For a standard 30-amp circuit, you need 10 AWG copper wire paired with a 30-amp breaker. This assumes THHN/THWN-2 insulation, a 75°C termination rating, 30°C ambient temperature, and no more than three current-carrying conductors in the conduit. If your run exceeds 100 feet, you must upsize to 8 AWG to prevent excessive voltage drop.
- Material: Copper (Aluminum requires different sizing, covered below)
- Insulation: THHN/THWN-2 or XHHW-2 (90°C rated wire, but terminated at 75°C)
- Temperature Column: 75°C column per NEC 110.14(C)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Baseline: 10 AWG Copper and the NEC Ampacity Table
Beginners often ask why we cannot use 12 AWG wire for a 30-amp circuit, especially since 12 AWG is commonly used for 20-amp circuits. The answer lies in the NFPA 70 (National Electrical Code) ampacity tables and the physics of thermal limits.
Under NEC Table 310.16, 12 AWG copper wire has an ampacity of 25 amps in the 75°C column and only 20 amps in the 60°C column. If you pull 30 amps through 12 AWG wire, the wire will overheat, melt its insulation, and potentially start a fire long before a 30-amp breaker trips. The breaker is there to protect the wire, not the appliance. Therefore, the wire's ampacity must equal or exceed the breaker's rating.
Furthermore, while THHN wire is technically rated for 90°C, NEC 110.14(C) dictates that we must size the wire based on the lowest temperature rating of any connected component. Since almost all standard residential breakers and receptacles are rated for 75°C terminations, we are legally restricted to using the 75°C column for our final ampacity sizing.
| AWG Size | 60°C Column (NM-B) | 75°C Column (THHN Terminations) | 90°C Column (Derating Only) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG (Target) | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
As shown in the table, 10 AWG copper in the 75°C column is rated for 35 amps. This safely exceeds our 30-amp breaker requirement, giving us a 5-amp thermal buffer at the terminations.
Voltage Drop: When 10 AWG Fails the Distance Test
Ampacity tells us the wire won't catch fire, but it doesn't guarantee your equipment will actually run. Voltage drop is the silent killer of 30-amp circuits, particularly for high-draw appliances like RV air conditioners, welders, and air compressors. The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% total from the utility to the furthest outlet.
To calculate voltage drop, we use the formula: VD = (2 × K × I × D) / CM
- K = 12.9 (resistivity constant for copper)
- I = 30 (current in amps)
- D = One-way distance in feet
- CM = Circular mils of the wire (10,380 for 10 AWG; 16,510 for 8 AWG)
Scenario A: 240V Circuit (e.g., Dryer or Water Heater) at 100 Feet
Using 10 AWG at 100 feet: VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45V.
On a 240V circuit, a 7.45V drop is 3.1%. This is marginally over the ideal 3% branch circuit recommendation but well within the 5% absolute maximum. 10 AWG is acceptable here.
Scenario B: 120V Circuit (e.g., TT-30 RV Receptacle) at 100 Feet
Using the exact same 7.45V drop on a 120V circuit yields a 6.2% voltage drop. This exceeds the 5% maximum limit. Your RV's air conditioner compressor will struggle to start, draw locked-rotor current, and potentially trip the breaker or burn out the motor. You must upsize to 8 AWG copper for any 120V, 30-amp run exceeding 75 feet.
Derating and Material Swaps: What Changes the Answer
The 10 AWG baseline assumes perfect conditions. In the real world, jobsite constraints force us to adjust our wire size. Here is a decision framework for when 10 AWG is no longer sufficient.
| Condition | NEC Rule | Required Wire Size |
|---|---|---|
| Continuous Load (On for 3+ hours, e.g., EV charger, baseboard heat) |
NEC 210.20(A) & 215.2(A)(1) require 125% multiplier. 30A × 1.25 = 37.5A. | 8 AWG Copper (and a 40A breaker) |
| Conduit Bundling (4 to 6 current-carrying conductors in one pipe) |
NEC 310.15(C)(1) requires 80% derating. 30A / 0.8 = 37.5A required 90°C ampacity. | 10 AWG Copper (10 AWG at 90°C is 40A, so it barely passes, but 8 AWG is highly recommended for heat dissipation). |
| Heavy Bundling (7 to 9 current-carrying conductors) |
NEC 310.15(C)(1) requires 70% derating. 30A / 0.7 = 42.8A required 90°C ampacity. | 8 AWG Copper (10 AWG at 90°C is only 40A and will fail inspection). |
| High Ambient Heat (Attic or rooftop conduit at 46-50°C / 115-122°F) |
NEC 310.15(B)(1) requires 0.82 correction factor. 30A / 0.82 = 36.5A required. | 8 AWG Copper (Provides necessary thermal headroom). |
| Aluminum Wire (Using SER cable or XHHW-2 AL for cost savings) |
NEC 310.16. Aluminum has higher resistance and is prone to creep at small gauges. | 8 AWG Aluminum (Never use 10 AWG aluminum for branch circuits; it is too brittle for standard screw terminations). |
A common and dangerous mistake is treating copper and aluminum interchangeably. While 10 AWG aluminum technically shows 30A in the 75°C column of Table 310.16, industry experts and AHJs universally reject 10 AWG aluminum for branch circuits due to mechanical fragility and oxidation risks at the lug. If your project calls for aluminum to save money on a long run, step up to 8 AWG.
When an Engineer or AHJ Must Confirm Your Sizing
While the guidelines above cover 95% of residential and light commercial 30-amp circuits, certain scenarios cross the threshold from standard DIY/trade practice into engineered design. You must pull a permit and have your calculations reviewed by the local AHJ or a licensed Professional Engineer (PE) in the following situations:
- Service Entrance Conductors: If this 30-amp circuit is actually a feeder to a subpanel (e.g., a detached garage), the sizing rules change. Feeders have different grounding requirements (NEC 250.32), and if the subpanel contains a mix of continuous and non-continuous loads, the feeder calculation (NEC 220) dictates the size, not just the breaker handle.
- Complex Voltage Drop Scenarios: If your run exceeds 200 feet, standard tables are insufficient. An engineer will need to calculate the exact impedance of the specific cable manufacturer's product, account for AC reactance (X_L), and potentially specify parallel runs or 6 AWG wire to keep the voltage drop under 3%.
- Harmonic Loads: If the 30-amp circuit is feeding a large Variable Frequency Drive (VFD) or a bank of non-linear LED drivers, the neutral conductor may carry additive triplen harmonics. In these cases, the neutral must be sized equal to or larger than the phase conductors, and the 90°C column derating becomes the limiting factor.
By anchoring your 30-amp circuit design to 10 AWG copper, verifying the 75°C termination limits, and running a quick voltage drop calculation for your specific distance and voltage, you ensure a safe, code-compliant installation that will perform reliably for decades.
