You need 10 AWG copper wire for a standard 30 amp breaker. This applies to THHN/THWN-2 insulation in a standard raceway at 30°C ambient temperature. If you are using aluminum wire, you must step up to 8 AWG. Never use 12 AWG or 14 AWG, as they will overheat and cause a fire before the breaker trips.
The Baseline: NEC Ampacity and the 10 AWG Standard
When determining what size wire for a 30 amp breaker is required, the National Electrical Code (NEC) Table 310.16 is the definitive starting point. This table dictates the allowable ampacity of conductors based on their material, size, and insulation temperature rating.
A common trap for DIYers and junior apprentices is looking at the 90°C column because modern THHN wire is rated for 90°C. However, NEC 110.14(C) requires that the ampacity of the circuit be determined by the lowest temperature rating of any connected termination, conductor, or device. Since almost all standard residential and commercial breakers and receptacles are rated for 75°C terminations, you must use the 75°C column for your final sizing.
| AWG Size | 60°C Column | 75°C Column (Standard Terminations) | 90°C Column (THHN Insulation) |
|---|---|---|---|
| 14 AWG | 15A | - | - |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Notice that 10 AWG in the 75°C column is rated for 35A. Because 35A is greater than your 30A breaker, 10 AWG copper is the minimum legal and safe size. (Note: 12 AWG in the 90°C column shows 30A, but because your breaker lugs are 75°C, you cannot legally use 12 AWG on a 30A breaker).
Assumptions Block: When 10 AWG Copper Actually Works
Wire sizing is never a one-size-fits-all number; it is a calculation based on environmental and installation variables. The 10 AWG recommendation holds true only if the following baseline assumptions are met:
- Material: Solid or stranded copper (not aluminum or copper-clad aluminum).
- Insulation: THHN/THWN-2 or XHHW-2.
- Termination Rating: 75°C (standard for modern Square D, Eaton, and Siemens breakers).
- Ambient Temperature: 30°C (86°F) or lower. If running through an attic in a southern summer, ambient temperatures can exceed 45°C, requiring derating.
- Conduit Fill: No more than 3 current-carrying conductors in a single raceway. If you pull 4 to 6 current-carrying conductors, NEC 310.15(C)(1) requires an 80% derating factor.
Voltage Drop: The Hidden Reason to Upsize to 8 AWG
The NEC ampacity tables tell you the minimum size to prevent a fire. They do not guarantee your equipment will actually work properly at the end of the line. According to EC&M's NEC analysis, while voltage drop is largely a recommendation (NEC Informational Note 210.19(A)) rather than a strict mandate for most branch circuits, exceeding a 3% drop on a branch circuit causes motors to overheat, lights to dim, and electronics to brown out.
Let us run a voltage drop check at a stated distance of 100 feet for a 240V, 30A load (like a standard RV receptacle or a heavy-duty air compressor) using 10 AWG copper.
- Formula: VD = (2 × K × I × D) / CM
- K (Copper Resistivity): 12.9 ohms per mil-foot
- I (Current): 30 Amps
- D (Distance): 100 feet
- CM (Circular Mils for 10 AWG): 10,380
Calculation: VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts.
On a 240V circuit, a 7.45V drop is 3.1%. This slightly exceeds the recommended 3% maximum. If this were a 120V circuit, the drop would be a massive 6.2%. Therefore, if your one-way wire run exceeds 90 feet on a 240V circuit, or 45 feet on a 120V circuit, you must upsize to 8 AWG copper to maintain power quality. You can verify your specific run using the Southwire Voltage Drop Calculator.
Decision Tree: Finalizing Your Wire and Breaker Pick
Use this decision path to lock in your exact materials list before heading to the supply house. Do not deviate from the final output.
| Condition / Variable | If YES / TRUE | If NO / FALSE |
|---|---|---|
| Is the load continuous (runs for 3 hours or more)? | Stop. You cannot use a 30A breaker. A 30A breaker is only rated for 24A continuous. Upsize to a 40A breaker and 8 AWG wire. | Proceed to next step. |
| Are you using Aluminum wire (e.g., SER cable or AL USE-2)? | Select 8 AWG Aluminum (rated 40A at 75°C). Terminate with antioxidant paste. | Proceed to next step (assuming Copper). |
| Are there 4-6 current-carrying conductors in the conduit? | Apply 80% derating to the 90°C column. 10 AWG (40A × 0.8 = 32A) still passes, but 8 AWG is highly recommended for heat dissipation. | Proceed to next step. |
| Is the one-way run longer than 90 ft (240V) or 45 ft (120V)? | Select 8 AWG Copper THHN to mitigate voltage drop. | Select 10 AWG Copper THHN. |
Why Not Smaller? The Physics of Breaker Coordination
A frequent question on job sites is why we cannot use 12 AWG wire on a 30A breaker if the load only draws 20 amps. The answer lies in overcurrent protective device (OCPD) coordination.
A circuit breaker has two tripping mechanisms: a thermal bimetallic strip for slow overloads, and a magnetic solenoid for instantaneous short circuits. A standard 30A breaker will hold 30A indefinitely. It will not trip at 31A. In fact, it may take several minutes to trip at 40A.
If you use 12 AWG wire (rated for 20A) on a 30A breaker, and a fault causes the circuit to draw 28A, the wire will begin to overheat, melt its insulation, and potentially ignite surrounding framing. The 30A breaker will simply sit there, completely unaware of the danger, because 28A is below its trip threshold. The wire effectively becomes an unprotected, unfused heating element. Sizing the wire to match or exceed the breaker rating ensures the breaker always acts as the deliberate weak link in the system.
When to Call an Engineer or the AHJ
While the guidelines above cover standard branch circuits, specific edge cases require professional verification. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector) under the following conditions:
- High Ambient Environments: If the conduit runs through a boiler room, above a kiln, or in an unventilated attic where ambient temperatures regularly exceed 45°C (113°F), standard ampacity tables no longer apply, and severe derating is required.
- Motor Starting Currents: If the 30A breaker is protecting a motor with a massive locked-rotor amperage (LRA), NEC Article 430 allows for different sizing rules for the conductors versus the OCPD to prevent nuisance tripping during startup.
- Aluminum Terminations: If you are transitioning from 8 AWG aluminum feeder to a 30A breaker that does not explicitly list 'AL' or 'ALR' on its terminal lug stamping, the AHJ will fail the inspection. You must use a copper pigtail or a rated transition lug.
Always treat NEC-style guidance as the baseline minimum for safety and performance. Your local inspector has the final authority on code compliance, and local amendments may require larger wire sizes or specific conduit types depending on your municipality.
