Data reviewed: 20 October 2026 · Engineering Desk Tools · How we check calculations
How to size wire for amps (NEC method)
- Find the load current. Single-phase: I = W ÷ (V × PF). Three-phase: I = W ÷ (1.732 × V × PF).
- Add 25 % for continuous loads (running three hours or more), such as EV chargers, heaters and lighting in stores.
- Pick the ampacity column. Start from the wire insulation column (60, 75 or 90 °C) and apply the temperature correction factor and the bundling factor for more than three current-carrying conductors.
- Check the terminals. The result may not exceed the ampacity in the column of the lowest-rated termination, usually 60 °C or 75 °C.
- Apply the small-conductor limits of 240.4(D): 14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A.
- Check voltage drop and go up a size if the run is long.
What size wire for 15, 20, 30, 40, 50, 60 and 100 amps
The 60 °C column applies only up to 100 A (NEC 110.14(C)). Continuous loads, hot locations, bundled cables and long runs need a larger wire; use the calculator above. Dwelling services and feeders may use the smaller sizes allowed by 310.12.
Voltage drop formula for wire size
Single-phase: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM
K is 12.9 Ω·cmil/ft for copper and 21.2 for aluminum, L the one-way length in feet and CM the conductor area in circular mils (12 AWG = 6,530; 10 AWG = 10,380; 6 AWG = 26,240; 1/0 = 105,600). Example: 20 A at 120 V over 50 ft of 12 AWG copper gives 2 × 12.9 × 20 × 50 ÷ 6,530 ≈ 3.95 V, or 3.3 %. That is above 3 %, so the calculator moves up to 10 AWG (2.1 %). For metric cables use the voltage drop calculator or the cable size calculator (mm²).
Wire ampacity chart (NEC Table 310.16)
Source: NFPA 70 (NEC) Table 310.16, ampacities of insulated conductors rated up to 2000 V. 60 °C: TW, UF (and NM-B cable ampacity); 75 °C: THW, THWN, XHHW, USE; 90 °C: THHN, THWN-2, XHHW-2. 14–10 AWG are limited by 240.4(D).
Temperature and bundling correction
Table 310.16 assumes 86 °F (30 °C) and no more than three current-carrying conductors. Above 86 °F multiply by the correction factor, for example 0.91 for 90 °C wire at 96–104 °F, and 0.82 at 114–122 °F. With 4–6 current-carrying conductors in one raceway or cable use 80 %, with 7–9 use 70 % and with 10–20 use 50 %. The calculator applies both factors automatically.
Wire size questions
What size wire for 100 amp service?
For a 120/240 V single-phase dwelling service, NEC 310.12 lets you size the service conductors for 83 % of the rating: 4 AWG copper or 2 AWG aluminum for 100 A. For a 100 A feeder or circuit that is not a dwelling service, use Table 310.16 directly: 3 AWG copper or 1 AWG aluminum at 75 °C.
What size wire for 200 amp service?
Under NEC 310.12 a 200 A dwelling service needs 2/0 AWG copper or 4/0 AWG aluminum. Other 200 A feeders use Table 310.16 at 75 °C: 3/0 AWG copper or 250 kcmil aluminum.
What gauge wire for 30 amps?
10 AWG copper (or 8 AWG aluminum) for a 30 A circuit. 10 AWG copper is limited to a 30 A breaker by NEC 240.4(D). On long runs, check voltage drop and go up to 8 AWG if needed.
What size wire for 50 amps?
At 60 °C terminals, 6 AWG copper (55 A); at 75 °C terminals, 8 AWG copper (50 A). Aluminum needs 4 AWG at 60 °C or 6 AWG at 75 °C. Many 50 A range and EV circuits use 6 AWG copper.
What gauge wire for 20 amps?
12 AWG copper or 10 AWG aluminum. 12 AWG copper is limited to a 20 A breaker by NEC 240.4(D).
What is the difference between the 60 °C, 75 °C and 90 °C columns?
They are the insulation temperature ratings. You may start derating from the wire’s own column (for example 90 °C for THHN), but the final ampacity cannot exceed the column of the lowest-rated termination. Equipment rated 100 A or less often has 60 °C terminals unless marked 75 °C.
How much voltage drop is allowed?
The NEC informational notes recommend no more than 3 % on a branch circuit and 5 % total for feeder plus branch circuit. It is a recommendation for efficiency, not a mandatory rule, unless your local code or specification makes it one.