For a standard 30 amp breaker, use 10 AWG copper wire or 8 AWG aluminum wire. The breaker must be rated exactly at 30A. This assumes copper THHN/THWN-2 in a raceway or NM-B cable, evaluated at the 75°C temperature column per NEC Table 310.16, with an ambient temperature of 30°C (86°F).
- Material: Copper (primary), Aluminum (secondary)
- Temperature Column: 75°C (Standard for 30A breakers per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway (EMT/PVC) or NM-B cable, maximum 3 current-carrying conductors
The Baseline: Why 10 AWG Copper and 8 AWG Aluminum?
Sizing wire isn't about guessing; it is a strict exercise in thermal management. According to the NFPA National Electrical Code (NEC), the ampacity of a conductor must be equal to or greater than the rating of the overcurrent protective device (OCPD) protecting it.
Most standard 30-amp breakers (like the Square D Homeline or Eaton BR series) feature termination lugs rated for 75°C. Therefore, we must look at the 75°C column of NEC Table 310.16, even if your wire insulation (like THHN) is rated for 90°C. At 75°C, 10 AWG copper has a base ampacity of 35A, and 8 AWG aluminum has a base ampacity of 40A. Both safely exceed the 30A breaker threshold.
Why not use one size smaller (12 AWG)? A 12 AWG copper wire maxes out at 25A in the 75°C column. If you pull a sustained 28A load on a 12 AWG wire protected by a 30A breaker, the breaker will not trip, but the wire will overheat. Over time, this degrades the insulation, creates a fire hazard, and causes excessive voltage drop. You must step up to 10 AWG to provide that 5A thermal buffer.
| Conductor | Insulation Type | Base Ampacity (75°C) | Min AWG for 30A | Max Run (3% drop @ 240V) |
|---|---|---|---|---|
| Copper | THHN / THWN-2 | 35A | 10 AWG | ~95 feet |
| Copper | XHHW-2 | 35A | 10 AWG | ~95 feet |
| Aluminum | THHN / THWN-2 | 40A | 8 AWG | ~75 feet |
| Aluminum | XHHW-2 | 40A | 8 AWG | ~75 feet |
Voltage Drop: When 10 AWG Isn't Enough
Ampacity tells you if the wire will melt; voltage drop tells you if your equipment will actually run. The NEC recommends keeping voltage drop under 3% for branch circuits. While 10 AWG copper handles 30A thermally, its electrical resistance becomes a problem over long distances.
Let's run the math using the standard Southwire voltage drop formula: VD = (2 × K × I × D) / CM.
- K (Copper constant) = 12.9
- I (Current) = 30A
- D (One-way distance) = 100 feet
- CM (Circular mils for 10 AWG) = 10,380
VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts.
If this is a 240V circuit (like a dryer or EV charger), 7.45V represents a 3.1% drop. This is borderline acceptable, but pushing it. If this is a 120V circuit (like a 30A RV receptacle or heavy 120V shop tool), that same 7.45V represents a 6.2% drop. Your tools will run hot, motors will stall, and electronics may brown out. For 120V circuits over 45 feet, you must step up to 8 AWG copper.
| Circuit Voltage | One-Way Run Length | Max Load | Required Copper Wire |
|---|---|---|---|
| 240V | Under 95 feet | 30A | 10 AWG |
| 240V | 95 to 150 feet | 30A | 8 AWG |
| 120V | Under 45 feet | 30A | 10 AWG |
| 120V | 45 to 75 feet | 30A | 8 AWG |
Derating and Edge Cases: What Changes the Answer?
The baseline assumes ideal conditions. On a real jobsite, heat buildup in conduits and high ambient temperatures force you to derate the wire's ampacity. Never treat aluminum and copper interchangeably; aluminum requires specific anti-oxidant paste (like Noalox) and breakers explicitly marked 'AL/CU'.
If you pull more than three current-carrying conductors through a single raceway (conduit), NEC Table 310.15(C)(1) mandates an adjustment factor. For 4 to 6 conductors, you must multiply the base ampacity by 80%.
- 10 AWG Copper (35A) × 0.80 = 28A. This is now below your 30A breaker. It fails.
- You must step up to 8 AWG Copper (50A × 0.80 = 40A) to maintain compliance.
Ambient Temperature Corrections: If your conduit runs through an unventilated attic where summer temperatures hit 40°C (104°F), you apply a correction factor of 0.88 to the 90°C column (for derating purposes only). 10 AWG THHN (40A at 90°C) × 0.88 = 35.2A. It still passes the 30A threshold, but if you combine high ambient heat and conductor bundling, 10 AWG will quickly become unsafe. When in doubt, pull 8 AWG.
When to Call an Engineer or the AHJ
While 10 AWG copper covers 90% of residential 30A applications (dryers, standard EV chargers, water heaters), specific load profiles require professional verification and a change in wire size.
1. Continuous Loads (The 125% Rule)
NEC Article 210.20 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. If your 30A load is continuous (e.g., a commercial space heater or a heavy-duty server rack), you must size both the breaker and the wire at 125% of the load.
30A × 1.25 = 37.5A.
A 10 AWG wire (35A) is now undersized. You must use 8 AWG copper and a 40A breaker.
2. Motor Circuits (NEC Article 430)
Motors have massive inrush currents. For a 30A motor circuit, the branch circuit conductors are sized at 125% of the motor's Full Load Amps (FLA), but the breaker can be sized much higher (up to 250% of FLA) to allow the motor to start without tripping. If you are wiring a 3HP or 5HP shop motor, do not use standard branch circuit rules; consult an electrician to calculate the specific FLA and OCPD sizing.
3. Local AHJ Amendments
Always remember that the NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or city inspector may have regional amendments—such as mandating copper-only for certain indoor branch circuits or requiring larger grounding conductors in high-resistivity soil. Always pull a permit and verify your wire sizing with the local inspector before closing up walls.
