Conductor sizing
Find the wire gauge a load actually needs. We size it two ways — NEC 310.16 ampacity and voltage drop — then recommend whichever is larger, for copper or aluminum, single- or three-phase.
Recommended wire size
—Enter a load, length, and voltage to size the conductor.
| Ampacity needs | — |
|---|---|
| Voltage drop needs | — |
| Recommended (larger of the two) | — |
| Ampacity of recommended | — |
| Voltage drop of recommended | — |
Wire sizing answers two questions at once. Ampacity asks whether the conductor can carry the current without overheating — that comes from NEC Table 310.16 at your terminal temperature rating. Voltage drop asks whether the wire is big enough that the load still gets usable voltage over the distance — that uses NEC Chapter 9 Table 8 resistance and the common 3% target. The right size is the larger of the two, which is exactly what this tool reports.
On short runs, ampacity usually governs: a 50 A load needs at least 8 AWG copper at 75 °C, full stop. On long runs, voltage drop takes over — that same 50 A over 100 ft at 240 V wants 6 AWG to stay under 3%, even though 8 AWG could carry it. Picking the larger size satisfies both. The calculator labels which constraint is driving the recommendation so you can see why.
Ampacity per NEC 310.16; voltage drop per NEC Ch.9 Table 8. Default 75 °C terminations.
You size for two things and take the larger. First, ampacity: the conductor must carry the load current per NEC Table 310.16 at your terminal temperature rating (usually 75 °C). Second, voltage drop: on longer runs the wire often has to be bigger than ampacity alone requires to keep drop within about 3%. The recommended size is whichever of the two is thicker.
Use the column that matches your equipment terminations. Most breakers and lugs are rated 75 °C, so 75 °C is the common default. The 60 °C column applies to some terminations and small conductors, and the 90 °C column is generally used only as a starting point for derating, not for the final ampacity at the terminals. When in doubt, size to 75 °C and confirm the terminal rating.
Yes. NEC 240.4(D) caps the overcurrent device for small conductors regardless of the higher figures in the ampacity table: 15 A for 14 AWG copper, 20 A for 12 AWG copper, and 30 A for 10 AWG copper. The calculator honours these caps so it never tells you a 12 AWG wire is good for 25 A.
Aluminum has lower ampacity and higher resistance than copper of the same size, so an aluminum conductor is usually one or two AWG sizes larger than copper for the same load and run. Switch the material in the calculator to compare them directly. Use connectors and antioxidant rated for aluminum (AL or CU-AL).
It estimates ampacity from NEC 310.16 (30 °C ambient, three or fewer current-carrying conductors) and voltage drop from NEC Chapter 9 Table 8. It does not apply ambient-temperature or conductor-count derating, the 125% continuous-load factor, conduit fill, or full terminal-rating logic, and it is not a substitute for a licensed electrician or your local code (AHJ).
Estimate only, based on NEC 310.16 ampacity and NEC Chapter 9 Table 8 voltage drop. This is not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal ratings. Always verify against the current NEC and local code before wiring. Based on NEC 310.16 and NEC Ch.9 Table 8.