The correct wire gauge for 30 amps is 10 AWG copper wire protected by a 30-amp breaker. This baseline assumes THHN/THWN-2 or NM-B insulation, a maximum 30°C (86°F) ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (unless explicitly noted as aluminum below).
- Temperature Column: 75°C for THHN/THWN-2 in conduit; 60°C for NM-B (Romex) cable.
- Ambient Temp: 30°C (86°F). Higher temperatures require derating.
- Conduit Fill: Maximum 3 current-carrying conductors.
- Code: NEC-style guidance; your local AHJ (Authority Having Jurisdiction) has final authority.
The Baseline: Why 10 AWG Copper?
You cannot use a smaller wire for a 30-amp circuit because 12 AWG copper is physically incapable of safely carrying the load under National Electrical Code (NEC) rules. While 12 AWG has an ampacity of 25A in the 60°C column and 30A in the 90°C column, NEC Article 240.4(D) strictly limits 12 AWG branch circuits to a 20-amp overcurrent device.
When we look at NEC Table 310.16, 10 AWG copper wire in the 75°C column (which applies to THHN in conduit) is rated for 35 amps. However, NEC 240.4(D)(7) explicitly caps 10 AWG branch circuits at a 30-amp breaker. This makes 10 AWG the exact, code-compliant minimum for a standard 30A non-continuous load.
The Decision Tree: When to Upsize to 8 AWG
A 30-amp breaker doesn't automatically mean 10 AWG wire. Real-world conditions like continuous loads, conduit bundling, and long distances force you to upsize to 8 AWG. Use this decision matrix to lock in your final wire size.
| Scenario Condition | Math / Code Rule | Required Wire Size (Copper) |
|---|---|---|
| Standard load (under 3 hours), under 90 ft, 1-3 conductors | Base ampacity 35A (75°C col). Breaker capped at 30A. | 10 AWG |
| Continuous load (runs 3+ hours, e.g., EV charger, heater) | NEC 210.20(A): 30A x 1.25 = 37.5A minimum wire ampacity. | 8 AWG (Rated 50A at 75°C) |
| 4 to 6 current-carrying conductors in one conduit | NEC 310.15(C)(1): 35A x 0.80 derating = 28A (too low for 30A breaker). | 8 AWG (50A x 0.80 = 40A) |
| Run length exceeds 90 feet (at 240V) | Voltage drop exceeds 3% (see math below). | 8 AWG |
Voltage Drop: The 90-Foot Threshold
The NEC recommends a maximum 3% voltage drop for branch circuits to ensure equipment operates efficiently and motors don't overheat. For a 240V circuit, a 3% drop is 7.2 volts.
Let's run the exact voltage drop math for a 30-amp load on 10 AWG copper wire using the standard formula: VD = (2 × K × I × D) / CM.
- K (Copper resistivity) = 12.9
- I (Current) = 30 Amps
- CM (Circular mils for 10 AWG) = 10,380
- VD (Max allowable drop) = 7.2V
Solving for Distance (D): D = (7.2 × 10,380) / (2 × 12.9 × 30).
D = 74,736 / 774 = 96.5 feet.
At exactly 96 feet, you hit the 3% limit. In practice, you should upsize to 8 AWG copper for any 30-amp run exceeding 90 feet. If you are running a 120V 30-amp circuit (like a heavy-duty RV receptacle), the math changes drastically: your max drop is only 3.6V, meaning you must upsize to 8 AWG at just 48 feet. You can verify your specific run parameters using the Southwire Voltage Drop Calculator.
Aluminum vs. Copper for 30-Amp Feeders
Aluminum and copper are not interchangeable. Aluminum has higher resistance and expands/contracts more under thermal cycling, requiring a larger cross-sectional area to carry the same current safely.
If you are using aluminum wire (such as XHHW or THWN-2), the minimum wire gauge for 30 amps is 8 AWG. According to NEC Table 310.16, 8 AWG aluminum in the 75°C column is rated for 40 amps, which safely covers the 30-amp breaker requirement.
Field rules for aluminum:
- Never use 60°C rated terminals with aluminum; ensure your breaker and lugs are explicitly marked AL/CU and rated for 75°C.
- Apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before terminating to prevent galvanic corrosion and high-resistance joints.
- Torque the lugs to the manufacturer's exact specification (typically around 20 to 35 in-lbs for standard 30A breakers). Hand-tightening aluminum is a primary cause of outlet fires.
When to Call an Engineer or the AHJ
While the decision tree above covers 95% of residential and light commercial 30-amp circuits, specific environmental and load conditions require professional sign-off.
- High Ambient Temperatures: If your conduit runs through an attic in Arizona or Texas where ambient temperatures regularly exceed 104°F (40°C), you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). A 10 AWG wire in a 50°C attic loses roughly 20% of its ampacity, dropping it below the 30A threshold and forcing an upsize.
- Complex Conduit Fill: If you are pulling more than six current-carrying conductors in a single raceway, the derating factors compound rapidly. An engineer should calculate the exact thermal dissipation.
- Service Entrance or Meter Work: If this 30-amp circuit originates directly from the line side of your main service disconnect or involves meter loop modifications, stop. This requires a licensed electrician and utility coordination.
Final Verification and Testing Steps
Once you have selected your wire (10 AWG copper for standard, 8 AWG for continuous/long runs) and installed the 30-amp breaker, do not energize the panel blindly.
- Torque Check: Verify all terminal screws are torqued to the breaker manufacturer's spec using a calibrated inch-pound torque screwdriver. NEC 110.14(D) mandates this for all terminations.
- Continuity and Short Test: Before turning the breaker on, use a multimeter in continuity mode. Check Line-to-Neutral, Line-to-Ground, and Line-to-Line (for 240V). You should read 'OL' (open loop) or infinite resistance. If you read near zero ohms, you have a dead short—do not energize.
- Voltage Verification: Energize the breaker and measure at the receptacle or load end. For a 240V circuit, expect 228V to 252V. If you read significantly lower, re-evaluate your voltage drop calculations and connection integrity.
