For a standard 240V 30 amp circuit, use 10 AWG copper wire with a 30A double-pole breaker. This assumes THHN/THWN-2 insulation in conduit at 30°C ambient. If using aluminum, step up to 8 AWG. This baseline applies to typical non-continuous loads like standard workshop equipment or short-run EV chargers.
- Material: Copper (unless aluminum is explicitly specified)
- Temperature Column: 75°C (for THHN in conduit) or 60°C (for NM-B Romex)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
- Load Type: Non-continuous (operates for less than 3 hours at a time)
The Baseline Sizing: Why 10 AWG Copper and Not Smaller?
When determining what size wire for 240v 30 amp applications, the National Electrical Code (NEC) Table 310.16 is the ultimate authority. For 10 AWG copper wire, the ampacity is 30A in the 60°C column and 35A in the 75°C column. Because your breaker and receptacle (like a NEMA 14-30 or 10-30) are typically rated for 75°C terminations, 10 AWG safely handles the 30A load with a 5A buffer under the 75°C column.
Why not use 12 AWG to save money? 12 AWG copper is rated for only 20A (60°C) or 25A (75°C). Pushing 30 amps through 12 AWG wire will cause the insulation to degrade, the voltage to drop significantly, and the breaker to eventually trip due to thermal overload. It is a severe fire hazard.
A common DIY mistake involves mixing up NM-B (Romex) and THHN. Even if you strip THHN wires out of a Romex jacket, NEC 110.14(C) dictates that NM-B cable assemblies must always be sized using the 60°C column, regardless of the 90°C printing on the individual wire jackets. Fortunately, 10 AWG is rated exactly 30A at 60°C, so it remains the correct minimum size for both conduit and Romex installations.
| AWG Size | Copper (60°C) | Copper (75°C) | Aluminum (60°C) | Aluminum (75°C) |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | - | - |
| 12 AWG | 20A | 25A | 15A | 20A |
| 10 AWG | 30A | 35A | 25A | 30A |
| 8 AWG | 40A | 50A | 30A | 40A |
| 6 AWG | 55A | 65A | 40A | 50A |
Source: NFPA 70 National Electrical Code, Table 310.16. Always verify against your local AHJ's adopted code year.
Variables That Force a Larger Wire Size
The 10 AWG baseline assumes perfect conditions. In real-world jobsite scenarios, three primary variables will force you to upsize to 8 AWG copper or 6 AWG aluminum.
1. Voltage Drop Over Distance
The NEC recommends keeping voltage drop under 3% for branch circuits (and 5% combined for feeder + branch). For a 240V circuit, a 3% drop means you cannot lose more than 7.2 volts from the panel to the receptacle.
Let's run the math for a 30A load on 10 AWG copper (Circular Mils = 10,380; K constant for copper = 12.9):
- At 50 feet: Voltage Drop = (2 × 12.9 × 30A × 50ft) / 10,380 = 3.73V (1.5%). This is perfectly acceptable.
- At 100 feet: Voltage Drop = (2 × 12.9 × 30A × 100ft) / 10,380 = 7.45V (3.1%). This exceeds the 3% recommendation.
The Fix: If your one-way wire run from the panel to the 240V receptacle exceeds 90 feet, you must step up to 8 AWG copper to mitigate voltage drop, even though the breaker remains 30A.
2. Conduit Bundling and Derating
If you are pulling multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires ampacity derating when you have 4 to 6 current-carrying conductors in one raceway. The derating factor is 80%.
If you use 10 AWG THHN (rated 35A at 75°C) and apply the 80% derating factor: 35A × 0.80 = 28A. Because 28A is less than your 30A breaker and load, 10 AWG fails the inspection. You must pull 8 AWG copper (50A × 0.80 = 40A) to maintain compliance in a bundled conduit.
3. Aluminum Wire Substitutions
Aluminum is cheaper than copper, but it has higher resistance and expands/contracts more under thermal load. While the 75°C column technically allows 10 AWG aluminum for 30A, the mechanical fragility of 10 AWG aluminum and the strict termination torque requirements make it a poor choice for small branch circuits. Furthermore, many modern 30A breakers and receptacles are not explicitly tested or listed (CU/AL) for 10 AWG aluminum. Always use a minimum of 8 AWG aluminum for a 30A circuit, apply an antioxidant compound (like Noalox) to the stripped ends, and use a calibrated torque screwdriver to terminate the lugs to the manufacturer's exact inch-pound specification.
Decision Tree: When to Upgrade Your AWG or Call an Engineer
Before you buy your wire and pull your permit, run your specific application through this decision framework. If you hit any of the 'Upsize' or 'Consult' conditions, 10 AWG is no longer your answer.
| Condition / Application | Standard 10 AWG Copper? | Required Action & Wire Size |
|---|---|---|
| Standard 240V tool / compressor (Non-continuous) | ✅ YES | Use 10 AWG Copper, 30A Breaker. |
| Run length exceeds 90 feet | ❌ NO | Upsize to 8 AWG Copper for voltage drop. |
| Continuous Load (EV Charger, Heater running 3+ hours) | ❌ NO | NEC 210.20(A) requires 125% sizing. 30A × 1.25 = 37.5A. Use 8 AWG Copper and a 40A Breaker. |
| 4 to 6 current-carrying wires in one conduit | ❌ NO | Apply 80% derating. Upsize to 8 AWG Copper. |
| Ambient temp exceeds 30°C (e.g., hot attic space) | ❌ NO | Apply Table 310.15(B)(1) temperature correction. Usually requires upsizing to 8 AWG Copper. |
| Using Aluminum Wire | ❌ NO | Use 8 AWG Aluminum minimum, apply antioxidant, torque to spec. |
The Continuous Load Trap (EV Chargers)
The most common code violation in 2026 involves home EV chargers. Many homeowners buy a 30A Level 2 charger and wire it with 10 AWG on a 30A breaker. However, EV charging is defined by the NEC as a continuous load because it operates for more than three hours continuously.
Under NEC 210.20(A), branch circuit overcurrent devices for continuous loads must be rated at 125% of the continuous load. If your EV charger pulls a true 30A continuously, 30A × 1.25 = 37.5A. You are legally required to install a 40A breaker and run 8 AWG copper wire. If the EV charger is internally limited to draw a maximum of 24A (which is 80% of 30A), then a 30A breaker and 10 AWG wire are compliant. Always check the equipment nameplate for the 'Maximum Continuous Current' rating, not just the breaker size the manual suggests.
When the AHJ or an Engineer Must Confirm
While the Copper Development Association and NEC tables provide the math, the local Authority Having Jurisdiction (AHJ) has the final say. You must consult your local electrical inspector or a licensed Professional Engineer (PE) if:
- You are routing wire through areas with extreme ambient heat (like above industrial ovens or in unventilated metal roof spaces in high-summer climates).
- You are combining solar inverter backfeed currents with standard utility branch circuits in a shared conduit.
- Your local municipality has amended the NEC to require aluminum-to-copper pigtailing or specific connector types for 240V receptacles.
Sizing wire correctly is not just about preventing a tripped breaker; it is about ensuring the insulation does not slowly cook and fail inside your walls over a decade of use. Stick to 10 AWG copper for standard, short, non-continuous runs, and step up to 8 AWG the moment distance, bundling, or continuous duty enters the equation.
