You need 10 AWG copper wire for a 30 amp breaker in standard residential applications. This assumes THHN/THWN-2 insulation, 75°C terminations, a maximum of three current-carrying conductors in the raceway, and a 30°C (86°F) ambient temperature. Never use 12 AWG or 14 AWG; they will overheat and melt before a 30A breaker trips.
- Material: Solid or stranded copper (aluminum requires different sizing, covered below).
- Temperature Column: 75°C for terminations (standard for modern breakers and receptacles); 60°C if using NM-B (Romex) cable.
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: 3 or fewer current-carrying conductors in the raceway or cable.
The Baseline: 10 AWG Copper for 30 Amps
When wiring a NEMA 14-30 receptacle for a dryer, a TT-30R for an RV, or a 30A subpanel feeder, 10 AWG copper is your baseline. However, the physical wire you pull dictates how you apply the National Electrical Code (NEC) ampacity tables.
If you are pulling individual THHN/THWN-2 conductors through EMT or PVC conduit, the wire itself is rated in the 90°C column of NEC Table 310.16. In the 90°C column, 10 AWG copper is rated for 40 amps. However, because the lugs on your breaker and receptacle are almost certainly rated for 75°C, you must use the 75°C column for your final ampacity check. In the 75°C column, 10 AWG is rated for 35 amps. Since 35A is greater than your 30A load and breaker size, 10 AWG passes.
If you are using NM-B (Romex) cable, NEC 334.80 strictly limits the ampacity to the 60°C column, regardless of the fact that the individual wires inside the sheath might have 90°C insulation. In the 60°C column, 10 AWG copper is rated for exactly 30 amps. This perfectly matches a 30A breaker, meaning NM-B 10/2 or 10/3 is also fully code-compliant for this circuit.
30 Amp Wire Sizing Matrix & Ampacity Data
Wire sizing is not a one-size-fits-all lookup. The table below maps out real-world scenarios for a 30A circuit, showing how material, insulation, and installation method change the required AWG.
| Wire Size & Material | Insulation / Cable Type | NEC Temp Column Used | Base Ampacity | Max Length for <3% Drop (240V) |
|---|---|---|---|---|
| 10 AWG Copper | THHN / THWN-2 (Conduit) | 75°C (Terminal limit) | 35A | ~95 feet |
| 10 AWG Copper | NM-B (Romex) | 60°C (NEC 334.80) | 30A | ~95 feet |
| 8 AWG Aluminum | XHHW-2 / THWN-2 | 75°C | 40A | ~75 feet |
| 8 AWG Copper | THHN (Upsized for Drop) | 75°C | 50A | ~150 feet |
Row-by-Row Notes: Notice that 8 AWG aluminum provides 40A of ampacity at 75°C, making it a valid, cost-effective alternative to 10 AWG copper for longer conduit runs where copper pricing is prohibitive. However, aluminum requires larger conduit, specific anti-oxidant paste (like Noalox), and torque-rated terminations explicitly marked AL/CU. Never terminate aluminum wire directly into a breaker or receptacle lug that is only rated for copper.
Why 10 AWG? The NEC 240.4(D) Rule and Terminal Limits
A common question on the jobsite is: "If 12 AWG THHN is rated for 30 amps in the 90°C column, why can't I use it on a 30A breaker?"
The answer lies in NEC 240.4(D), which dictates specific overcurrent protection limits for small conductors. Regardless of the 90°C ampacity, the NEC hard-caps the breaker size for 12 AWG copper at 20 amps, and 14 AWG copper at 15 amps. This rule exists because the physical mass of a 12 AWG wire cannot dissipate heat fast enough under sustained fault conditions to prevent the insulation from degrading, even if the breaker eventually trips. Therefore, 10 AWG is the absolute minimum size permitted by code for a 30A overcurrent device.
Furthermore, you must respect the weakest link in the circuit. Modern THHN wire is incredibly heat-resistant, but the brass and steel lugs inside a standard 30A breaker or a NEMA 14-30 receptacle will soften, warp, or lose spring tension at 90°C. The Copper Development Association and NEC 110.14(C) require you to size the wire based on the lowest temperature rating of any connected termination. Since almost all residential 30A devices are rated 75°C, your wire's final allowable ampacity is locked to the 75°C column.
When to Upsize: Voltage Drop, Derating, and Aluminum
Ampacity tables tell you what size wire will prevent a fire. Voltage drop calculations tell you what size wire will actually make your equipment run correctly. NEC 310.15(B) provides information on voltage drop, and while it is technically a "recommendation" rather than a strict mandate in all residential cases, exceeding a 3% drop on a branch circuit will cause motors to overheat, electronics to brown out, and heaters to underperform.
If you are running a 240V, 30A circuit (like a kiln or EV charger) 100 feet from the panel using 10 AWG copper, the voltage drop is calculated as:
VD = (2 × K × I × L) / Circular MilsVD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts7.45V is 3.1% of 240V. This slightly exceeds the 3% recommendation. If your run exceeds 95 feet on a 240V circuit, or 45 feet on a 120V circuit (like an RV TT-30), you must upsize to 8 AWG copper to maintain optimal performance.
Conduit Bundling and Derating: If you are pulling multiple circuits through the same conduit, the wires heat each other up. NEC 310.15(C)(1) requires derating when you have 4 or more current-carrying conductors. If you have 4 to 6 conductors in a pipe, you must multiply the base ampacity by 80%. For 10 AWG THHN (base 90°C ampacity of 40A), 40A × 0.80 = 32A. Since 32A is still above your 30A breaker, 10 AWG survives. But if you pull 7 to 9 conductors (derated to 70%), 40A × 0.70 = 28A. Your 10 AWG wire is now legally limited to 28A, and you must upsize to 8 AWG.
When to Call an Engineer or the AHJ
While 10 AWG covers 90% of standard 30A residential branch circuits, specific load profiles require professional verification. You must consult a licensed engineer or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- Continuous Loads: If your 30A load will run for 3 hours or more without interruption (e.g., a commercial kiln, a continuous-duty air compressor, or an EV charger set to maximum output), NEC 210.20(A) requires the branch circuit to be sized at 125% of the continuous load. A true 30A continuous load requires a 40A breaker and 8 AWG wire. Do not rely on the breaker's nominal rating for continuous thermal limits.
- High Ambient Temperatures: If the conduit runs through an attic in a hot climate where ambient temperatures regularly exceed 104°F (40°C) or 122°F (50°C), you must apply the temperature correction factors in NEC Table 310.15(B)(1)(1). At 50°C ambient, the 90°C ampacity of 10 AWG (40A) is multiplied by 0.82, dropping it to 32.8A. If the attic reaches 59°C (138°F), the derating factor drops to 0.71, reducing ampacity to 28.4A, forcing an upsize to 8 AWG.
- Service Entrance or Feeder Upgrades: If this 30A circuit is part of a larger subpanel feeder calculation or involves complex utility interconnections (like a solar inverter backfeed), the standard branch-circuit rules do not apply in isolation. An engineer must perform a full load calculation and fault-current analysis to ensure the wire's short-circuit withstand rating is not exceeded.
Always verify your local amendments. While the NEC provides the baseline framework, local inspectors have the final say on whether a specific installation method, like running NM-B through insulated wall cavities, meets the safety threshold for your specific municipality.
