Key takeaways
- Wire is sized by two limits — ampacity (the current it can safely carry) and voltage drop over the run.
- Find both gauges and use the larger; long runs are usually driven by voltage drop, not ampacity.
- Aluminum needs about two AWG steps larger than copper for the same job (K = 21.2 vs 12.9).
- The NEC 240.4(D) small-conductor rule caps #14 at 15 A, #12 at 20 A, and #10 at 30 A.
How wire is sized
Wire is sized by two limits — ampacity, the current it can safely carry, and voltage drop over the run length. The wire must satisfy both, so you find the gauge each limit demands and use whichever is larger.
On short runs ampacity usually wins; on long runs voltage drop forces a bigger wire. This tool picks whichever is larger.
Worked example: 20 A, 120 V, 100 ft of copper
Take the page defaults — a 20 A load at 120 V on a 100 ft copper run, held to 3% drop. Ampacity alone is easy: a 20 A circuit needs #12 copper (the small-conductor rule caps #12 at exactly 20 A). But run the voltage-drop formula: (2 × 12.9 × 20 × 100) ÷ (120 × 0.03) = 51,600 ÷ 3.6 = 14,333 circular mils. The smallest gauge that big is #8 copper (16,510 cmil). On a run this long voltage drop, not ampacity, sets the wire — you'd pull #8 copper, two sizes up from the #12 ampacity alone would call for.
AWG ampacity and the 240.4(D) limits
Copper and aluminum ampacities at 75°C (NEC Table 310.16), with the 240.4(D) small-conductor overcurrent limits shown where they apply:
| AWG | Copper (75°C) | Aluminum (75°C) | 240.4(D) max breaker |
|---|---|---|---|
| #14 | 20 A | — | 15 A (Cu) |
| #12 | 25 A | 20 A | 20 A (Cu) |
| #10 | 35 A | 30 A | 30 A (Cu) |
| #8 | 50 A | 40 A | — |
| #6 | 65 A | 50 A | — |
| #4 | 85 A | 65 A | — |
| #2 | 115 A | 90 A | — |
| #1/0 | 150 A | — | — |
| #4/0 | 230 A | 180 A | — |
The small-conductor rule
NEC 240.4(D) overrides the table for the smallest wires: #14 is limited to 15 A, #12 to 20 A, and #10 to 30 A regardless of their higher table ampacity. That's why a 20 A circuit needs #12, not #14 — even though #14 copper shows 20 A in the table, its breaker may not exceed 15 A. The calculator applies these caps automatically. See the full list of calculators.
Frequently asked questions
What size wire do I need?
The larger of the ampacity-rated gauge and the voltage-drop gauge for your run length. Long runs are usually driven by voltage drop.
How is voltage drop figured?
Required circular mils = (2 × K × amps × distance) ÷ (volts × VD%), with K = 12.9 for copper and 21.2 for aluminum; match to the next AWG up.
What size wire for a 50 amp subpanel 100 ft away?
At 240 V over 100 ft, ampacity needs #8 copper, but 3% voltage drop needs about 17,900 circular mils — step up to #6 copper (or #4 aluminum).
What wire for a 100 ft 30 amp run?
At 240 V, #10 copper (capped at 30 A by 240.4(D)) is borderline on voltage drop, so #8 copper is safer. At 120 V the drop doubles — use #8.
Why does a 20 A circuit need #12, not #14?
NEC 240.4(D) caps #14 copper at 15 A regardless of its 20 A table ampacity, so a 20 A breaker requires at least #12.
Can I use aluminum instead of copper?
Yes, but size up about two AWG steps (K = 21.2 vs 12.9) and use terminations and connectors rated for aluminum.
Ampacities are from NEC (NFPA 70) Table 310.16 at 75°C, with the 240.4(D) small-conductor rule applied to #14, #12, and #10.
Last reviewed June 2026