For a standard 30 amp circuit, you need 10 AWG copper wire paired with a 30-amp double-pole (240V) or single-pole (120V) breaker. This assumes THHN/THWN-2 insulation in a raceway, a 75°C temperature rating, and an ambient temperature of 30°C (86°F). If your run exceeds 100 feet, you must upsize to 8 AWG to mitigate voltage drop.
Baseline Assumptions and the 10 AWG Rule
Before pulling any wire from the spool, we have to establish the environmental and material baselines. Electrical sizing is not a one-size-fits-all lookup; it is a calculation bound by physics and the National Electrical Code (NEC). If your installation deviates from the assumptions below, the 10 AWG baseline changes.
- Material: Solid or stranded copper (not aluminum or copper-clad aluminum).
- Insulation: THHN/THWN-2 (rated for 90°C in dry/damp locations).
- Termination Temperature: 75°C column (standard for modern breakers like Square D QO or Eaton BR and standard receptacles).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: Not more than 3 current-carrying conductors in a single raceway.
Under these exact conditions, we look at NEC Table 310.16 to find our allowable ampacities. The table below highlights why 10 AWG is the correct starting point, while illustrating the danger of relying on the wrong temperature column.
| AWG Size | 60°C Column (14-2 AWG Terminations) | 75°C Column (Standard Breaker Lugs) | 90°C Column (THHN Wire Insulation) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Why 10 AWG Copper and Not 12 AWG?
A common bench or jobsite mistake is looking at the 90°C column, seeing that 12 AWG THHN is rated for 30A, and attempting to use it on a 30-amp breaker. This is a direct violation of NEC 110.14(C) regarding temperature limitations of equipment terminations.
While the insulation on a spool of Southwire SIMpull THHN can handle 90°C, the lugs inside your breaker panel and your 30-amp receptacle (like a NEMA L14-30R or 10-30R dryer outlet) are typically only rated for 75°C. The NEC requires you to size the wire based on the lowest temperature rating of any connected component.
Looking at the 75°C column in the table above, 12 AWG is only rated for 25A. If you push 30A through it, the breaker lugs will overheat, potentially degrading the internal thermal-magnetic trip mechanism or melting the lug insulation. 10 AWG copper, rated at 35A in the 75°C column, provides a safe 5A buffer above the 30A breaker trip threshold, keeping terminations cool and code-compliant.
Variables That Force an Upsize: Length, Bundling, and Aluminum
The 10 AWG rule holds up perfectly in a short, single-circuit run inside a climate-controlled house. But real-world installations introduce resistance, heat, and material changes that force you to move to 8 AWG or larger.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for branch circuits. Let us run the math on a 100-foot run of 10 AWG copper carrying a full 30A load. Using the standard voltage drop formula (VD = 2 × K × I × L / Circular Mils, where K=12.9 for copper and 10 AWG = 10,380 CM):
- 240V Circuit (e.g., Dryer, Water Heater): The drop is 7.45V. (7.45 / 240 = 3.1%). This is borderline but generally acceptable for resistive heating loads.
- 120V Circuit (e.g., 30A RV Receptacle): The drop is still 7.45V, but now (7.45 / 120 = 6.2%). This exceeds the 3% recommendation and can cause RV air conditioner compressors to stall and overheat. Fix: Upsize to 8 AWG (VD drops to 4.6V / 3.8%) or 6 AWG (VD drops to 2.9V / 2.4%) for long 120V RV runs.
2. Conduit Bundling and Derating
If you are pulling multiple circuits through a single piece of EMT conduit, the wires heat each other up. NEC Chapter 9 requires ampacity derating when you have more than three current-carrying conductors.
Here is where the 90°C column finally becomes useful. We use the 90°C rating (40A for 10 AWG) to apply the derating factor, then verify it still covers the 30A breaker.
| Current-Carrying Conductors | Adjustment Factor | Derated Ampacity | Passes 30A Breaker? |
|---|---|---|---|
| 1 to 3 | 100% | 40A | Yes (Use 75°C col: 35A) |
| 4 to 6 | 80% | 32A | Yes (32A > 30A) |
| 7 to 9 | 70% | 28A | No. Must upsize to 8 AWG. |
3. Aluminum Wire Substitutions
If you are feeding a 30-amp subpanel or running a long underground feeder in PVC where copper is cost-prohibitive, you might switch to aluminum (like XHHW-2 or USE-2). Aluminum has higher resistance and expands more under heat. You cannot use 10 AWG aluminum for 30 amps. In the 75°C column, 8 AWG aluminum is rated for 40A, making it the minimum legal size for a 30-amp aluminum feeder. Always apply an antioxidant compound (like Noalox) to aluminum terminations to prevent galvanic corrosion and high-resistance arcing.
When an Engineer or the AHJ Must Confirm
While the guidelines above cover 95% of residential and light-commercial 30-amp circuits, specific load profiles require professional verification. Your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer must sign off on the following edge cases:
- Continuous Loads: If the 30-amp load will run for 3 hours or more continuously (e.g., commercial HVAC, EV chargers, server racks), NEC Article 210.20 requires the branch circuit to be sized at 125% of the continuous load. A 30A continuous load requires a breaker sized at 37.5A (next standard size up is 40A), which mandates 8 AWG copper wire, not 10 AWG.
- High Ambient Temperatures: If your conduit runs through an attic that routinely exceeds 30°C (86°F) in the summer, or is mounted on a hot roof deck, you must apply ambient temperature correction factors from the bottom of NEC Table 310.16. A 10 AWG wire in a 40°C attic loses roughly 12% of its ampacity, dropping it below the safe threshold for a 30A breaker.
- Motor Circuits: If the 30-amp breaker is protecting a large motor (like a well pump or air compressor), the wire is sized based on the motor's Full Load Amps (FLA) multiplied by 125%, while the breaker is sized for short-circuit and ground-fault protection (often 250% of FLA). In these cases, the breaker size and wire size intentionally mismatch, and standard branch-circuit rules do not apply.
For standard resistive loads, short-run receptacles, and typical appliance hookups, 10 AWG copper with a 30-amp breaker remains the gold standard. Verify your distances, check your conduit fill, and torque your lugs to the manufacturer's specifications (usually around 20-25 in-lbs for standard 30A residential breakers) to ensure a safe, long-lasting installation.
