You need 10 AWG copper wire and a 30A breaker for a standard 30-amp circuit. This assumes copper conductors with 75°C insulation (like THHN) in an ambient temperature of 30°C (86°F). If you are using aluminum wire, you must step up to 8 AWG. Never use 12 AWG or 14 AWG on a 30A breaker.
- Material: Copper (unless aluminum is explicitly specified)
- Temperature Column: 75°C (Standard for most modern breakers and terminals)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Standard conduit or NM-B cable, up to 3 current-carrying conductors
Note: NEC-style guidance provided here is for educational purposes. Your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Baseline: 10 AWG Copper and NEC Ampacity Rules
To understand why 10 AWG is the correct gauge for a 30 amp breaker, we have to look at how the National Electrical Code (NEC) matches wire ampacity to overcurrent protection. According to NEC Table 310.16, a 10 AWG copper wire with 75°C insulation (such as THHN or THWN-2) has an allowable ampacity of 35 amps. Because the 35A wire rating exceeds the 30A breaker rating, the breaker will safely trip before the wire can overheat.
Why not use 12 AWG, which is cheaper and easier to bend? Under the 75°C column, 12 AWG copper is rated for 25 amps. More importantly, NEC Article 240.4(D) places a hard, specific limitation on small conductors: 12 AWG copper is strictly limited to 20 amps of overcurrent protection, regardless of the insulation temperature rating. If you put 12 AWG wire on a 30A breaker and pull 28 amps through it, the breaker will not trip, but the wire will act like a toaster element, melting the insulation and creating a severe fire hazard.
| AWG Size | 75°C Ampacity | Max Standard Breaker Size | Common Use Case |
|---|---|---|---|
| 14 AWG | 20A | 15A (per 240.4(D)) | Lighting, general receptacles |
| 12 AWG | 25A | 20A (per 240.4(D)) | Kitchen/bathroom receptacles |
| 10 AWG | 35A | 30A / 35A | Dryers, RV outlets, water heaters |
| 8 AWG | 50A | 40A / 50A | EV chargers, subpanel feeders |
When 10 AWG Fails: Voltage Drop and Derating Factors
Ampacity tables assume ideal conditions. In the real world, physics and installation environments force you to upsize your wire. The two main culprits that change the answer from 10 AWG to 8 AWG are voltage drop and conductor bundling.
The Voltage Drop Check
The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently. Let us run a voltage drop check for a 120V, 30-amp RV outlet located 80 feet from the panel. Using the standard formula and the Southwire voltage drop parameters for 10 AWG copper (Circular Mil area = 10,380):
- Calculation: VD = (2 × 12.9 × 30A × 80ft) / 10,380 = 5.96V
- Percentage: 5.96V / 120V = 4.97% drop
At nearly 5%, this exceeds the 3% recommendation. Your RV air conditioner might struggle to start, and the compressor could overheat. To fix this, you must upsize to 8 AWG copper for this specific 80-foot run, which brings the drop down to a much safer ~3.1% (and 6 AWG drops it below 2%).
Bundling and Ambient Derating
If you are pulling multiple circuits through the same conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires derating when you have more than three current-carrying conductors in a raceway. If you have 4 to 6 conductors, you must multiply the wire's ampacity by 80%.
For 10 AWG (35A at 75°C), an 80% derating factor drops the allowable ampacity to 28A. If your actual load is a full 30 amps, 10 AWG is now undersized, and you must pull 8 AWG wire to maintain code compliance.
| Condition | 10 AWG Copper Status | Action Required |
|---|---|---|
| Standard run < 50ft, 3 conductors | Pass (35A > 30A) | Use 10 AWG, 30A breaker |
| 120V run > 60ft at full 30A load | Fails Voltage Drop (>3%) | Upsize to 8 AWG or 6 AWG |
| 4-6 conductors in single conduit | Fails Derating (28A < 30A) | Upsize to 8 AWG |
| Ambient temp > 104°F (40°C) | Fails Temp Correction | Upsize to 8 AWG |
Aluminum vs. Copper for 30-Amp Circuits
Copper and aluminum are not interchangeable. Aluminum has a higher resistance and expands/contracts more under thermal cycling, which can loosen terminations over time if not installed correctly.
For a 30-amp circuit using aluminum conductors, you must use 8 AWG aluminum. While 10 AWG aluminum technically shows a 30A rating in the 75°C column of Table 310.16, most electrical inspectors and practical bench experience dictate using 8 AWG (rated 40A at 75°C) for aluminum branch circuits. This provides a thermal buffer for the terminations and accounts for the higher voltage drop inherent to aluminum.
If you are terminating aluminum wire, you must use an anti-oxidant compound (like Noalox) on the stripped conductor and torque the breaker lugs to the exact inch-pound specification printed on the breaker label. Aluminum creeps over time; a lug torqued by 'feel' will eventually loosen, arc, and melt the breaker.
When an Engineer or AHJ Must Confirm: You must consult a licensed professional engineer or your local AHJ if your 30A circuit involves service entrance conductors, runs longer than 200 feet underground, or powers specialized industrial equipment with high inrush currents. Local amendments frequently override baseline NEC tables for agricultural or specialized commercial installations.
FAQ: Common Questions About 30 Amp Wire Sizing
Can I use 12 AWG wire on a 30 amp breaker?
No. Doing so is a direct violation of NEC 240.4(D) and a severe fire hazard. 12 AWG copper is legally limited to a maximum 20A breaker. If a 30A breaker is used, a 28A fault or load will not trip the breaker, but it will cause the 12 AWG wire to overheat, melt its insulation, and potentially ignite surrounding framing. Always match the breaker to the wire's ampacity limits.
What gauge for 30 amp 240V vs 120V?
The baseline wire gauge for the conductors remains 10 AWG copper for both 240V and 120V 30-amp circuits, because ampacity is based on current (amps), not voltage. However, the voltage dictates your voltage drop tolerance. A 240V circuit (like a dryer) can run much further on 10 AWG before hitting a 3% drop compared to a 120V circuit (like an RV outlet). For long 120V runs, you will likely need to upsize to 8 AWG, whereas the same distance on 240V might be perfectly fine with 10 AWG.
Does a 30 amp continuous load change the wire size?
Yes, significantly. The NEC defines a continuous load as one that operates at maximum current for 3 hours or more (like a commercial water heater or EV charger). For continuous loads, NEC Article 210.20 requires the overcurrent device and wire to be sized at 125% of the load. A 30A continuous load requires 37.5A of capacity (30 × 1.25). Because 10 AWG is only rated for 35A, it is too small. You must upsize to 8 AWG copper (50A at 75°C) and use a 40A breaker to safely handle a 30A continuous load.
