The short answer
A 40 amp continuous load at 240 V on a 50 ft run needs a 50 amp breaker and 6 AWG copper NM-B cable. The same load in THHN wire in conduit needs 8 AWG. For the classic breaker sizes, use 12 AWG copper on 20 amps, 10 AWG on 30 amps and 6 AWG on 50 amps.
Wire gauge chart
Allowable current for each wire size, from NFPA 70 (NEC) 2023 Table 310.16 as commonly published, with the 240.4(D) limits on small wire applied. The last columns show the largest standard breaker and the usual use.
| AWG | Cu 60/75°C | Al 60/75°C | Breaker | Common use |
|---|---|---|---|---|
| 14 | 15/15 | — | 15 A | Lighting |
| 12 | 20/20 | 15/15 | 20 A | Outlets, kitchen and general circuits |
| 10 | 30/30 | 25/25 | 30 A | Dryer, water heater, window AC |
| 8 | 40/50 | 35/40 | 40 A | Electric range, large AC |
| 6 | 55/65 | 40/50 | 50 A | EV charger, large range, hot tub |
| 4 | 70/85 | 55/65 | 70 A | Big EV charger, subpanel |
| 3 | 85/100 | 65/75 | 80 A | Subpanel |
| 2 | 95/115 | 75/90 | 90 A | Subpanel, small service |
| 1 | 110/130 | 85/100 | 110 A | Subpanel, small service |
| 1/0 | 125/150 | 100/120 | 125 A | Subpanel, service |
| 2/0 | 145/175 | 115/135 | 125 A | Service |
| 3/0 | 165/200 | 130/155 | 150 A | Service |
| 4/0 | 195/230 | 150/180 | 175 A | Service |
Cu is copper, Al is aluminum. The 60°C column is for NM-B cable, the 75°C column for THHN in conduit. Breaker is the largest standard size for copper NM-B.
How it works
The formula
We follow the same steps an electrician does: size the breaker for the load, pick the smallest wire the breaker allows, then check the voltage drop and go up a size if it is too high.
breaker = next standard size from 15 to 200 A at or above the load used
wire = smallest gauge whose rating is at least the breaker
drop (V) = 2 × K × amps × feet ÷ circular mils
drop (%) = drop (V) ÷ volts × 100
Assumptions
- Ampacities are from NFPA 70 (NEC) 2023 Table 310.16 as commonly published, for no more than three current-carrying wires at 86°F (30°C).
- NM-B cable uses the 60°C column. NEC 334.80 limits it there, even though the wires inside are rated 90°C.
- THHN/THWN-2 in conduit uses the 75°C column. That needs breakers and terminals marked 75°C, which most are. If yours are rated only 60°C (some older ones under 100 A), use the NM-B result.
- NEC 240.4(D) limits small wire: copper 14 AWG to 15 A, 12 AWG to 20 A, 10 AWG to 30 A; aluminum 12 AWG to 15 A and 10 AWG to 25 A.
- Aluminum starts at 12 AWG here. Aluminum branch circuits smaller than 8 AWG are unusual, so most homes use copper for them.
- The wire must be rated for the whole breaker. We don't round up to the next breaker size, which the code sometimes allows, so the result is on the safe side.
- Voltage drop is figured on your load amps and a single-phase circuit. At 208 V that means two legs of a 208Y/120 system. K is 12.9 for copper and 21.2 for aluminum, the resistance in ohm-circular mils per foot at about 75°C.
- If the drop is over your limit, we go up one gauge at a time until it isn't. Bigger wire also needs conduit and terminals that fit it.
- Local code and your inspector decide. Heat, bundling and long conduit runs can lower the allowed current.
Worked example: 40 amp continuous load, 150 ft
40 A at 240 V, continuous, 150 ft one way, copper THHN in conduit, 3% limit.
- Load used: 40 × 1.25 = 50 A, so the breaker is 50 A.
- The 75°C column: 8 AWG copper is rated 50 A, enough for the breaker.
- Drop on 8 AWG: 2 × 12.9 × 40 × 150 ÷ 16,510 = 9.4 V, which is 3.9% of 240 V. That is over 3%.
- Drop on 6 AWG: 2 × 12.9 × 40 × 150 ÷ 26,240 = 5.9 V, or 2.5%. That passes.
- Use 6 AWG copper on a 50 A breaker. The wire is one size bigger than the breaker needs, because of the long run.
Too big or too small
- Too small: it overheats. Wire under the load gets hot, which can melt insulation and start a fire. A breaker bigger than the wire won't trip in time to stop it.
- Too small: voltage drop. On a long run the load gets less voltage. Lights dim, motors and compressors run hot, and chargers can fault.
- Too big: costs more. Each step up in gauge costs more per foot, and the thick wire is stiff and hard to fit in boxes and under breaker screws.
- Too big a breaker: a breaker above the wire's rating leaves the wire unprotected. Always size the breaker first, then the wire for the breaker.
- Between sizes? Go up one gauge. It is cheap protection, and it lets you add load later.
Questions people ask
What size wire do I need for a 50 amp breaker?
Use 6 AWG copper NM-B cable (55 A) or 8 AWG copper THHN wire in conduit (50 A). Aluminum needs 6 AWG in conduit. For a long run, go bigger: at 100 ft and 240 V a 50 amp load needs 6 AWG copper to keep the voltage drop under 3%.
What size wire do I need for a 30 amp breaker?
10 AWG copper, in NM-B cable or in conduit. Even though 10 AWG THHN is listed for 35 A at 75°C, the code caps it at 30 A on a breaker (240.4(D)). Aluminum needs 8 AWG.
What size wire do I need for a 20 amp breaker?
12 AWG copper. That is the standard wire for 20 amp kitchen, garage and general outlets. In aluminum, 10 AWG is the smallest that carries 25 A, but aluminum this small is unusual in homes.
Can I use 14 gauge wire on a 20 amp breaker?
No. 14 AWG copper is limited to a 15 amp breaker, whatever the insulation. A 20 amp circuit needs 12 AWG. A bigger wire on a smaller breaker is fine; a smaller wire on a bigger breaker can overheat without tripping.
How far can I run a wire before voltage drop is a problem?
It depends on the wire, the amps and the voltage. A 20 amp load on 12 AWG copper at 120 V stays under 3% drop for about 45 ft one way. Past that, step up to the next gauge. Voltage drop doesn't trip a breaker, but motors run hot and lights dim.
Why does a continuous load need a bigger breaker?
A continuous load runs for 3 hours or more, such as an EV charger or a space heater. The code (NEC 210.19 and 210.20) says the breaker and wire must carry 125% of it, because breakers run warm under a steady load. A 40 amp continuous load needs a 50 amp breaker.