For a standard 30 amp breaker, use 10 AWG copper wire with a 75°C or 90°C insulation rating (like THHN or THWN-2). The breaker must be rated for 30A. This assumes a standard residential branch circuit under 50 feet in length, installed in a standard conduit or NM-B cable at 30°C ambient temperature.

Baseline Assumptions for This Guide:
  • Material: Copper (Aluminum requires different sizing)
  • Temperature Column: 75°C termination ratings per NEC 110.14(C)
  • Ambient Temperature: 30°C (86°F) or lower
  • Installation Method: Single circuit in standard conduit, NM-B cable, or direct burial
  • Load Type: Non-continuous (operates for less than 3 hours at a time)

The Baseline: Why 10 AWG Copper is the Standard

When determining what size wire for 30 amp overcurrent protection, 10 AWG copper is the universal baseline. However, the reasoning goes deeper than simply matching the breaker size to the wire's absolute melting point.

Under NEC Table 310.16, 10 AWG copper wire with 75°C insulation (like THWN) has an allowable ampacity of 35A. With 90°C insulation (like THHN), it jumps to 40A. So why can't you use a 35A or 40A breaker?

The answer lies in NEC 240.4(D), which imposes strict overcurrent protection limits on small conductors. Specifically, NEC 240.4(D)(7) mandates that the overcurrent device for 10 AWG copper shall not exceed 30A. Even though the wire's thermal insulation can handle 35A or 40A, the NEC caps the breaker at 30A to protect the wire from transient surges and to account for the physical limitations of terminating small wires under high-current lugs. Conversely, you cannot use 12 AWG wire; NEC 240.4(D)(6) caps 12 AWG at 20A. Therefore, 10 AWG is the exact, code-mandated minimum for a 30A breaker.

NEC Ampacity Tables and Temperature Columns

A common mistake on the workbench is reading the 90°C column on Copper Development Association ampacity charts and assuming that rating applies to the entire circuit. It does not. The allowable ampacity is dictated by the lowest temperature rating of any connected component, which is almost always the breaker or receptacle termination.

Table 310.16 Ampacities for Copper Conductors (Not more than 3 current-carrying)
Wire Size (AWG) 60°C Column (NM-B) 75°C Column (THWN) 90°C Column (THHN) Max Breaker (NEC 240.4)
10 AWG 30A 35A 40A 30A
8 AWG 40A 50A 55A 40A / 50A*
6 AWG 55A 65A 75A 60A / 70A*

*Note: 8 AWG and 6 AWG do not fall under the strict 240.4(D) small conductor rules, allowing you to use standard breaker sizes matching the 75°C column.

Termination Rule (NEC 110.14(C)): Most modern 30A breakers and NEMA receptacles (like a 14-30R dryer outlet or L14-30R generator inlet) are rated for 75°C. If you use 90°C THHN wire in conduit, you must still size the wire based on the 75°C column for ampacity purposes, though you can use the 90°C column for derating adjustments.

Voltage Drop: When 10 AWG Isn't Enough

Ampacity tables assume the wire can handle the heat, but they ignore voltage drop. The NEC recommends (via Informational Note in 210.19) a maximum 3% voltage drop for branch circuits. If your 30A run is long, 10 AWG will cause the voltage at the load to sag, leading to overheating motors, tripped RV inverters, or poorly performing heaters.

Let's calculate the maximum run length for 10 AWG copper carrying a full 30A load, using the standard voltage drop formula: Length = (CM × Voltage Drop) / (2 × K × I)

  • CM (Circular Mils for 10 AWG): 10,380
  • K (Copper Resistivity): 12.9
  • I (Current): 30A

Scenario A: 240V Circuit (e.g., Dryer, Water Heater, EV Charger)
Max allowable drop (3% of 240V) = 7.2V.
Length = (10,380 × 7.2) / (2 × 12.9 × 30) = 96.5 feet.

Scenario B: 120V Circuit (e.g., RV TT-30 Receptacle)
Max allowable drop (3% of 120V) = 3.6V.
Length = (10,380 × 3.6) / (2 × 12.9 × 30) = 48.2 feet.

