How to use the wire size calculator
Enter the current the circuit will carry, whether it's a continuous load, the system voltage and the one-way distance. The calculator finds the smallest conductor that both carries the current under NEC Table 310.16 and keeps the voltage drop inside your limit, and tells you which of the two decided it.
"Auto" picks the temperature column the way NEC 110.14(C) usually requires: 60 °C for circuits of 100 A or less, 75 °C above that. Choose 75 °C only if the breaker, the device and every lug are marked for it.
How the size is chosen
design current = load × 1.25 if continuous
ampacity = Table 310.16 at the chosen column, capped by 240.4(D) for 14-10 AWG
voltage drop = 2 (or 1.732 for three-phase) × L × I × R ÷ 1,000
result = smallest size with ampacity ≥ design current and drop ≤ limit
Worked example: a 40 A load at 240 V, 100 ft away
- Not continuous, so design current is 40 A; auto uses the 60 °C column.
- 8 AWG copper is rated 40 A at 60 °C, enough for the load.
- Voltage drop on 8 AWG: 2 × 100 × 40 × 0.778 ÷ 1,000 = 6.22 V, which is 2.59%.
- Both checks pass, so the answer is 8 AWG copper.
Stretch the same circuit to 150 ft and the drop on 8 AWG is 3.9%, so the calculator moves up to 6 AWG for voltage drop.
Common copper sizes (Table 310.16 with 240.4(D))
| AWG | 60 °C | 75 °C | 90 °C | Max breaker (240.4(D)) |
|---|---|---|---|---|
| 14 | 15 | 20 | 25 | 15 A |
| 12 | 20 | 25 | 30 | 20 A |
| 10 | 30 | 35 | 40 | 30 A |
| 8 | 40 | 50 | 55 | — |
| 6 | 55 | 65 | 75 | — |
| 4 | 70 | 85 | 95 | — |
| 2 | 95 | 115 | 130 | — |
| 1/0 | 125 | 150 | 170 | — |
Amps, not more than three current-carrying conductors, 30 °C ambient.
When you need a bigger wire than this
- More than three current-carrying conductors in a raceway: ampacity is reduced (80% for 4-6 conductors, 70% for 7-9, and so on, per 310.15(C)(1)).
- Hot locations: attics and rooftops above 30 °C need the correction factors in 310.15(B).
- Specific equipment: motors, air conditioners, EV chargers and dwelling services have their own sizing rules in the NEC. Use the nameplate and the matching article.
- Cable types: NM cable (Romex) is limited to the 60 °C ampacity for its final rating.
Questions people ask
What size wire do I need for 20 amps?
12 AWG copper for a 20 A circuit. NEC 240.4(D) limits 12 AWG copper to a 20 A breaker even though Table 310.16 lists it higher. On long runs voltage drop can push you to 10 AWG or larger; at 120 V, a 20 A load past about 45 ft one way exceeds 3% on 12 AWG.
What size wire for 30 amps?
10 AWG copper. Its 60 °C ampacity is 30 A and 240.4(D) caps its breaker at 30 A. In aluminum, 8 AWG (35 A at 60 °C) is the usual choice.
What size wire for 50 amps?
6 AWG copper at 60 °C (55 A), or 8 AWG copper (50 A) only where the breaker and all terminations are rated 75 °C. 50 A circuits for ranges and EV chargers commonly use 6 AWG copper.
Which temperature column should I use?
NEC 110.14(C) generally limits circuits of 100 A or less, or 14 through 1 AWG conductors, to the 60 °C column unless the equipment is listed for 75 °C. Above 100 A, the 75 °C column is usual. The 90 °C column is used for derating, not for the final ampacity, unless every termination is rated 90 °C.
Does a longer run need thicker wire?
Yes, because of voltage drop. The wire must carry the current safely regardless of length, and then long runs need a larger size so the voltage at the load stays within about 3%.
Sources and assumptions
- NFPA 70 (NEC) Table 310.16 (formerly 310.15(B)(16)): allowable ampacities at 60/75/90 °C, 2011 table pages; 2023 copper values confirmed against HELUKABEL's NEC 2023 ampacity tables
- NEC 240.4(D): 14 AWG Cu 15 A, 12 AWG Cu 20 A, 10 AWG Cu 30 A, 12 AWG Al 15 A, 10 AWG Al 25 A
- NEC Chapter 9 Table 8 resistance for voltage drop
- Continuous loads (3 hours or more) are sized at 125% per NEC 210.19(A) and 215.2(A). Correction for ambient temperature and adjustment for more than three conductors are not applied here.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.