That gap is why swapping a 30-amp dryer outlet for a 50-amp range without touching the cable goes wrong: volts don’t set the gauge. The load in amps, the conductor’s temperature rating, and the wiring method do — and together they decide what size wire for 240V circuits require.
What follows is the sizing order, the copper ampacity numbers from NEC Table 310.16, and the specific answers for 30-, 40-, and 50-amp runs.
Wire Size For 240V: The Numbers That Decide It
Wire gauge follows amps, not volts. You size the conductor to the current it has to carry, read from the temperature column that matches your wire type and terminations, and the voltage only has to match what the load needs.
The order that works:
- Find the load’s amps. A 240V label on a nameplate says nothing about gauge by itself — amps are the input.
- Match the temperature column. Wire, breakers, and receptacles carry 60°C, 75°C, or 90°C ratings, and you read ampacity from the lowest one in the chain.
- Multiply by 125% for continuous loads. Anything running three hours or more gets sized at 125% — a 40-amp continuous load needs a conductor good for 50 amps.
- Pick the gauge that clears it. Choose the smallest copper size whose ampacity in your column meets or beats the required current.
- Check the wiring method. NM-B (Romex) is capped at its 60°C ampacity, so it needs a larger gauge than THHN in conduit carrying the same load.
Step two is where most DIY runs go sideways. Reading a number out of the 90°C column and landing it on a 75°C breaker terminal leaves the circuit undersized while the chart still looks fine — and those table values assume a moderate ambient temperature and a limited number of current-carrying conductors in the raceway. A hot attic or a stuffed conduit can blow past both.
If you’d rather buy cable than calculate it, our tested picks for 240V wire cover the gauges and jacket types worth ordering for 30-, 40-, and 50-amp runs.
Copper Ampacity At A Glance
Copper ampacity climbs with wire size and with the temperature rating of the insulation and terminations. Two columns cover nearly every home 240V run: 60°C, the ceiling for NM-B, and 75°C, which is what most breakers and receptacles are rated for.
| Copper Gauge | Ampacity At 60°C / 75°C | Common 240V Use |
|---|---|---|
| 14 AWG | 15A / 20A | Control circuits only |
| 12 AWG | 20A / 25A | 20A baseboard heater |
| 10 AWG | 30A / 35A | 30A dryer or compressor |
| 8 AWG | 40A / 50A | 40A range, 40A EV charger |
| 6 AWG | 55A / 65A | 50A range or EV charger |
| 4 AWG | 70A / 85A | 60A subpanel feed |
| 2 AWG | 95A / 115A | 100A subpanel feed |
| 1/0 AWG | 125A / 150A | 125A feeder or service |
Those figures are the NEC Table 310.16 ampacity values, and the column you read from isn’t a preference — it’s set by the weakest link among the wire, the terminations, and the installation. NM-B stops at 60°C, so 6 AWG cable tops out at 55 amps even though the same copper in THHN rates 65 amps at 75°C.
Aluminum runs lower at every size — 6 AWG aluminum is listed at 40 amps at 60°C and 50 amps at 75°C in the same table — so an aluminum conductor usually steps up a size or two, and the breaker and receptacle both have to be marked for aluminum.
What Size Wire For A 50-Amp 240V Circuit?
NM-B needs the larger gauge because its ampacity stops at the 60°C column, where 6 AWG is rated 55 amps, while 8 AWG reads exactly 50 amps in the 75°C column.
The same logic answers the other common sizes:
- 30 amps: 10 AWG copper either way — 30 amps at 60°C, 35 amps at 75°C.
Voltage doesn’t move any of these numbers. A 120V circuit at the same amps takes the same gauge, and going to 240V doesn’t buy smaller wire — it halves the current for the same wattage.
One honest limit: these are table summaries, not the NEC text itself, and ambient temperature, conduit fill, and local amendments can each push the required gauge up. A licensed electrician’s sign-off is the cheap part of a 240V run.
Working order for the job:
- Write down the load’s amp rating, not its voltage.
- Add 25% if it runs three hours or longer.
- Note the wire type and read only its temperature column.
- Take the smallest copper gauge that clears that number.
- Confirm the breaker and receptacle match the wire’s material and temperature rating.
FAQs
Is 12-Gauge Wire Ever Acceptable On A 240V Circuit?
Yes, if the circuit is 20 amps. NEC Table 310.16 lists 12 AWG copper at 20 amps in the 60°C column and 25 amps at 75°C, so a 20-amp 240V baseboard heater or a small 240V tool circuit can run on it. Anything drawing more than 20 amps needs a larger gauge — voltage doesn’t change the limit.
Do Aluminum Conductors Need A Different Gauge Than Copper?
Aluminum ampacity is lower than copper at the same size. 6 AWG aluminum is listed at 40 amps in the 60°C column and 50 amps at 75°C, where 6 AWG copper reads 55 and 65 amps. An aluminum run usually steps up a size or two, and the breaker and receptacle must be rated for aluminum.
Why Does Romex Need A Bigger Gauge Than THHN In Conduit?
Because NM-B is limited to its 60°C ampacity, while THHN in conduit can be read from the 75°C column. The terminations still have to be rated for the higher column.
References & Sources
- Thomas & Betts. “NEC Table 310.16 — Ampacities of Insulated Conductors.” Source of the copper and aluminum ampacity values by temperature column.
- EcoFlow. “Wire Gauge Amp Chart: NEC Ampacity Table for Copper and Aluminum.” Cross-checks the ampacity figures and the copper-versus-aluminum sizing gap.