You need 10 AWG copper wire and a 30-amp double-pole breaker for a standard 30-amp circuit. This assumes THHN/THWN-2 insulation in a raceway at 30°C ambient. If using aluminum, step up to 8 AWG. Never use 12 AWG; it will overheat and melt before the breaker clears a fault.

The Baseline Spec Sheet: 10 AWG Copper & 30A Breaker

Before pulling any wire through conduit, we have to lock in the baseline assumptions. Wire ampacity is not a fixed number; it changes based on insulation temperature rating, ambient heat, and how many wires are bundled together. The recommendation of 10 AWG copper relies on the following strict parameters:

Baseline Assumptions for this Guide:
  • Conductor Material: Copper (Aluminum requires different sizing)
  • Insulation Type: THHN/THWN-2 or XHHW-2 (90°C rated wire)
  • Termination Temperature: 75°C (Standard for modern breakers and receptacles)
  • Ambient Temperature: 30°C (86°F) or lower
  • Installation Method: Raceway (conduit) or NM-B cable, maximum 3 current-carrying conductors

According to NEC Table 310.16, 10 AWG copper wire in the 75°C column has an allowable ampacity of 35 amps. However, NEC Article 240.4(D) establishes strict overcurrent protection limits for small conductors. It explicitly caps the breaker size for 10 AWG copper at 30 amps, regardless of the 35A thermal rating. This is a hard code limit designed to protect the wire from sustained overloads that wouldn't quite trip a larger breaker but would slowly degrade the insulation.

Why 10 AWG and Not 12 AWG? (The Physics & Code)

A common mistake on the jobsite is assuming that because a 20-amp circuit uses 12 AWG, a 30-amp circuit can just 'push' 12 AWG a little harder. This is physically dangerous and a direct code violation.

Wire heating follows the formula P = I²R (Power loss equals current squared times resistance). If you push 30 amps through a 12 AWG wire (which has a higher resistance per foot than 10 AWG), you aren't just generating 50% more heat than a 20-amp load. Because the current is squared, you are generating 2.25 times the heat.

Fire Hazard Warning: 12 AWG copper is strictly limited to 20-amp overcurrent protection under NEC 240.4(D). If you place a 30-amp breaker on 12 AWG wire and pull a 28-amp load, the wire will operate at roughly 140% of its safe thermal capacity. The 30-amp breaker will never trip, but the wire insulation will bake, become brittle, and eventually cause an arc fault or short circuit inside your walls.

By stepping up to 10 AWG, you increase the cross-sectional area of the copper by roughly 60% compared to 12 AWG. This drops the resistance, keeps the I²R heating well within the 75°C termination limits of your breaker lugs, and satisfies the code.

Voltage Drop: When 10 AWG Fails the Distance Test

Ampacity tells you if the wire will melt. Voltage drop tells you if your equipment will actually run. The NEC recommends (via Informational Note in 210.19(A)(1)) that branch circuit voltage drop not exceed 3%. This is where the nominal voltage of your 30-amp circuit completely changes the wire sizing decision.

Let's run the math using the standard voltage drop formula: VD = (2 × K × I × L) / CM.
K = 12.9 (copper), I = 30A, CM = 10,380 (circular mils for 10 AWG).

Scenario A: 240V Electric Dryer or Subpanel (100-foot run)

  • Calculation: (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts dropped.
  • Percentage: 7.45V / 240V = 3.1% drop.
  • Verdict: At 100 feet on a 240V circuit, 10 AWG slightly exceeds the 3% recommendation. You should step up to 8 AWG copper to ensure the dryer heating elements or subpanel busbars receive adequate voltage.

Scenario B: 120V RV Receptacle (TT-30R) (50-foot run)

  • Calculation: (2 × 12.9 × 30 × 50) / 10,380 = 3.72 Volts dropped.
  • Percentage: 3.72V / 120V = 3.1% drop.
  • Verdict: Because the baseline voltage is half, the acceptable voltage drop window is also half. At just 50 feet, a 120V 30-amp RV circuit on 10 AWG wire fails the 3% rule. For a 50-foot RV cord or trench run, you must use 8 AWG copper (or 6 AWG if extending past 70 feet).

For a deeper look into conductor properties and circular mil values, the Copper Development Association (CDA) maintains excellent reference charts for exact wire dimensions and resistance metrics.

Decision Tree: Adjusting for Aluminum, Bundling, and Heat

Real-world installations rarely match the baseline spec sheet perfectly. Use this decision matrix to find your exact wire and breaker combination based on your specific jobsite conditions.

Jobsite Condition Required Wire Size Breaker Size Why This Change?
Standard run < 80ft, Copper 10 AWG 30A Baseline NEC 310.16 and 240.4(D) compliance.
Standard run < 80ft, Aluminum 8 AWG 30A Aluminum has higher resistance; 10 AWG Al is only rated 30A at 60°C, but terminations require 75°C margins.
Run is 80ft to 130ft (240V) 8 AWG Copper 30A Mitigates >3% voltage drop over long distances.
4 to 6 current-carrying conductors in one conduit 8 AWG Copper 30A NEC 310.15(C)(1) requires an 80% derating factor. 10 AWG derates to 28A (too low); 8 AWG derates to 40A (safe).
Ambient temp 41°C - 45°C (e.g., hot attic) 8 AWG Copper 30A THHN 90°C column requires an 87% temperature correction factor. 8 AWG provides the necessary thermal headroom.

When to Call an Engineer or the AHJ

The guidelines above cover standard resistive loads (heaters, dryers, RV outlets) and general branch circuits. However, there are two specific scenarios where 'what gauge wire for a 30 amp circuit' becomes a trick question, requiring a licensed professional or your local Authority Having Jurisdiction (AHJ) to sign off on the design.

1. Continuous Loads (The 125% Rule)

If your 30-amp load will run continuously for 3 hours or more (like a commercial kiln, a large aquarium heater array, or EV charging equipment), NEC Article 210.20(A) requires the overcurrent device to be rated at 125% of the continuous load.

A 30-amp continuous load requires a 37.5-amp breaker (rounded up to 40A). Therefore, you are no longer building a '30-amp circuit'; you are building a 40-amp circuit, which requires 8 AWG copper wire and a 40-amp breaker. Never put a 30A continuous load on a 30A breaker.

2. Motor Circuits (Inverse Time Breakers)

Under NEC Article 430, motor circuits are sized entirely differently. A motor drawing 24 amps full-load (FLA) requires wire sized at 125% of FLA (30A -> 10 AWG wire). However, to handle the massive inrush current when the motor starts, the breaker can be sized up to 250% of the FLA. In this scenario, you might legally see 10 AWG wire protected by a 60-amp breaker. This looks like a massive code violation to an untrained eye, but it is standard motor protection. If you are wiring a 3HP or 5HP air compressor, consult the manufacturer's spec sheet and have an electrician verify the breaker sizing.

The Default Pick: If you are wiring a standard 240V dryer, a 120V TT-30R RV receptacle, or a generic 30A subpanel feed under 80 feet in a climate-controlled space, buy a spool of 10 AWG THHN copper (or 10/3 NM-B Romex) and a 30-amp double-pole breaker. It is the safest, most code-compliant baseline for the job.