For a standard 30-amp circuit, use 10 AWG copper wire with a 30-amp breaker. If using aluminum, step up to 8 AWG. This assumes copper THHN/THWN-2 insulation, 75°C terminations, and 30°C ambient.
- Material: Solid or stranded copper (unless aluminum is explicitly stated).
- Insulation: THHN/THWN-2 (rated for 90°C in dry locations, 75°C in wet).
- Termination Rating: 75°C (standard for modern breakers and receptacles).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single circuit in a raceway (conduit) or standard NM-B cable, with no more than 3 current-carrying conductors.
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Baseline Sizing and NEC Ampacity Table
To understand why 10 AWG is the correct choice, we have to look at NEC Table 310.16, which dictates the allowable ampacities for insulated conductors. The table is divided into temperature columns (60°C, 75°C, and 90°C). While THHN wire is physically rated for 90°C, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component, which is almost always the 75°C terminal lugs on your breaker or receptacle.
| AWG Size | 60°C Column | 75°C Column (Standard) | 90°C Column (Derating only) |
|---|---|---|---|
| 14 AWG | 15A | — | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Looking at the 75°C column, 10 AWG copper is rated for 35 amps. Because 35A is greater than our 30A load and 30A breaker, 10 AWG is the minimum legal and safe size for standard runs.
Why 10 AWG Copper? (And Why 12 AWG Fails)
A common mistake on the workbench or jobsite is looking at the 90°C column, seeing that 12 AWG is rated for 30A, and attempting to use it on a 30-amp breaker. This is a severe code violation and a fire hazard.
The breaker's primary job is to protect the wire from melting, not the appliance. If you use 12 AWG wire (which is limited to 20A at 60°C or 25A at 75°C for terminations) on a 30-amp breaker, the breaker will not trip until the current exceeds 30 amps. At 28 amps, the 12 AWG wire is being pushed beyond its terminal thermal limits, but the breaker remains closed. Over time, this causes the termination lugs to overheat, oxidize, and eventually arc or ignite.
Furthermore, standard NM-B (Romex) cable is legally restricted to the 60°C column regardless of its internal wire insulation, per NEC 334.80. In the 60°C column, 10 AWG is rated for exactly 30A, making it the absolute floor for a 30A circuit using standard indoor cable.
Voltage Drop: The Hidden Gauge Killer
Ampacity tables assume a short run. When wire gets longer, resistance increases, causing voltage to drop before it reaches the load. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total from the service entrance. For a 120V circuit, a 3% drop is 3.6V. For 240V, it is 7.2V.
We calculate voltage drop using the formula: VD = (2 × K × I × L) / CM
- K = 12.9 (resistivity constant for copper)
- I = 30 Amps
- L = One-way length in feet
- CM = Circular mils of the wire (10 AWG = 10,380 CM; 8 AWG = 16,510 CM per the Engineering Toolbox AWG standards)
Scenario A: 50-Foot Run (120V)
VD = (2 × 12.9 × 30 × 50) / 10,380 = 3.73V.
Percentage: 3.73 / 120 = 3.1%. This is acceptable and within standard tolerances.
Scenario B: 100-Foot Run (120V)
VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.46V.
Percentage: 7.46 / 120 = 6.2%. This fails the 3% branch circuit recommendation. You must step up to 8 AWG to compensate for the distance.
| One-Way Distance | 120V Circuit | 240V Circuit | Required AWG |
|---|---|---|---|
| Up to 45 ft | Pass (<3%) | Pass (<1.5%) | 10 AWG |
| 46 to 90 ft | Fail (>3%) | Pass (<3%) | 10 AWG (240V only) 8 AWG (120V) |
| 91 to 140 ft | Fail (>3%) | Fail (>3%) | 8 AWG (Both) |
Derating Factors: Bundling, Heat, and Aluminum
The baseline 10 AWG answer evaporates the moment you change the physical environment of the wire. Here is what forces you to upsize.
Conduit Bundling (More Than 3 Conductors)
When you pull multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) mandates derating. If you have 4 to 6 current-carrying conductors in a raceway, you must multiply the wire's ampacity by 80%.
For derating, we are allowed to use the 90°C column. 10 AWG at 90°C is 40A. 40A × 0.80 = 32A. This still clears the 30A hurdle. However, if you have 7 to 9 conductors, the derating factor drops to 70%. 40A × 0.70 = 28A. Your 10 AWG wire is now legally limited to 28 amps. You must upsize to 8 AWG to maintain a 30A circuit in a heavily packed conduit.
Ambient Temperature
If your conduit runs through an attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors. At 40°C (104°F), the correction factor for 90°C wire is 0.91. 40A × 0.91 = 36.4A. You are still safe with 10 AWG, but if the attic hits 50°C (122°F), the factor drops to 0.82, yielding 32.8A. You are running out of thermal headroom, and an inspector may require 8 AWG.
Aluminum Conductors
Aluminum has higher resistance and expands/contracts more than copper under thermal cycling. You cannot use the same gauge for aluminum as you do for copper. For a 30-amp circuit using aluminum (like SER cable or XHHW-2 in conduit), you must use 8 AWG aluminum, which is rated for 40A in the 75°C column. Never use 10 AWG aluminum for a 30A load; it is only rated for 25A at 75°C.
Continuous Loads and AHJ Sign-Off
The final variable is the nature of the load itself. The NEC defines a continuous load as any load where the maximum current is expected to continue for 3 hours or more. Examples include EV chargers, hardwired space heaters, and commercial lighting.
Under NEC 210.20(A), overcurrent protection for continuous loads must be rated at 125% of the continuous current. If your 30-amp load is continuous (e.g., a 30A EV charger drawing 24A continuously, or a 30A heater), you must size the breaker and wire for 30A × 1.25 = 37.5A.
A 30-amp breaker and 10 AWG wire will fail this requirement. For a 30A continuous load, you must install a 40-amp breaker and pull 8 AWG copper wire.
- Complex Derating: If you are combining high ambient temperatures with conduit bundling (e.g., 6 wires in a hot attic), the math compounds. Have a professional engineer or your local inspector verify the final ampacity.
- Local Amendments: Some municipalities ban aluminum entirely for branch circuits or require 125% sizing for all EV chargers regardless of the manufacturer's stated draw. Always check local codes before pulling wire.
- Service Entrance Work: If this 30A circuit requires adding a new subpanel or upgrading your main service, this is licensed electrician territory.
Sizing wire correctly is about matching the breaker to the wire's weakest thermal link, then adjusting for distance and environment. Stick to 10 AWG copper for standard, short-run 30A circuits, step up to 8 AWG for long distances or aluminum, and always respect the 125% continuous load rule.
