The correct wire gauge for a 30 amp 220v circuit is 10 AWG copper wire, protected by a 30-amp double-pole breaker. This assumes THHN/THWN-2 insulation in standard conduit at 30°C ambient. If your run exceeds 85 feet, step up to 8 AWG copper to maintain voltage drop below 3%.
- Conductor: Copper (not aluminum)
- Insulation: THHN/THWN-2 in EMT conduit, or NM-B (Romex) cable
- Temperature Column: 75°C for THHN terminations, 60°C for NM-B
- Ambient Temperature: 30°C (86°F) or lower
- Load Type: Non-continuous (runs for less than 3 hours at a time)
The Baseline Spec: 10 AWG Copper and the 240V Reality
When wiring a dedicated 220V circuit—commonly used for welders, air compressors, or heavy-duty power tools—you are actually working with a nominal 240V residential split-phase system. The "220V" label is a legacy term; modern US utility transformers deliver 240V across the two hot legs.
For a standard 30-amp receptacle (like a NEMA 6-30R or L6-30R), you need a 30-amp double-pole breaker and two current-carrying conductors. Using 10 AWG copper wire is the industry standard here. You will pull a black hot, a red hot (or a white wire re-identified with red/black tape at both ends per NEC 200.7), and a 10 AWG bare or green equipment grounding conductor. If you are wiring a 120/240V appliance like a dryer that requires a neutral, you will add a 10 AWG white neutral and use a 4-prong NEMA 14-30R receptacle.
Ampacity and Voltage Drop: The Math Behind the Pick
Wire sizing is governed by two distinct limits: thermal ampacity (how much heat the wire can safely dissipate) and voltage drop (how much voltage is lost to resistance over distance).
Thermal Ampacity (NEC Table 310.16)
According to the National Electrical Code (NFPA 70), 10 AWG copper wire has different ampacities depending on the insulation and termination temperatures. Under NEC 110.14(C), you must size your wire based on the lowest temperature rating of any connected termination, device, or conductor. Most standard 30A breakers and receptacles are rated for 75°C.
| Wire Size (AWG) | Material | 60°C Column (NM-B Cable) | 75°C Column (THHN Terminations) | 90°C Column (Conduit Derating Only) |
|---|---|---|---|---|
| 12 AWG | Copper | 20A | 25A | 30A |
| 10 AWG | Copper | 30A | 35A | 40A |
| 8 AWG | Copper | 40A | 50A | 55A |
Looking at the 10 AWG row in the 75°C column, the wire is rated for 35 amps. Because the overcurrent protective device (the breaker) is 30 amps, the wire is fully protected. If you are using NM-B (Romex) cable, you are forced into the 60°C column, where 10 AWG is rated for exactly 30 amps. This is perfectly legal for a 30A breaker under the next-size-up rule, but it leaves zero thermal headroom.
Voltage Drop at Distance
Ampacity tells you the wire won't melt; voltage drop tells you your tool will actually run correctly. The NEC recommends a maximum 3% voltage drop on branch circuits. Using the standard Southwire voltage drop formula for single-phase AC:
Voltage Drop = (2 × K × I × D) / Circular Mils
- K (Copper resistance constant) = 12.9 ohms
- I (Current) = 30 amps
- D (Distance) = 100 feet
- Circular Mils (10 AWG) = 10,380
At 100 feet, the drop is 7.45 volts. Divided by 240V, that is a 3.1% drop, which exceeds the 3% recommendation. To stay under 3%, the maximum distance for 10 AWG copper at a full 30A load is 85 feet. Beyond that, you must step up to 8 AWG.
Decision Tree: When to Step Up to 8 AWG
Use this decision path to finalize your material list. Follow the conditions down to your concrete pick.
| Condition / Scenario | If True... | Concrete Pick |
|---|---|---|
| Total one-way wire run is under 85 feet | Ampacity and VD are satisfied | 10 AWG Copper + 30A Breaker |
| Total one-way wire run is 86 to 130 feet | VD exceeds 3% on 10 AWG | 8 AWG Copper + 30A Breaker |
| Run is over 130 feet | VD exceeds 3% on 8 AWG | 6 AWG Copper + 30A Breaker |
| More than 3 current-carrying conductors in conduit | NEC 310.15(C)(1) requires 80% derating | 8 AWG Copper (THHN) + 30A Breaker |
| Load runs continuously for 3+ hours | NEC 210.20(A) requires 125% sizing (37.5A) | 8 AWG Copper + 40A Breaker (if load allows) or 30A breaker with oversized wire |
| Using Aluminum wire instead of Copper | Aluminum has higher resistance and lower ampacity | 8 AWG Aluminum + 30A Breaker |
Why 12 AWG Fails and Aluminum Changes the Rules
A common and dangerous mistake on the jobsite is assuming that because a 12 AWG wire physically fits into a 30A breaker lug, it is safe to use. 12 AWG copper is rated for 20 amps (60°C column) or 25 amps (75°C column). If you pull 30 amps through 12 AWG wire, the 30-amp breaker will not trip, but the wire will overheat, potentially melting the insulation and starting a fire inside the wall cavity before the breaker ever senses a fault. The breaker protects the wire, not the appliance; you must never use a wire with an ampacity lower than the breaker rating.
Continuous Loads and AHJ Sign-Off
The baseline 10 AWG recommendation assumes a non-continuous load—meaning the equipment cycles off or runs for less than three hours at a time. This covers most welders, air compressors, and table saws.
However, if your 30-amp circuit is feeding a continuous load (like a 240V baseboard heater, a kiln, or an EV charger configured to draw exactly 24 amps continuously), NEC Article 210.20(A) mandates that the branch circuit be rated at 125% of the continuous load. A 30A continuous load requires a circuit rated for 37.5 amps. In this scenario, 10 AWG copper (35A at 75°C) is illegal. You must step up to 8 AWG copper, and if the load truly is 30A continuous, you must install a 40-amp breaker, provided the equipment nameplate allows it.
Whenever you are dealing with continuous loads, high-ambient environments (like running conduit across an unventilated attic in a southern summer where temperatures exceed 110°F), or complex conduit bundling, the standard tables no longer apply cleanly. In these edge cases, the derating factors compound rapidly. This is the exact threshold where you must stop guessing, pull out the full NEC Article 310 derating tables, and have your calculations confirmed by a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ) before pulling wire.
