub-panel feeders, EV charger runs.
• XHHW — 90 °C dry / 75 °C wet. Heavier insulation, sometimes used on service entrance.

Derating for ambient temperature (310.15(B)): ampacity drops in hot environments. A garage attic that hits 130 °F in summer drops conductor ampacity by ~20%. NJ inspectors don't generally enforce derating on residential branch circuits, but commercial work in attics or rooftops absolutely needs it calculated.

For voltage drop on long runs (which often drives upsizing well above the ampacity-required size), use the free Malfettone Voltage Drop Calculator at /tools/voltage-drop.