If your 120V RV outlet is 70 feet from the panel, 10 AWG will yield a 4.3% voltage drop. You must upsize to 8 AWG copper to maintain the 3% threshold. You can verify these calculations using the Southwire Voltage Drop Calculator.

Decision Tree: Sizing for Your Specific 30A Run

Use this decision matrix to lock in your exact wire size and part type. Follow the path that matches your installation parameters.

Installation Condition Required Wire Size Recommended Insulation / Cable
120V or 240V, under 48 ft (120V) or 96 ft (240V), 30°C ambient 10 AWG Copper 10/2 NM-B (Romex) or 10 AWG THHN in conduit
120V run between 48 ft and 75 ft 8 AWG Copper 8/2 NM-B or 8 AWG THHN
240V run between 96 ft and 150 ft 8 AWG Copper 8/3 NM-B or 8 AWG THHN
Installed in an attic or environment > 113°F (45°C) 8 AWG Copper THHN/THWN-2 (NM-B is not rated for high-heat derating)
Using Aluminum wire (any standard distance) 8 AWG Aluminum XHHW-2 or USE-2 (Must use anti-oxidant paste)

Aluminum, Bundling, and Environmental Derating

The baseline answer changes immediately if you alter the material or the physical environment of the wire. Never treat aluminum and copper interchangeably.

The Aluminum Factor

Aluminum has higher resistance and expands/contracts differently than copper. Under the 75°C column, 10 AWG aluminum is only rated for 25A. Because 25A is below your 30A breaker, 10 AWG aluminum is illegal for a 30A circuit. You must step up to 8 AWG aluminum, which carries a 75°C ampacity of 40A. Always apply an antioxidant compound (like Noalox) to aluminum terminations to prevent galvanic corrosion and high-resistance heating at the lug.

Conduit Bundling (NEC 310.15(C)(1))

Wires generate heat. When you pull multiple circuits through a single conduit, they heat each other up, requiring ampacity derating. If you have 4 to 6 current-carrying conductors in a conduit, you must apply an 80% derating factor.

Here is where the 90°C THHN column saves you. 10 AWG THHN is rated 40A at 90°C.
40A × 0.80 (derating factor) = 32A.
Because 32A is still greater than 30A, 10 AWG THHN survives the derating and can still be used on a 30A breaker. However, if you have 7 to 9 conductors in the pipe (70% derating), 40A × 0.70 = 28A. At 28A, the wire is undersized for a 30A breaker, and you must pull 8 AWG THHN.

When to Call an Engineer or the AHJ

While 10 AWG copper covers 90% of residential 30A applications, specific load profiles require professional sign-off and larger wire. Refer to the NFPA 70 (National Electrical Code) guidelines for the following edge cases:

Continuous Load Warning (NEC 210.20(A)): If your 30A load will run continuously for 3 hours or more (e.g., a commercial heater, a continuous-duty EV charger, or server rack cooling), the circuit must be derated to 80%. A 30A continuous load requires a 40A breaker (30A × 1.25 = 37.5A, rounded up to 40A). Consequently, you must use 8 AWG copper wire. Never put a continuous 30A load on a 30A breaker.

Additionally, consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if:

  • You are pulling wire through areas with extreme ambient heat (e.g., inside a boiler room or directly above a kiln) where standard 30°C/86°F derating tables do not apply.
  • You are designing a feeder for a subpanel where the 30A represents a calculated load rather than a single branch circuit appliance.
  • Local municipal codes have amended NEC 240.4(D) or voltage drop recommendations into strict, enforceable law (as seen in some California and New York jurisdictions).

The Final Verdict: Buy 10 AWG copper (THHN in conduit or 10/2 NM-B) for standard 30A runs under 50 feet. If your 120V run exceeds 48 feet, your 240V run exceeds 96 feet, or you are wiring a 3-hour continuous load, buy 8 AWG copper. Do not guess; measure the run, calculate the drop, and pull the correct gauge the first time.