The correct wire gauge for a standard 30 amp circuit is 10 AWG copper paired with a 30-amp breaker. This assumes 60°C or 75°C rated copper conductors, up to 3 current-carrying conductors in a raceway, and an ambient temperature of 30°C (86°F).
The Baseline: 10 AWG Copper and the Temperature Column
To understand why 10 AWG is the mandatory baseline, we have to look at how the National Electrical Code (NEC) rates wire ampacity. Wire isn’t just rated by its physical thickness; it is rated by the thermal limits of its insulation and the terminals it connects to.
- Material: Copper (Aluminum is addressed separately below)
- Temperature Column: 60°C (for NM-B cable) or 75°C (for THHN in conduit)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit/Cable Type: Standard EMT conduit or NM-B (Romex) romex cable
- Conductor Count: Maximum 3 current-carrying conductors in the raceway
According to Cerrowire’s standard NEC Table 310.16 ampacity charts, a 10 AWG copper conductor has the following allowable ampacities based on its insulation temperature rating:
| Wire Gauge | 60°C Column (NM-B) | 75°C Column (THHN/THWN) | 90°C Column (THHN) |
|---|---|---|---|
| 12 AWG | 20 Amps | 25 Amps | 30 Amps |
| 10 AWG | 30 Amps | 35 Amps | 40 Amps |
| 8 AWG | 40 Amps | 50 Amps | 55 Amps |
Why 10 AWG and Not 12 AWG?
A common mistake is looking at the 90°C column, seeing that 12 AWG is rated for 30 amps, and attempting to use it on a 30-amp breaker. This violates NEC 110.14(C), which governs termination temperatures. Standard 30-amp breakers and receptacles are typically rated for 75°C terminations, and NM-B cable is legally restricted to the 60°C column regardless of the fact that its internal THHN wires are rated for 90°C. Therefore, 12 AWG maxes out at 20 amps (NM-B) or 25 amps (THHN terminations). Pushing 30 amps through 12 AWG wire will cause the insulation to degrade, melt, and potentially ignite before the 30-amp breaker’s thermal trip mechanism engages.
Voltage Drop: When 10 AWG Isn’t Enough
Ampacity tables tell you what size wire prevents a fire. Voltage drop calculations tell you what size wire actually delivers usable power to the load. The NEC recommends a maximum 3% voltage drop for branch circuits.
Let’s run the math on a 240V, 30-amp circuit (like a NEMA 14-30 dryer outlet) using 10 AWG copper at a distance of 100 feet from the panel.
- Formula: VD = (2 × K × I × L) / CM
- K (Copper resistivity): 12.9
- I (Current): 30 Amps
- L (One-way length): 100 feet
- CM (Circular Mils for 10 AWG): 10,380
Calculation: VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts.
Percentage: 7.45V / 240V = 3.1%.
At exactly 100 feet on a 240V circuit, 10 AWG is borderline, slightly exceeding the 3% recommendation. If your run is 125 feet, the drop hits 3.9%, and you must upgrade to 8 AWG copper.
Note on 120V RV Circuits: If you are wiring a 120V 30-amp TT-30 RV receptacle, that same 7.45V drop represents a 6.2% loss on a 120V system. For 120V 30-amp circuits over 50 feet, 8 AWG or even 6 AWG is required to maintain voltage integrity.
Decision Tree: Adjusting for Real-World Conditions
Use this decision matrix to finalize your exact wire pick. Start at the top and stop when your specific installation scenario is met.
| Installation Condition | Required Action / Wire Size |
|---|---|
| Standard run < 80 ft, Copper, ≤ 3 conductors | 10 AWG Copper (Baseline) |
| Run is 80 ft to 150 ft (240V circuit) | Upgrade to 8 AWG Copper |
| Run is > 50 ft (120V TT-30 RV circuit) | Upgrade to 8 AWG Copper (or 6 AWG if > 100 ft) |
| 4 to 6 current-carrying conductors in one conduit | Derate ampacity by 80%. 10 AWG (35A × 0.8 = 28A) fails. Use 8 AWG Copper. |
| Ambient temperature exceeds 30°C (86°F) | Apply NEC Table 310.15(B)(1) correction factors. Usually requires 8 AWG Copper. |
| Using Aluminum wire instead of Copper | Use 8 AWG Aluminum minimum (See section below). |
| Load is Continuous (runs for 3+ hours) | Calculate at 125%. A 30A continuous load requires a 40A circuit (8 AWG Copper + 40A breaker). |
Aluminum vs. Copper: The 8 AWG Mandate
Copper and aluminum are not interchangeable. Aluminum has a higher electrical resistance and expands/contracts more under thermal cycling, which historically caused loose terminations and fires if not installed with specific anti-oxidant paste and CO/ALR rated terminals.
If you choose to use aluminum wire (often done for long feeder runs to save money, though less common for 30-amp branch circuits), 10 AWG aluminum is not permitted for standard branch circuit wiring in most jurisdictions due to mechanical fragility at terminations. You must step up to 8 AWG aluminum.
According to the 75°C column of NEC Table 310.16, 8 AWG aluminum is rated for 40 amps. This safely covers your 30-amp breaker requirement while providing the physical mass needed for a secure mechanical connection at the breaker lug. Always apply a UL-listed antioxidant compound (like Noalox) to stripped aluminum conductors before termination to prevent galvanic corrosion.
Continuous Loads and AHJ Overrides
The most misunderstood concept in breaker sizing is the continuous load rule (NEC Article 210.20). A continuous load is any load where the maximum current is expected to continue for 3 hours or more. Examples include commercial lighting, server racks, or heavy-duty space heaters.
If your 30-amp load is continuous, you cannot use a 30-amp breaker. The NEC requires the branch circuit to be sized at 125% of the continuous load.
- Load: 30 Amps (Continuous)
- Required Circuit Rating: 30A × 1.25 = 37.5 Amps
- Next Standard Breaker Size: 40 Amps (per NEC 240.6)
- Required Wire Gauge: 8 AWG Copper (rated 40A at 60°C / 50A at 75°C)
Conversely, if you are locked into an existing 30-amp breaker, the maximum continuous load you can legally and safely place on that circuit is 24 amps (30A / 1.25). If you are wiring a dedicated circuit for a specific piece of equipment, always check the manufacturer’s nameplate for the "Minimum Circuit Ampacity" (MCA) and "Maximum Overcurrent Protection" (MOP) values. The MCA dictates your wire gauge; the MOP dictates your breaker size. When in doubt, or if your installation involves complex bundling, high ambient heat in an attic, or utility interconnections, consult a licensed electrical engineer or your local AHJ inspector before pulling wire.
