For a standard 220-volt, 30-amp circuit, you need 10 AWG copper wire protected by a 30-amp double-pole breaker. This assumes standard THHN/THWN-2 insulation in a conduit or NM-B (Romex) cable, rated at the 75°C column, with an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together.
- Material: Copper (Aluminum requires different sizing, covered below).
- Temperature Rating: 75°C column (standard for most residential breakers and terminals).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single cable in open air, or individual THHN conductors in a conduit with 3 or fewer current-carrying wires.
The Core Sizing Rules: Why 10 AWG and Not Smaller?
The National Electrical Code (NEC) dictates wire sizing based on ampacity tables and overcurrent protection limits. According to the Cerro Wire NEC Ampacity Charts (which reflect NEC Table 310.16), 10 AWG copper wire with 75°C insulation (like THHW or the 75°C rating of THHN/THWN-2) has an ampacity of 35 amps. Since 35A is greater than your 30A load and breaker, 10 AWG is legally and physically sufficient.
Why not use 12 AWG wire? This is a common DIY trap. If you look at the 90°C column of the ampacity table, 12 AWG copper is rated for 30 amps. However, NEC Article 240.4(D) establishes strict limits for small conductors. It explicitly states that the overcurrent protection for 12 AWG copper shall not exceed 20 amps, regardless of the 90°C insulation rating. You cannot legally or safely put a 30-amp breaker on 12 AWG wire. The breaker would allow 30 amps to flow, but the wire's termination points (which are rarely rated for 90°C) and the conductor itself could overheat before the breaker trips.
| Property | Value / Rating |
|---|---|
| Wire Gauge (AWG) | 10 AWG |
| Material | Solid or Stranded Copper |
| Insulation Type | THHN/THWN-2 or NM-B (Romex) |
| Ampacity (75°C Column) | 35 Amps |
| Max Overcurrent Protection | 30 Amps (Standard Breaker Size) |
| Circular Mils (CM) | 10,380 |
Variables That Force an Upsize (Length, Bundling, Aluminum)
The baseline 10 AWG answer only holds true if your installation matches the assumptions perfectly. Real-world jobsite conditions frequently force you to upsize to 8 AWG copper. Here is the decision framework for when 10 AWG is no longer acceptable.
| Condition | NEC Rule | Action Required |
|---|---|---|
| 4 to 6 current-carrying conductors in the same conduit | NEC 310.15(C)(1) | Derate ampacity to 80%. 35A × 0.80 = 28A. Since 28A < 30A, you must upsize to 8 AWG. |
| Ambient temperature exceeds 86°F (30°C) | NEC 310.15(B)(1) | Apply temperature correction factors. In a 110°F attic, 10 AWG derates below 30A. Upsize to 8 AWG. |
| Using Aluminum wire instead of Copper | NEC 310.16 / 110.14 | 10 AWG Al is only 30A at 75°C. Due to mechanical fragility and terminal compatibility, use 8 AWG Al minimum. |
| Run length exceeds 95 feet | NEC 310.15(B) / Info Note | Voltage drop exceeds 3% at full load. Upsize to 8 AWG to maintain equipment efficiency. |
A note on Aluminum vs. Copper: Never treat these interchangeably. Aluminum has a higher resistance per circular mil and expands/contracts more under heat. While 10 AWG aluminum is technically rated for 30 amps at 75°C, most electricians will not pull 10 AWG aluminum for a 30A feeder. The wire is mechanically stiff, prone to breaking if nicked, and many 30A receptacle terminals are only rated for copper or require specific CO/ALR markings. If you are running aluminum (like SER cable for a subpanel feed), standard practice is to use 8 AWG aluminum for a 30A circuit to ensure mechanical durability and terminal compatibility.
Voltage Drop Verification at Distance
While the NEC does not strictly mandate voltage drop limits for standard branch circuits (it is an 'Informational Note' recommendation), keeping voltage drop under 3% is critical for the lifespan of 220V appliances like compressors, welders, and dryers. A motor running on low voltage will draw higher amperage to compensate, leading to overheating.
