Ampacity is not the whole story
Most "what gauge do I need" answers stop at ampacity — look up the amps, read off a wire size, done. That is correct for a short run and incomplete for a long one. A 20A circuit only needs 12 AWG copper to stay under its 25A rating, but 12 AWG run 50 ft one-way at 120V loses 3.2% of the source voltage — over the 3% NEC 210.19(A) recommendation for a branch circuit alone. The honest minimum for that run is 10 AWG, one size up from what ampacity alone would suggest. Length, not just load, decides the wire.
Romex, THHN, and the column that trips people up
NEC Table 310.16 has three ampacity columns — 60°C, 75°C, and 90°C — and which one applies depends on insulation, not just wire size. Standard NM-B sheathed cable (Romex) uses conductors rated 90°C, but NEC 334.80 requires ordinary residential installation to be sized off the 60°C column regardless. That 90°C rating matters for other derating math, not for picking your ampacity here. THWN pulled through conduit is the usual 75°C case; THHN in a dry attic or crawlspace can use 90°C. Guessing the wrong column is the easiest way to under-size a Romex run.
Copper vs. aluminum
Aluminum costs less per foot but needs roughly two AWG sizes more than copper for the same ampacity — 10 AWG copper (35A at 75°C) only gets you to 8 AWG aluminum territory (40A), and NEC does not rate aluminum below 12 AWG at all. Aluminum branch-circuit wiring installed before the mid-1970s has a documented history of connection failures at devices never rated for it; modern practice reserves aluminum mostly for larger feeders and service conductors with connectors rated "CO/ALR" or "AL/CU." If you are working on existing aluminum branch wiring rather than sizing new feeder, stop and call a licensed electrician.
AWG wire size chart — ampacity and resistance at 75°C
The 75°C column, the rating most THWN-in-conduit and general-purpose work uses. Resistance is DC resistance at 75°C from NEC Chapter 9, Table 8 — what the calculator's voltage-drop math runs on regardless of which ampacity column you pick above.
| AWG | Cu ampacity | Al ampacity | Cu Ω/1000ft | Al Ω/1000ft |
|---|---|---|---|---|
| 14 | 20 A | — | 3.07 | — |
| 12 | 25 A | 20 A | 1.93 | 3.18 |
| 10 | 35 A | 30 A | 1.21 | 2 |
| 8 | 50 A | 40 A | 0.764 | 1.26 |
| 6 | 65 A | 50 A | 0.491 | 0.808 |
| 4 | 85 A | 65 A | 0.308 | 0.508 |
| 3 | 100 A | 75 A | 0.245 | 0.403 |
| 2 | 115 A | 90 A | 0.194 | 0.319 |
| 1 | 130 A | 100 A | 0.154 | 0.253 |
| 1/0 | 150 A | 120 A | 0.122 | 0.201 |
| 2/0 | 175 A | 135 A | 0.0967 | 0.159 |
| 3/0 | 200 A | 155 A | 0.0766 | 0.126 |
| 4/0 | 230 A | 180 A | 0.0608 | 0.1 |
Full 60/75/90°C columns for both materials, 14 AWG through 4/0, are in the calculator above and cited below.
What this tool does not do
No derating for more than three current-carrying conductors bundled in one raceway or cable (NEC Table 310.15(C)(1)), no check against a breaker or device's terminal temperature rating even when it is lower than the wire's own rating, and no sizing above 4/0 AWG. The voltage-drop percentages are NEC Informational Notes — recommended, not a national requirement — though a number of states and cities (California and New York City among them) enforce them locally. This tool estimates a single ordinary circuit; it is not a substitute for a licensed electrician on a subpanel, service entrance, or any run with bundled conductors.
For the underlying tables and the rules this tool applies, three guides go deeper than the calculator can: the NEC ampacity chart covers why the 90°C column is rarely the one you may use, wire size by amperage gives the direct answer for 20 through 200 amp circuits including the 83% dwelling service allowance, and the voltage drop formula works the arithmetic by hand.