A 1,800 W load on 120 V draws 15 A. By ampacity 14 AWG copper would do, but over a 50 ft run it drops 4.7 V — 3.9 %, more than the 3 % the NEC recommends for a branch circuit. 12 AWG drops 2.97 V (2.5 %), so the circuit is 12 AWG on a 15 A breaker. The calculator above picks the wire and the breaker for any load, voltage and distance.
How the calculator works
Enter the load in watts or amps, the voltage and phase, mark a continuous load, then pick copper or aluminum, the ampacity column and the one-way length. The result is the wire size by ampacity, by voltage drop and the larger of the two, its ampacity, the breaker and the actual drop.
Examples
| Circuit | Wire | Breaker |
|---|---|---|
| 15 A, 120 V, 50 ft, NM cable | 12 AWG | 15 A |
| 20 A, 120 V, 50 ft, NM cable | 10 AWG (drop) | 20 A |
| 30 A, 240 V, 50 ft, NM cable | 10 AWG | 30 A |
| 40 A EV charger, continuous, 75 °C | 8 AWG | 50 A |
| 48 A EV charger, continuous, 75 °C | 6 AWG | 60 A |
| 50 A, 240 V, NM cable | 6 AWG | 50 A |
Ampacity column
NEC Table 310.16 lists three columns, 60, 75 and 90 °C. NM-B (Romex) cable is limited to the 60 °C column; THHN in conduit on 75 °C terminals uses the 75 °C column. Small copper wire has its own limit whatever the column: 14 AWG on 15 A, 12 AWG on 20 A, 10 AWG on 30 A (NEC 240.4(D)).
Continuous loads and voltage drop
A load that runs three hours or more — an EV charger, a water heater, baseboard heat — is sized at 125 %: a 40 A charger needs a 50 A circuit. Long runs often decide the size by voltage drop rather than ampacity: 16 A at 120 V over 150 ft needs 6 AWG to stay under 3 %.
To size the load first, see the AC BTU Calculator, Heating BTU Calculator and Water Heater Size Calculator; lighting loads are in the Lumens Calculator.