For a standard 30-amp, 240V circuit, use 10 AWG copper wire on a 30A double-pole breaker. For a 50-amp, 240V circuit, use 6 AWG copper wire on a 50A breaker. These baseline answers assume standard residential conditions, but exact sizing requires verifying your specific load and installation environment.

WARNING: Mains Voltage Hazard. Working inside an electrical panel exposes you to lethal 240V and 120V potentials. Always de-energize the main breaker, use a lockout/tagout device if possible, and verify the bus bars are dead with a properly rated CAT III or CAT IV multimeter before touching any terminals. If you are unsure, hire a licensed electrician.

The Baseline Assumptions for 240V Sizing

Wire sizing is not a one-size-fits-all lookup. The answers provided in this guide rely on a specific set of baseline assumptions. If your project deviates from these, you must adjust your wire gauge accordingly.

  • Conductor Material: Copper (specifically, standard building wire, not aluminum).
  • Temperature Column: 75°C termination ratings (standard for modern breakers and receptacles).
  • Ambient Temperature: 30°C (86°F) or lower.
  • Installation Method: THHN/THWN-2 individual conductors in a conduit, or standard NM-B (Romex) cable.
  • System Type: Single-phase, 120/240V residential split-phase power.
  • Conductor Count: Maximum of three current-carrying conductors in a single raceway (no bundling derating required).

NEC Ampacity Rules: Why Not One Size Smaller?

A common mistake DIYers make is looking at the 90°C column on NEC Table 310.16 and assuming they can use a smaller wire. For example, 8 AWG copper is rated for 50A in the 90°C column. Why can't you use 8 AWG for a 50A range or EV charger?

The answer lies in termination temperature limits and cable type restrictions. NEC 110.14(C) dictates that the ampacity of a wire is limited by the lowest temperature rating of any connected device, terminal, or conductor in the circuit. Most standard residential breakers and NEMA receptacles are rated for 75°C terminations. Therefore, you must use the 75°C column, which limits 8 AWG copper to 50A. However, if you are using NM-B (Romex) cable, NEC 334.80 strictly forces you to use the 60°C column for ampacity, regardless of the fact that the wire's internal insulation is rated for 90°C. In the 60°C column, 8 AWG is only rated for 40A.

The NM-B Trap: If you are running Romex (NM-B) to a 50A NEMA 14-50 receptacle, 8 AWG is a code violation. You must use 6 AWG NM-B (rated 55A at 60°C) or switch to THHN conductors in a conduit to utilize the 75°C column.

Using a wire that is too small causes heat buildup at the breaker and receptacle terminals. Over time, this thermal cycling loosens the mechanical connections, increases resistance, and creates a severe fire hazard. Always size the wire to handle the breaker's maximum continuous trip threshold, not just the nominal load.

Voltage Drop: When Distance Forces an Upsize

Ampacity tables tell you what size wire will prevent a fire, but they do not guarantee your equipment will operate correctly. The Copper Development Association and NEC Informational Note 210.19(A)(1) recommend keeping voltage drop under 3% for branch circuits.

Let us run a voltage drop check for a highly common 240V scenario: a 40A continuous Level 2 EV charger on a 50A breaker, using 6 AWG copper wire. The formula for single-phase voltage drop is:

VD = (2 × K × I × L) / CM

  • K (Copper Resistivity): 12.9
  • I (Current): 40A (continuous load)
  • L (One-way Length): 100 feet
  • CM (Circular Mils for 6 AWG): 26,240

VD = (2 × 12.9 × 40 × 100) / 26,240 = 3.93 Volts.

Dividing 3.93V by 240V yields a 1.6% drop, which is well within the 3% limit. However, if that same EV charger is located 200 feet from the panel, the drop doubles to 7.86V (3.27%). At this distance, you must upsize to 4 AWG copper to maintain safe operating voltage and prevent the charger from faulting out during peak draw.

Voltage Drop Thresholds for 240V Circuits (Copper, 40A Load)
Wire Gauge (AWG) Max Distance for <3% Drop Voltage Drop at 100 ft Voltage Drop at 150 ft
8 AWG 63 feet 4.7% (Fail) 7.1% (Fail)
6 AWG 183 feet 1.6% (Pass) 2.4% (Pass)
4 AWG 290 feet 1.0% (Pass) 1.5% (Pass)

Decision Matrix: Picking Your Exact Wire and Breaker

Use this decision path to lock in your exact materials list. Follow the logic sequentially based on your specific appliance and installation method.

Appliance / Load Type Max Load (Amps) Required Breaker Wire (THHN in Conduit) Wire (NM-B Romex) Receptacle
Window AC / Small Tool 16A - 20A 20A Double-Pole 12 AWG 12 AWG NEMA 6-20
Electric Dryer 24A - 30A 30A Double-Pole 10 AWG 10 AWG NEMA 14-30
EV Charger / Range 32A - 40A 50A Double-Pole 8 AWG 6 AWG NEMA 14-50
Welder / HVAC Condenser 45A - 55A 60A Double-Pole 6 AWG 4 AWG Hardwired / Lug

The Continuous Load Rule: If your 240V load will run for 3 hours or more continuously (like an EV charger or a kiln), NEC 210.20(A) requires you to multiply the continuous load by 1.25 to size the breaker and wire. A 40A EV charger requires a 50A breaker (40 × 1.25 = 50). Never run a continuous 40A load on a 40A breaker; it will eventually nuisance-trip as the bimetallic strip heats up.

Variables That Change the Math (And When to Call the AHJ)

The copper-in-conduit baseline covers 80% of residential projects, but three major variables will force you to change your wire gauge:

  1. Aluminum Conductors: Aluminum has higher resistance and different thermal expansion properties than copper. If you are using AA-8000 series aluminum wire (common for large subpanel feeders due to cost), you must upsize. A 50A circuit requires 4 AWG aluminum, not 6 AWG. Furthermore, you must use an antioxidant compound (like Noalox) on aluminum terminations and torque them precisely to the manufacturer's inch-pound specifications to prevent arcing.
  2. Conduit Bundling (Derating): If you pull more than three current-carrying conductors through a single conduit, the trapped heat requires you to derate the wire's ampacity per NEC 310.15(C)(1). For 4-6 conductors, you must multiply the 90°C ampacity by 80%. If you are running two 240V circuits (four hot wires) in one EMT conduit, 10 AWG THHN derates to 28A, forcing you to upsize to 8 AWG to legally protect a 30A circuit.
  3. Ambient Temperature: If your conduit runs through an attic that reaches 120°F (49°C) in the summer, you must apply the temperature correction factors in Table 310.15(B)(1). At 114°F to 122°F, you must multiply the base ampacity by 0.82.

When to Call an Engineer or the AHJ: You must involve your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if you are sizing service entrance conductors over 200A, dealing with complex multi-stage derating (high ambient heat + bundling), or if your local municipality has amended the NEC to require AFCI/GFCI protection on 240V receptacles in specific zones. Local code always supersedes general guidance.

Final Recommendation: If you are wiring a standard 50A 240V outlet (NEMA 14-50) for an EV charger or welder in a garage, and the run is under 100 feet, buy 6 AWG copper THHN/THWN-2 (black, red, white, green) and a 50A double-pole breaker. Do not attempt to save money by using 8 AWG NM-B; the 60°C column restriction makes it a code violation and a thermal liability.