Let us calculate the maximum run length for 10 AWG copper on a 240V nominal circuit (colloquially called 220V) at a full 30-amp load.
- Target Voltage Drop (3% of 240V): 7.2 Volts
- K (Copper Resistivity): 12.9 ohms-cmil/ft
- I (Current): 30 Amps
- CM (Circular Mils for 10 AWG): 10,380
Using the standard single-phase voltage drop formula rearranged for length:
Length = (VD × CM) / (2 × K × I)
Length = (7.2 × 10,380) / (2 × 12.9 × 30)
Length = 74,736 / 774 = 96.5 feet
If your wire run from the breaker panel to the outlet is 95 feet or less, 10 AWG copper is perfectly fine. If the run is 100 feet or longer, you must upsize to 8 AWG copper to prevent the voltage drop from exceeding the 3% threshold under maximum load.
When an Engineer or the AHJ Must Confirm
There are specific scenarios where the standard '10 AWG on a 30A breaker' rule fails, and you must consult the National Electrical Code (NFPA 70) or your local inspector.
1. Continuous Loads (NEC Article 100 & 210.20): A continuous load is defined as any load where the maximum current is expected to continue for 3 hours or more. Examples include kilns, large baseboard heaters, or EV chargers. For continuous loads, the branch circuit must be sized at 125% of the load. If your 220V equipment draws 30 amps continuously, you must multiply 30A × 1.25 = 37.5 amps. You must then round up to the next standard breaker size, which is 40 amps. A 40-amp breaker requires 8 AWG copper wire.
2. High-Temperature Environments: If you are routing conduit through an attic in a southern climate where ambient temperatures regularly exceed 104°F (40°C), the temperature correction factors in NEC Table 310.15(B)(1) will severely derate 10 AWG wire. An electrical engineer or local AHJ must verify the derating calculations to ensure the wire will not melt its insulation.
Frequently Asked Questions
Can I use 10/2 Romex for a 220V 30-amp dryer outlet?
No. Modern NEC code (Article 250.140) requires a 4-wire setup for new dryer installations to separate the neutral and ground paths, eliminating the risk of chassis electrification. You must use 10/3 NM-B (Romex), which contains two hot wires (black and red), one neutral (white), and one bare equipment ground. Using 10/2 for a new dryer outlet is a code violation and a severe shock hazard.
What size wire for a 220 volt 30 amp welder?
Welders are governed by NEC Article 630, which allows for smaller wire sizes based on the machine's duty cycle. If your welder has a 30-amp plug but a duty cycle of 30% at its max output, the effective current draw is much lower, and the nameplate will specify the exact minimum circuit ampacity (MCA). However, if the nameplate specifies a 30A circuit, or if you are wiring a generic 30A twist-lock receptacle (like a NEMA L6-30R) for future use, stick to the standard 10 AWG copper rule.
Does the voltage (220V vs 240V) change the wire size?
No. '220V' is an outdated colloquial term; the actual nominal voltage in US residential split-phase systems is 240V (measured hot-to-hot). Wire sizing is determined by amperage (current), not voltage. The voltage only dictates the insulation rating (which must be at least 600V for standard THHN/NM-B) and the voltage drop calculation baseline. Whether you call it 220V, 230V, or 240V, a 30-amp load requires 10 AWG copper.
Can I use aluminum wire for a 30-amp 220V circuit?
Yes, but you must upsize. As noted in the decision tree, 10 AWG aluminum is technically rated for 30 amps at 75°C. However, because aluminum is brittle and prone to cold creep under terminal screws, standard industry practice is to use 8 AWG aluminum for a 30A feeder. Furthermore, you must ensure the breaker and receptacle terminals are explicitly rated for aluminum (marked as 'AL' or 'CU/AL') and apply an antioxidant compound like Noalox to the stripped wire ends before termination to prevent galvanic corrosion.
