For a standard 30-amp 240V circuit, use 10 AWG copper wire with a 30A double-pole breaker. For a 50-amp 240V circuit, step up to 6 AWG copper on a 50A breaker. These sizes assume standard residential conditions, but exact sizing depends on your specific load, distance, and insulation type.
Baseline Assumptions for This Guide
- Material: Copper (aluminum requires different, larger sizing).
- Temperature Rating: 75°C column (standard for modern breakers and terminals).
- Ambient Temperature: 30°C (86°F).
- Installation: Single circuit, maximum 3 current-carrying conductors in a raceway or standard NM-B cable.
Note: Sizing answers without stated assumptions are dangerous. Always verify your local AHJ requirements, as they supersede general guidance.
The Baseline: Sizing Copper Wire for 240V 30A to 50A Loads
A common misconception is that 240V requires a different wire thickness than 120V. It does not. Wire ampacity is strictly a function of current (Amps) and thermal limits, not voltage. The voltage rating of standard residential wire insulation (typically 600V) safely covers both 120V and 240V applications. The primary difference for 240V is that you are using two ungrounded (hot) conductors and a double-pole breaker, rather than one hot and one neutral.
To size the wire, we look at the ampacity tables in NFPA 70 (NEC) Table 310.16. For standard residential wiring, we use the 75°C column because modern breakers and terminals are rated for 75°C, even if the wire insulation (like THHN) is rated for 90°C. The 90°C column is only used as a starting point for derating calculations.
| AWG Size | Max Ampacity (75°C) | Standard Breaker Size | Common 240V Applications |
|---|---|---|---|
| 12 AWG | 20A | 20A | Baseboard heaters, small window ACs |
| 10 AWG | 30A | 30A | Clothes dryers, small welders, 24A EV chargers |
| 8 AWG | 40A | 40A | Large AC compressors, spa panels |
| 6 AWG | 55A | 50A | Electric ranges, 40A continuous EV chargers |
| 4 AWG | 70A | 70A | Large subpanels, heavy-duty shop equipment |
Voltage Drop: When Distance Forces a Larger Gauge
Ampacity tables tell you what size wire will prevent the insulation from melting. They do not guarantee that your equipment will receive adequate voltage at the end of a long run. The NEC recommends a maximum 3% voltage drop for branch circuits to ensure equipment operates efficiently and motors do not overheat.
Let us run a real-world voltage drop check for a 30A, 240V circuit (like a garage EV charger or welder) located 100 feet from the panel using 10 AWG copper.
The Formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 30A
- D (One-way distance) = 100 ft
- CM (Circular mils for 10 AWG) = 10,380
The Math: VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45V
The Percentage: (7.45V / 240V) × 100 = 3.1%
At 3.1%, we have slightly exceeded the recommended 3% threshold. While 10 AWG is legally safe from a thermal/fire perspective for a 30A breaker, the voltage drop will cause your welder to run hotter or your EV charger to throttle its charging speed. The fix: For any 30A 240V run exceeding 80 feet, bump the wire up to 8 AWG copper to keep the voltage drop under 2.5%.
Decision Tree: What Changes the Required Wire Size?
Why use 10 AWG for 30A and not one size smaller (12 AWG)? Because a 12 AWG wire under a 30A load will exceed its thermal rating, degrading the insulation over time and creating a fire hazard, while the 30A breaker will fail to trip because it is sized above the wire's ampacity. The breaker protects the wire, not the device.
However, the baseline table above changes when you alter the installation environment. Use this decision matrix to adjust your sizing.
| Condition | Effect on Wire | Required Action |
|---|---|---|
| Continuous Load (On for 3+ hours, e.g., EV charger) | Thermal buildup over time | Multiply load by 125% to size the breaker and wire. (e.g., A 40A EV charger requires a 50A breaker and 6 AWG wire). |
| High Ambient Temp (e.g., 110°F attic in summer) | Reduces wire's ability to dissipate heat | Apply NEC Table 310.15(B)(1) temperature correction factors. You may need to jump up one AWG size. |
| Conduit Bundling (4+ current-carrying conductors in one pipe) | Wires heat each other up | Apply NEC Table 310.15(C)(1) adjustment factors (e.g., 4-6 conductors requires derating to 80% of the 90°C column ampacity). |
| Using Aluminum Wire (e.g., SER cable for feeders) | Aluminum has higher resistance than copper | Never use copper tables for aluminum. You must jump up two AWG sizes (e.g., use 2 AWG aluminum instead of 4 AWG copper for 100A). |
When an Engineer or AHJ Must Confirm: If you are sizing service entrance conductors over 200A, dealing with complex parallel runs, or installing feeders in an environment with severe harmonic distortion or extreme ambient temperatures (above 40°C), stop and consult a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ). Standard residential tables do not safely cover these edge cases.
Frequently Asked Questions
What wire gauge for 240v 50 amp?
For a standard 50A non-continuous load (like an electric range or a welder), use 6 AWG copper wire on a 50A double-pole breaker. However, if your 50A load is an EV charger that runs continuously for more than 3 hours, NEC Article 210.20(A) requires you to size the circuit at 125% of the continuous load. A 50A continuous load requires a 62.5A circuit capacity, meaning you must step up to 4 AWG copper and a 70A breaker, or use a 6 AWG wire on a 50A breaker only if the charger's internal software limits the draw to 40A continuous.
Can I use 12 gauge wire for a 240v baseboard heater?
Yes, provided the total amperage of the heater(s) on that circuit does not exceed 20A, and the circuit is protected by a 20A double-pole breaker. Because 240V baseboard heaters do not require a neutral wire, you can use 12/2 NM-B cable. Connect the black wire to one pole of the breaker, the white wire to the other pole (and mark it with black or red electrical tape at both ends to indicate it is a hot conductor, not a neutral), and the bare copper to the ground bus and the heater chassis.
What size wire for 240v 100 amp subpanel?
This is where many DIYers make an expensive mistake by looking at the wrong column in the NEC. If you are running a 100A feeder to a subpanel in a detached garage or an addition, standard Table 310.16 dictates 3 AWG copper or 1 AWG aluminum. However, if this 100A feed is the main service feeder for an entire single-family dwelling, NEC Article 310.12(A) provides a specific allowance: you can use 4 AWG copper or 2 AWG aluminum. Always check if your installation qualifies for the dwelling service feeder rule before buying wire, as the price difference between 2 AWG and 1 AWG aluminum SER cable is significant.
