For a standard 240V residential circuit, the required wire gauge is dictated entirely by the breaker amperage and the insulation temperature rating, not the voltage itself. A 30A 240V circuit (like a standard dryer) requires 10 AWG copper, a 40A circuit (electric range) needs 8 AWG, and a 50A circuit (EV charger or welder) requires 6 AWG. Voltage determines the insulation thickness and the number of conductors (two hots plus ground/neutral), but amperage dictates the physical copper cross-section required to prevent overheating and nuisance tripping.
How to Read the NEC Wire Gauge Amperage Chart
The definitive source for wire ampacity in the United States is NEC Table 310.16 (formerly 310.15(B)(16)). When you look at this table, you will see three distinct temperature columns for copper: 60°C, 75°C, and 90°C. Understanding which column applies to your specific installation is where most DIYers and junior apprentices make critical errors.
Most modern THHN wire is rated for 90°C, but you almost never get to use the 90°C column for final breaker sizing. Under NEC 110.14(C), for circuits 100A or less, you must size the wire based on the 60°C column unless the equipment terminals are explicitly marked for 75°C. While most modern breakers are 75°C rated, many receptacles, disconnects, and hardwired appliance pigtails are only rated for 60°C. Always default to the lowest temperature rating of any component in the circuit loop.
Below is the complete reference table for copper and aluminum conductors based on the 2023/2026 NEC Table 310.16, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
| AWG / kcmil | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 75°C |
|---|---|---|---|---|
| 14 AWG | 15A* | 20A* | 25A | N/A |
| 12 AWG | 20A* | 25A* | 30A | N/A |
| 10 AWG | 30A | 35A | 40A | 30A |
| 8 AWG | 40A | 50A | 55A | 40A |
| 6 AWG | 55A | 65A | 75A | 50A |
| 4 AWG | 70A | 85A | 95A | 65A |
| 3 AWG | 85A | 100A | 110A | 75A |
| 2 AWG | 95A | 115A | 130A | 90A |
| 1 AWG | 110A | 130A | 145A | 100A |
| 1/0 AWG | 125A | 150A | 170A | 120A |
| 2/0 AWG | 145A | 175A | 195A | 135A |
| 3/0 AWG | 165A | 200A | 225A | 155A |
| 4/0 AWG | 195A | 230A | 260A | 180A |
*Note: While 14 AWG and 12 AWG have higher thermal limits at 75°C/90°C, NEC 240.4(D) strictly limits overcurrent protection for these small conductors to 15A and 20A respectively, regardless of the column used.
Quick-Jump Reference for Common 240V Appliances
If you are wiring a specific appliance and need a fast answer without cross-referencing the terminal ratings, use this bookmark-friendly quick-jump list. These assume standard residential NM-B (Romex) cable, which is strictly limited to the 60°C column.
- 20A 240V (Window AC / Baseboard Heat): 12 AWG Copper. (Requires 2-pole 20A breaker, 12/2 NM-B).
- 30A 240V (Dryer / Water Heater): 10 AWG Copper. (Requires 2-pole 30A breaker, 10/2 or 10/3 NM-B depending on neutral requirement).
- 40A 240V (Electric Range / Cooktop): 8 AWG Copper. (Requires 2-pole 40A breaker, 8/3 NM-B with ground).
- 50A 240V (EV Level 2 Charger / Welder / Hot Tub): 6 AWG Copper. (Requires 2-pole 50A breaker. Use 6/2 THHN in conduit for EV chargers, or 6/3 NM-B for ranges/tubs).
- 60A 240V (Subpanel Feed / Heavy Machinery): 4 AWG Copper or 2 AWG Aluminum. (Requires 2-pole 60A breaker. If using aluminum SER cable for a subpanel, ensure lugs are rated 75°C).
Derating Factors and What the Chart Cannot Tell You
The ampacities listed in NEC Table 310.16 are idealized baseline numbers. They assume exactly three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). In the real world, two major factors force you to upsize your wire.
Conductor Bundling and Derating
When you pull multiple circuits through the same conduit, the wires heat each other up. Under NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a single raceway, you must apply an 80% derating factor to the wire's 90°C base ampacity.
Worked Example: You are running two 240V circuits (4 hot wires total, no neutral) through a single 3/4-inch EMT conduit to a detached garage. You want to use 6 AWG THHN for a 50A EV charger. The 90°C base ampacity of 6 AWG is 75A. Applying the 80% derating factor (75A x 0.80) yields 60A. Since 60A is greater than your 50A breaker, 6 AWG is perfectly legal. However, if you tried to use 8 AWG THHN (90°C base 55A), derating drops it to 44A. You could no longer use it on a 40A breaker.
Voltage Drop and Distance
The NEC ampacity chart completely ignores distance. A 6 AWG wire can safely carry 55A for 10 feet or 1,000 feet without melting, but over long distances, the resistance of the copper will cause the voltage to sag. For 240V circuits, the industry standard (and NEC informational note recommendation) is to keep voltage drop under 3% for branch circuits, which equals a 7.2V drop. If your 50A EV charger is 150 feet from the panel, 6 AWG copper will experience roughly a 4.5% drop, starving the charger's internal power supply and potentially causing fault codes. For runs over 100 feet at high amperage, use a voltage drop calculator and upsize your wire by one or two gauges.
Frequently Asked Questions
What size wire do I need for a 30 amp 240 volt circuit?
You need 10 AWG copper wire. Under the 60°C column of NEC Table 310.16, 10 AWG is rated for exactly 30A. This is the standard size for residential electric dryers and point-of-use water heaters. If you are using THHN wire in conduit and your terminals are rated 75°C, 10 AWG is technically rated for 35A, but the breaker size dictates the circuit name, so it remains a 30A circuit protected by a 30A double-pole breaker.
Can I use 8 AWG wire on a 50 amp 240V breaker?
No. 8 AWG copper has a maximum ampacity of 40A in the 60°C column and 50A in the 75°C column. Even if you meet the strict requirements to use the 75°C column, NEC 240.4(B) does not allow you to round up to the next standard breaker size (60A) for 50A. You must use a minimum of 6 AWG copper (rated 55A at 60°C, 65A at 75°C) to safely and legally wire a 50A 240V circuit.
Does 240V require a different wire gauge than 120V for the same amperage?
No. Wire gauge is strictly a function of amperage (current) and thermal limits, not voltage. A 20A 120V circuit and a 20A 240V circuit both require 12 AWG copper wire. The difference is that the 240V circuit requires a double-pole breaker and two hot conductors (often sharing a single neutral if it's a 120/240V multi-wire branch circuit or appliance), whereas the 120V circuit uses a single-pole breaker and one hot conductor.
How do I calculate wire gauge for a 240V mini-split heat pump?
Mini-splits do not use standard breaker rules; they use the Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOP) printed on the outdoor condenser's spec sheet. If the MCA is 18A, you must use wire rated for at least 18A (12 AWG copper is rated 20A). If the MOP is 30A, you can legally protect that 12 AWG wire with a 30A breaker, bypassing the standard NEC 240.4(D) small conductor limits because HVAC equipment falls under the specific exceptions in NEC Article 440. Always size the wire to the MCA, and the breaker to the MOP.
