How the size is chosen
- Required ampacity. Continuous loads (running three hours or more) are multiplied by 125% per NEC 210.19(A) and 215.2(A).
- Temperature column. Per 110.14(C), circuits rated 100 A or less, or using 14–1 AWG, are limited to the 60 °C column unless all terminations are listed for 75 °C. Larger circuits use 75 °C.
- Ampacity. The smallest size in Table 310.16 that meets the required ampacity, also respecting the small-conductor limits in 240.4(D): 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A.
- Voltage drop. If that size drops more than your limit at the actual load current, the calculator steps up until it doesn't.
Ampacity reference (Table 310.16, copper)
| Size | 60 °C | 75 °C | OCPD limit |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 15 A |
| 12 AWG | 20 A | 25 A | 20 A |
| 10 AWG | 30 A | 35 A | 30 A |
| 8 AWG | 40 A | 50 A | |
| 6 AWG | 55 A | 65 A | |
| 4 AWG | 70 A | 85 A | |
| 2 AWG | 95 A | 115 A | |
| 1/0 AWG | 125 A | 150 A | |
| 4/0 AWG | 195 A | 230 A | |
| 250 kcmil | 215 A | 255 A | |
| 500 kcmil | 320 A | 380 A |
Worked example
A 40 A continuous load at 240 V, 100 ft away, on copper: 40 × 1.25 = 50 A required. At 100 A or less the 60 °C column applies, where 8 AWG is only 40 A, so 6 AWG (55 A) is the minimum. Voltage drop on 6 AWG at 40 A is 3.9 V, or 1.6%, so ampacity governs.
What this doesn't cover
Ampacity adjustment for more than three current-carrying conductors in a raceway (310.15(C)(1)), ambient temperature correction (310.15(B)), motor circuits (Article 430), and conductors in cable assemblies with their own limits such as NM cable (60 °C). Apply those before final sizing.
Frequently asked questions
- What wire size do I need for a 50 amp circuit?
- With 60 °C terminations, 6 AWG copper (55 A). With terminations listed for 75 °C, 8 AWG copper (50 A) is allowed. Long runs may need a larger size for voltage drop.
- Why does the calculator use 60 °C for small circuits?
- NEC 110.14(C)(1)(a) limits conductors on circuits of 100 A or less to their 60 °C ampacity unless every termination is listed for 75 °C. Most modern breakers are 75 °C rated, so choose 75 °C if you've confirmed that.
- Do I count the neutral as a current-carrying conductor?
- For derating, a neutral carrying only the unbalanced current of a single-phase or 3-phase wye circuit generally isn't counted. Neutrals of 3-phase 4-wire wye circuits feeding mostly nonlinear loads are counted.
Quick answers
- What size wire for 20 amps at 100 feet?
- What size wire for 30 amps at 100 feet?
- What size wire for 40 amps at 100 feet?
- What size wire for 20 amps at 50 feet?
- What size wire for 50 amps at 100 feet?
- What size wire for 30 amps at 50 feet?
- What size wire for 60 amps at 100 feet?
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.