For a standard 30-amp circuit, you need 10 AWG copper wire or 8 AWG aluminum wire paired with a 30-amp breaker. This assumes standard THHN/THWN-2 insulation in a 75°C termination environment. Never use 12 AWG or 14 AWG, as they will overheat and trip the breaker or cause a fire.
- Material: Copper (default) / Aluminum (noted separately)
- Temperature Column: 75°C (standard for modern breakers and receptacles)
- Ambient Temperature: 30°C (86°F)
- Conduit/Raceway: Standard NM-B cable or THHN in PVC/EMT conduit
- Phases: Single-phase, standard residential (120V or 240V)
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Core Ampacity Rules: Why 10 AWG Copper?
To understand why 10 AWG is the minimum, we look at NEC Table 310.16, which dictates the allowable ampacity of insulated conductors. While modern THHN/THWN-2 wire is rated for 90°C, the National Electrical Code (NEC) 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected device, terminal, or splice.
Almost all standard 30-amp breakers and receptacles (like the NEMA 14-30 dryer outlet or L14-30 generator inlet) are rated for 75°C terminations. Therefore, we must use the 75°C column for sizing.
| Wire Gauge (AWG) | 60°C Column | 75°C Column (Used for Sizing) | 90°C Column (Used for Derating) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Why not use one size smaller (12 AWG)?
A 12 AWG copper wire is rated for 25 amps in the 75°C column. If you connect it to a 30-amp breaker, the breaker will not trip until the current exceeds 30 amps. If a fault or heavy load draws 28 amps, the 12 AWG wire will overheat, melt its insulation, and potentially start a fire inside the wall long before the breaker ever trips. The breaker's primary job is to protect the wire, not the appliance.
When the Standard Answer Changes: Voltage Drop and Derating
The 10 AWG rule holds true for short runs, but physics takes over on longer circuits. The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently and safely. Let us run a voltage drop check for a common 30-amp scenario: a 240V electric dryer or RV outlet located 100 feet from the panel.
Using the standard voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 30A
- D (Distance) = 100 ft
- CM (Circular mils for 10 AWG) = 10,380
VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts.
Percentage drop: (7.45 / 240) × 100 = 3.1%. This slightly exceeds the 3% NEC informational recommendation. To keep the drop under 3% at 100 feet, you must step up to 8 AWG copper.
| One-Way Distance | Required Wire Size | Reasoning |
|---|---|---|
| Under 70 feet | 10 AWG | Ampacity and voltage drop are within safe limits. |
| 70 to 120 feet | 8 AWG | 10 AWG exceeds 3% voltage drop; 8 AWG keeps it under 2%. |
| Over 120 feet | 6 AWG | Significant distance requires larger mass to prevent severe voltage sag. |
Conduit Bundling and Derating
If you are pulling THHN wires through a conduit and have more than three current-carrying conductors in the same pipe, you must apply an adjustment factor per NEC Table 310.15(C)(1). For 4 to 6 conductors, you multiply the 90°C ampacity by 80%.
For 10 AWG, the 90°C rating is 40A. 40A × 0.80 = 32A. While 32A still technically covers a 30A breaker, if you add a seventh conductor, the derating drops to 70% (40A × 0.70 = 28A). At that point, 10 AWG is illegal, and you must upgrade to 8 AWG.
Copper vs. Aluminum and Continuous Loads
Aluminum wire is lighter and cheaper, but it has higher resistance and expands/contracts more under heat. For a 30-amp circuit, 8 AWG aluminum is the standard recommendation. While 10 AWG aluminum is technically rated for 30A at 75°C, using 8 AWG provides a crucial buffer for voltage drop and ensures the wire physically fits securely into lugs designed for larger gauge feeders.
The 125% Continuous Load Rule
If your 30-amp load is considered 'continuous' by the NEC—meaning it is expected to run at maximum current for 3 hours or more (like a heavy duty space heater, a continuous-duty air compressor, or EV charging equipment)—you must size the wire and breaker for 125% of the load.
30A × 1.25 = 37.5A. A standard 30-amp breaker is no longer sufficient; you need a 40-amp breaker and 8 AWG copper wire. When dealing with continuous loads, complex solar backfeed calculations, or heavy derating scenarios, this is the exact point where you must stop and have a licensed professional engineer or your local AHJ confirm your calculations.
Frequently Asked Questions
Can I use 10 AWG wire on a 40-amp breaker?
No. The breaker must always be sized to protect the weakest point of the wire. 10 AWG copper is rated for a maximum of 30 amps in standard residential applications. If you place it on a 40-amp breaker, a 35-amp fault will melt the wire inside your walls without tripping the breaker. If your appliance requires a 40-amp breaker, you must use a minimum of 8 AWG copper wire.
What size wire do I need for a 30-amp RV outlet (NEMA TT-30)?
A NEMA TT-30 receptacle is a 120V, 30-amp outlet. Because it operates at 120V rather than 240V, voltage drop is twice as aggressive over the same distance. While 10 AWG copper is the code minimum for short runs, a 50-foot run at 120V and 30A yields a 3.7% voltage drop. For RV pedestals, it is highly recommended to use 8 AWG copper to ensure sensitive RV electronics do not suffer from low-voltage brownouts.
Does the equipment grounding wire need to be 10 AWG too?
Yes, in most cases. According to NEC Table 250.122, the minimum size equipment grounding conductor is based on the rating of the overcurrent device. For a 30-amp breaker, Table 250.122 explicitly requires a minimum of 10 AWG copper (or 8 AWG aluminum). If you are using NM-B (Romex) cable, the bare copper ground included inside a 10/2 or 10/3 cable perfectly meets this requirement.
What if I am wiring a 30-amp 12V DC system (like solar or marine)?
DC wiring follows entirely different rules due to the low voltage. A 30-amp draw on a 12V system over just 10 feet will cause massive voltage drop if you use standard AC wire sizes. For a 12V, 30A DC circuit, you typically need 4 AWG or 2 AWG copper wire to keep the voltage drop under 3% and prevent the wire from acting as a heating element. Always use a DC-specific voltage drop calculator for low-voltage systems.
