The correct wire gauge for a 30 amp breaker is 10 AWG copper wire. This applies to standard branch circuits using THHN in conduit or NM-B cable. You must pair this with a 30A overcurrent protective device. However, long runs or high ambient temperatures require upsizing to 8 AWG.
Baseline Assumptions for This Guide
- Material: Copper conductors (Aluminum requires different sizing, covered below).
- Temperature Column: 75°C for THHN/THWN-2 in conduit; 60°C for NM-B (Romex) per NEC 334.80.
- Ambient Temperature: 30°C (86°F) or lower.
- Installation: Single circuit, no more than 3 current-carrying conductors in a raceway.
The Baseline Sizing: 10 AWG Copper and the 240.4(D) Rule
When sizing wire, most DIYers look at NEC Table 310.16 and stop reading. But understanding why 10 AWG is the answer requires looking at two different NEC rules interacting at once.
| Insulation Type | Temp Rating | Base Ampacity | Max Breaker Size (NEC 240.4(D)) |
|---|---|---|---|
| THHN / THWN-2 | 90°C (Terminations rated 75°C) | 35A (at 75°C column) | 30A |
| NM-B (Romex) | 60°C (per NEC 334.80) | 30A (at 60°C column) | 30A |
Notice the discrepancy in the THHN row. The wire can physically handle 35 amps at the 75°C termination rating. So why can't you put it on a 35A breaker? Because NEC Article 240.4(D) specifically restricts small conductors. It dictates that 10 AWG copper cannot be protected by an overcurrent device larger than 30 amps, regardless of its calculated ampacity.
Why not one size smaller? You might wonder if 12 AWG (rated 20A at 60°C) could work if the actual load is only 25A. It cannot. If a fault or sustained overload pulls 29A, a 30A breaker will not trip immediately, but 12 AWG wire will overheat, melt its insulation, and start a fire inside your walls. The breaker protects the wire, not the appliance.
When 10 AWG Fails: Voltage Drop and Derating
The baseline answer assumes a short run in a cool environment. In the real world, physics and conduit fill change the math.
The Voltage Drop Check
Let's run the math for a 30A, 240V circuit (like a standard electric dryer or water heater) at a distance of 100 feet using 10 AWG copper. Using the standard voltage drop formula (VD = 2 × L × I × K / CM):
- Length (L): 100 ft
- Current (I): 30A
- K (Copper): 12.9
- Circular Mils (10 AWG): 10,380
VD = (2 × 100 × 30 × 12.9) / 10,380 = 7.45 Volts.
At 240V, a 7.45V drop is 3.1%. The NEC recommends a maximum of 3% voltage drop on branch circuits for reasonable efficiency. At 100 feet, 10 AWG is right on the ragged edge. If your run is 120 feet, the drop hits 3.7%, and your appliance will run hot and inefficiently.
| One-Way Distance | Recommended Wire Gauge | Voltage Drop |
|---|---|---|
| Under 50 ft | 10 AWG Copper | < 1.6% (Excellent) |
| 50 ft to 100 ft | 10 AWG Copper | 1.6% - 3.1% (Acceptable) |
| 100 ft to 150 ft | 8 AWG Copper | 2.0% - 3.0% (Excellent) |
| Over 150 ft | 6 AWG Copper | < 3.2% (Acceptable) |
Bundling and Ambient Temperature Derating
If you pull multiple circuits through the same conduit, the wires heat each other up. NEC 310.15(C)(1) requires derating. If you have 4 to 6 current-carrying conductors in a single raceway, you must multiply the base ampacity by 80%.
For 10 AWG THHN (90°C column base ampacity is 40A), 40A × 0.80 = 32A. You can still use a 30A breaker. But if you have 7 to 9 conductors, the derating factor drops to 70%. 40A × 0.70 = 28A. Your 10 AWG wire is now only good for 28 amps, meaning you must upsize to 8 AWG to legally use a 30A breaker.
Aluminum vs. Copper for 30A Circuits
Aluminum and copper are not interchangeable. Aluminum has higher resistance and expands/contracts more under thermal load, which historically caused loose terminations and fires.
If you are running a 30A feeder (for example, to a subpanel) and choose aluminum to save money, you must use 8 AWG Aluminum. 8 AWG aluminum is rated 40A at 75°C, giving you plenty of headroom for a 30A breaker. Never use 10 AWG aluminum for a 30A circuit; it is too close to the thermal limit and highly susceptible to voltage drop.
Termination Warning: If you use aluminum wire, the breaker lugs and terminal bars must be explicitly rated for aluminum (marked AL or AL/CU). You must also apply an anti-oxidant compound like Noalox to the stripped wire before torquing the lug to the manufacturer's specification (usually 20-25 in-lbs for small breakers). If your breaker is only marked CU, you must use copper wire or splice to a copper pigtail inside an approved junction box.
When to Involve an Engineer or the AHJ
While 10 AWG covers 90% of residential 30A branch circuits, certain conditions require a professional review or a call to your local Authority Having Jurisdiction (AHJ):
- Continuous Loads: Under NEC 210.20, if your 30A load will run continuously for 3 hours or more (like a heavy-duty space heater, a kiln, or an EV charger), you must multiply the load by 125%. 30A × 1.25 = 37.5A. You now need a 40A breaker and 8 AWG copper wire. The appliance nameplate will dictate if it is a continuous load.
- High Ambient Temperatures: If the conduit runs through an attic in a hot climate where ambient temperatures regularly exceed 113°F (45°C), you must apply temperature correction factors from Table 310.16, which will severely reduce the ampacity of 10 AWG wire.
- Complex Derating: If you are pulling wire through underground conduit with multiple other circuits, the combined thermal mass and soil conditions may require an engineered voltage drop and ampacity study.
Frequently Asked Questions
Can I use 8 AWG wire on a 30 amp breaker?
Yes. The NEC allows you to use a larger wire on a smaller breaker, provided the wire physically fits into the breaker's terminal lug. In fact, upsizing to 8 AWG is the correct and necessary fix for long runs (over 100 feet at 240V) to mitigate voltage drop. Just ensure you strip the wire correctly and do not fold it back on itself just to make it fit the lug; if it doesn't fit cleanly, use a 10 AWG pigtail in a junction box, though this is rare for 8 AWG into standard 30A lugs.
What wire gauge for a 30 amp 240V circuit?
The baseline wire gauge for a 30 amp 240V circuit is exactly the same as a 120V circuit: 10 AWG copper. The breaker size and wire ampacity are dictated by the current (Amps), not the voltage. However, because the voltage is doubled to 240V, the percentage of voltage drop is halved. This means a 10 AWG wire can run twice as far on a 240V circuit (up to 100 feet) before you are forced to upsize to 8 AWG, compared to a 120V circuit.
Does a 30 amp RV outlet (TT-30) require a different wire size?
A standard RV TT-30 receptacle is 120V, not 240V. Because it operates at 120V and pulls a full 30A, voltage drop is a major factor. 10 AWG copper is only acceptable for a TT-30 if the run from the panel is less than 40 feet. If your RV pad is 60 feet away from the panel, you must run 8 AWG copper to prevent the RV's air conditioner from browning out and damaging its compressor motor during startup. Always verify the receptacle type; a NEMA 14-30 is 240V, while a TT-30 is 120V.
