For a standard 30 amp circuit, use 10 AWG copper wire paired with a 30-amp breaker. This baseline assumes THHN/THWN-2 insulation rated in the 75°C column. If your one-way cable run exceeds 50 feet, you must step up to 8 AWG copper to keep voltage drop under 3%.
- Material: Solid or stranded copper (not aluminum or copper-clad aluminum).
- Temperature Rating: 75°C column (standard for modern breakers and terminals).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Standard EMT conduit, PVC, or NM-B (Romex) cable in a wall cavity.
- Load Type: Non-continuous load (under 3 hours of maximum draw).
The Baseline Spec: 10 AWG Copper and the 75°C Rule
Sizing wire is not about guessing; it is about matching the wire's thermal limits to the breaker's trip curve. According to the National Electrical Code (NEC), the breaker protects the wire from overheating. If the wire draws more current than its ampacity, the breaker must trip before the insulation melts.
Modern circuit breakers (like Square D QO or Eaton BR series) and terminal lugs are rated for 75°C. Therefore, we use the 75°C column in the NEC ampacity tables (specifically NEC Table 310.16). In this column, 10 AWG copper is rated for 35 amps. Because standard breakers do not come in 35A sizes, NEC 240.4(B) allows you to round down to the next standard breaker size, which is 30 amps.
| Wire Size (AWG) | Insulation Type | Temp Rating | Ampacity | Max Standard Breaker |
|---|---|---|---|---|
| 12 AWG | THHN / THWN-2 | 75°C | 25A | 20A |
| 10 AWG | THHN / THWN-2 | 75°C | 35A | 30A |
| 8 AWG | THHN / THWN-2 | 75°C | 50A | 50A |
Note: If you are using NM-B (Romex) cable, the jacket is rated for 60°C. However, 10 AWG in the 60°C column is exactly 30A, so 10 AWG NM-B is still perfectly legal and safe for a 30A breaker.
Voltage Drop: When to Upsize to 8 AWG
Ampacity tells you what the wire can handle without catching fire. Voltage drop tells you if your equipment will actually run correctly at the end of the line. The NEC recommends keeping voltage drop under 3% for branch circuits to ensure motors and appliances operate efficiently without drawing excess current to compensate for low voltage.
Using the Southwire Voltage Drop Calculator and standard AC resistance values for copper, here is where 10 AWG fails the 3% test at a full 30A load:
- For a 120V, 30A circuit: 10 AWG copper hits a 3% voltage drop (3.6V) at roughly 55 feet.
- For a 240V, 30A circuit: 10 AWG copper hits a 3% voltage drop (7.2V) at roughly 110 feet.
Decision Tree: Finalizing Your Wire and Breaker Pick
Use this decision path to lock in your exact materials list before heading to the supply house. Do not skip steps.
| Condition / Scenario | Required Wire Size (Copper) | Required Breaker Size |
|---|---|---|
| Run is under 50 ft (120V) or 100 ft (240V), standard ambient temp, non-continuous load. | 10 AWG | 30A |
| Run is 50-85 ft (120V) or 100-175 ft (240V). | 8 AWG | 30A (or 40A if load demands) |
| Load is continuous (runs 3+ hours, e.g., EV charger, kiln, heater). NEC requires 125% sizing. | 8 AWG (30A x 1.25 = 37.5A) | 40A |
| More than 3 current-carrying conductors in a single conduit (requires 80% derating). | 8 AWG (50A x 0.8 = 40A) | 30A or 40A |
Why Not 12 AWG or Aluminum?
A common mistake on the bench or jobsite is grabbing 12 AWG because it is cheaper and easier to bend, or using aluminum because it is left over from a feeder run. Both are dangerous if applied incorrectly to a 30A circuit.
Why 12 AWG is a Fire Hazard on a 30A Breaker
12 AWG copper is rated for 25A in the 75°C column, and 20A in the 60°C column. If you pull 30 amps through 12 AWG wire, the wire will heat up past its insulation rating. The 30-amp breaker will not trip, because it is designed to carry 30 amps indefinitely. The wire will eventually melt its insulation, short out, and start a fire inside the wall cavity. The breaker must always be sized to the weakest link in the circuit.
The Aluminum Trap
Aluminum wire has lower conductivity and higher thermal expansion than copper. You cannot use the same AWG sizes interchangeably.
- 10 AWG Aluminum is only rated for 25A in the 75°C column. It is illegal and unsafe to put 10 AWG aluminum on a 30A breaker.
- To carry 30 amps with aluminum, you must use 8 AWG aluminum (rated 40A at 75°C).
Furthermore, aluminum requires anti-oxidant paste (like Noalox) at terminations and specific torque settings to prevent loose connections from arcing over time. Unless you are running long outdoor feeder lines where copper pricing is prohibitive, stick to copper for 30A branch circuits.
When to Call an Engineer or the AHJ
While the NEC provides clear baseline rules (referenced via the NFPA 70 standard development guidelines), real-world conditions sometimes break the standard tables. You must consult a licensed professional engineer or your local Authority Having Jurisdiction (AHJ / electrical inspector) in the following scenarios:
- High Ambient Temperatures: If your conduit runs through an attic in the Southwest US where ambient temperatures exceed 40°C (104°F), you must apply temperature correction factors. At 41-45°C, you multiply the base ampacity by 0.82. A 10 AWG wire (35A) drops to 28.7A, meaning it can no longer legally be used on a 30A breaker. You must upsize.
- Conduit Fill and Bundling: If you are pulling multiple circuits through a single PVC conduit, the heat generated by adjacent wires cannot dissipate. If you have 4 to 6 current-carrying conductors in one raceway, NEC 310.15(C)(1) requires an 80% derating factor. Again, 10 AWG drops below 30A, forcing an upsize to 8 AWG.
- Specialty Equipment: Some 30A equipment (like specific welders or HVAC compressors) have Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOP) ratings printed on the nameplate that override standard NEC branch circuit rules. Always defer to the manufacturer's nameplate data.
