For a standard 30-amp, 220-volt circuit (like a dryer), use 10 AWG copper wire and a 30-amp double-pole breaker. For a 50-amp, 220-volt circuit (like an electric range), use 6 AWG copper and a 50-amp breaker. The answer to what size wire for 220 volt depends strictly on amperage.
- Material: Copper conductors (Aluminum requires different sizing, addressed below).
- 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 in EMT conduit, or standard NM-B cable (with a critical 60°C caveat noted below).
- Conductor Count: Maximum of 3 current-carrying conductors in a single raceway.
The Core Sizing Matrix for 220V/240V Loads
In North American residential split-phase systems, 220V, 230V, and 240V are nominally identical. The voltage dictates the insulation rating (always use 600V-rated wire like THHN or NM-B), but the amperage dictates the physical wire thickness (AWG). Sizing is governed by NEC Table 310.16, which maps wire gauge to allowable ampacity based on insulation temperature limits.
| Common 220V/240V Load | Max Ampacity | Min Copper AWG (75°C) | Breaker Size (2-Pole) |
|---|---|---|---|
| Window AC / Small Welder / Compressor | 20A | 12 AWG | 20A |
| Electric Dryer / Water Heater | 30A | 10 AWG | 30A |
| EV Charger (Level 2) / Hot Tub | 40A / 50A* | 8 AWG / 6 AWG* | 40A / 50A* |
| Electric Range / Large MIG Welder | 50A | 6 AWG | 50A |
| Subpanel Feeder (Small Workshop) | 60A | 6 AWG (or 4 AWG) | 60A |
*Note on EV Chargers: A 40A continuous EV load requires wire and a breaker sized at 125% (50A). See the AHJ section below for continuous load rules.
Why You Can't Downsize the Wire (The Thermal Limit)
A common question on the bench is why we can't use 12 AWG wire on a 30A breaker if the run is "only a few feet." The answer lies in the physics of resistive heating and the mandate of NEC 240.4, which requires conductors to be protected against overcurrent in accordance with their ampacities.
When current flows through copper, it encounters resistance. This generates heat proportional to the square of the current (I²R). A 10 AWG wire has roughly 1.0 ohm of resistance per 1,000 feet, while a 12 AWG wire has about 1.6 ohms. If you pull 30A through 12 AWG wire, the smaller cross-sectional area cannot dissipate the heat fast enough. The PVC insulation (rated for 90°C max, but terminations limited to 75°C) will soften, degrade, and eventually melt, creating a short circuit or fire hazard long before a 30A breaker's thermal-magnetic trip curve reacts.
The breaker protects the wire, not the appliance. If the appliance demands 30A, the wire must be capable of carrying 30A indefinitely without exceeding its thermal rating. Downsizing the wire turns your wall cavity into a toaster.
Variables That Force a Wire Size Upgrade
The matrix above assumes perfect, short-run conditions. In the real world, three variables will force you to buy the next size up.
1. Voltage Drop at Distance
The NEC recommends (but does not strictly mandate for branch circuits) a maximum voltage drop of 3% for branch circuits and 5% total from the utility transformer. At 240V, a 3% drop is 7.2 volts. Let's run the math on a 50A electric range located 100 feet from the panel using 6 AWG copper.
Using the standard voltage drop formula: VD = (2 × L × I × R) / 1000
- L (Length): 100 feet
- I (Current): 50 Amps
- R (Resistance): 0.491 ohms/kFT for 6 AWG copper at 75°C
VD = (2 × 100 × 50 × 0.491) / 1000 = 4.91V
4.91V is a 2.04% drop. At 100 feet, 6 AWG is perfectly adequate. However, if that same range is in a detached garage 200 feet away, the drop doubles to 9.82V (4.09%). You must now upgrade to 4 AWG copper to keep the voltage drop under the 3% threshold, ensuring the range's control boards don't brownout during high-draw heating cycles. You can verify specific runs using the Southwire Voltage Drop Calculator.
2. Bundling and Conduit Derating
If you are pulling multiple 220V circuits through a single EMT conduit, the wires heat each other up. NEC 310.15(C)(1) mandates ampacity derating when you have more than three current-carrying conductors in a raceway.
- 4 to 6 conductors: Derate to 80% of the 90°C column ampacity.
- 7 to 9 conductors: Derate to 70%.
If you pull two 220V circuits (4 hot wires total, grounds do not count) using 10 AWG THHN (90°C rating = 40A), your adjusted ampacity is 40A × 0.80 = 32A. You can still use a 30A breaker. But if you try to run three 220V circuits (6 hot wires) with 10 AWG, the derating (80%) still leaves you at 32A, which is dangerously close to the limit if any load fluctuates. In high-fill conduits, always bump up a wire size.
3. Aluminum vs. Copper
Aluminum and copper are never interchangeable at the same AWG. Aluminum has higher resistance and expands/contracts more under heat. If you are running a 220V subpanel feeder using SER (Service Entrance Rated) aluminum cable to save money, you must use the aluminum column in NEC 310.16.
- To carry 50A in Copper: 6 AWG
- To carry 50A in Aluminum: 4 AWG
Furthermore, aluminum terminations require an anti-oxidant compound (like Noalox) and specific torque settings to prevent high-resistance joints that cause arcing and fires.
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential DIY and contractor scenarios, local Authority Having Jurisdiction (AHJ) inspectors and electrical engineers must sign off on specific edge cases.
Continuous Loads (The 125% Rule): NEC 210.20 defines a continuous load as one expected to run for 3 hours or more. EV chargers, baseboard heaters, and server racks fall into this category. If your EV charger draws a continuous 32A, you cannot use a 35A breaker and 10 AWG wire. You must multiply the load by 1.25 (32 × 1.25 = 40A). You must size the wire for 40A (8 AWG) and use a 40A breaker. If the charger draws 40A continuous, the math demands 50A wire (6 AWG) and a 50A breaker.
High Ambient Environments: If your conduit runs through an attic in the US South where ambient temperatures routinely exceed 104°F (40°C), the 30°C baseline assumption fails. You must apply the temperature correction factors in NEC Table 310.15(B)(1). A 10 AWG THHN wire in a 50°C attic loses nearly 20% of its ampacity, forcing an upgrade to 8 AWG for a 30A circuit.
Utility Interconnects: If your 220V circuit involves a grid-tied solar inverter or a whole-home backup generator transfer switch, the fault current available at the busbar may exceed the standard 10,000 AIC (Ampere Interrupting Capacity) rating of residential breakers. In these cases, an engineer must calculate the available fault current to specify breakers with higher AIC ratings (e.g., 22kA or 65kA) and verify the busbar bracing.
