The correct wire size for a 220V 30-amp circuit is 10 AWG copper, which safely carries the load without exceeding the insulation's temperature rating or causing the breaker to trip prematurely. While we colloquially say "220V," the modern nominal voltage in North America is 240V, but the voltage level does not change the required wire gauge—only the amperage does. For a standard residential run under 50 feet, you need a 2-conductor cable with a ground (like 10/2 NM-B) protected by a 30-amp double-pole breaker.

The Physics of Ampacity: Why 10 AWG?

Ampacity is not a measure of how much electricity a wire can "hold"; it is a measure of how much heat the wire can safely dissipate into its surroundings before the insulation degrades. The physical cross-sectional area of the copper dictates its electrical resistance. A smaller wire has higher resistance, converting electrical energy into heat. If that heat cannot dissipate fast enough inside a wall cavity or conduit, the insulation melts, leading to a short circuit or fire.

What wire size changes in a real circuit: Stepping up from 12 AWG to 10 AWG increases the copper cross-section by roughly 59%. This lowers the resistance per foot, directly reducing the voltage drop at the appliance and slashing the amount of waste heat trapped inside your wall insulation.

Worked Numeric Example: Heat Dissipation

Let us look at the actual physics using data from the Cerro Wire Ampacity Charts and standard copper resistance tables. The resistance of 10 AWG copper is roughly 1.018 ohms per 1,000 feet. For a 50-foot run (100 feet total for the out-and-back circuit), the resistance is 0.1018 ohms.

Assume a continuous 24A draw (which is 80% of 30A, the legal maximum for continuous loads under NFPA 70: National Electrical Code Article 210.20).

  • Voltage Drop: V = I × R (24A × 0.1018Ω = 2.44V drop)
  • Heat Dissipated in Wire: P = I² × R (576 × 0.1018 = 58.6 watts)

If you incorrectly used 12 AWG wire (1.588 ohms/kft), the resistance becomes 0.1588 ohms. The heat dissipation jumps to 91.4 watts—a 56% increase in heat trapped inside your wall. Furthermore, 12 AWG is only rated for 20A in the 60°C column. Pushing 24A through it will eventually trip a 20A breaker, or if you mistakenly paired it with a 30A breaker, the wire will overheat and fail before the breaker ever trips.

Where You Meet This in Practice

You will typically encounter the 240V 30-amp requirement in specific heavy-duty residential and workshop applications. Recognizing these helps you plan your rough-in correctly.

  • Electric Dryers (NEMA 14-30): Older or compact electric dryers often require a 30-amp circuit. This requires 10/3 NM-B (two hots, a neutral, and a ground) because the dryer uses 120V for the timer and motor, and 240V for the heating element.
  • Welders and Air Compressors (NEMA 6-30): Pure 240V tools do not require a neutral. A 10/2 NM-B cable (two hots and a ground) is perfectly suited here.
  • Level 2 EV Chargers: Many entry-level EV chargers draw 24 amps continuously. Because 24A is exactly 80% of 30A, they pair perfectly with a 30-amp double-pole breaker and 10 AWG wire. (Note: If your charger draws 32A continuous, you must step up to a 40A breaker and 8 AWG wire).
  • Window AC / PTAC Units: Large 240V through-the-wall heat pump units in apartments or additions frequently specify a 30-amp minimum circuit ampacity (MCA).
Pro-Tip for EV Chargers: Even if your current EV charger only pulls 24A (requiring 10 AWG), consider pulling 8 AWG wire and installing a 40-amp breaker if the conduit is already open. Future EV chargers will likely demand 32A or 48A, and re-pulling wire later costs triple the initial material savings.

Common Confusions and Code Traps

When sizing wire for 220V/240V circuits, DIYers and even junior apprentices frequently fall into a few specific traps that violate NEC-style guidance.

Confusion 1: Believing voltage dictates wire size.
Voltage does not determine the AWG; amperage does. A 120V 30-amp circuit (like an RV TT-30 outlet) and a 240V 30-amp circuit both require 10 AWG copper. The voltage only dictates the insulation thickness rating (usually 300V or 600V) and whether you need a single-pole or double-pole breaker.

Confusion 2: Sizing the breaker to the appliance, not the wire.
The breaker protects the wire, not the appliance. If your compressor nameplate says it draws 22 amps, you might be tempted to use 12 AWG wire (rated 20A) and a 30A breaker because the compressor has internal overload protection. This is a severe fire hazard. The 30A breaker will allow 28 amps to flow through the 20A-rated 12 AWG wire during a startup surge or fault, melting the insulation long before the breaker trips. Always match the breaker to the wire's ampacity limit.

Confusion 3: Ignoring the 60°C vs 75°C temperature column.
Under NEC Table 310.16, 10 AWG THHN wire in a conduit is technically rated for 35A or even 40A at higher temperature columns. However, NEC 110.14(C) requires you to size the circuit based on the lowest temperature rating of any termination in the circuit. Since most standard residential breakers and receptacles are rated for 60°C or 75°C, and standard NM-B cable is strictly limited to the 60°C column, 10 AWG is legally capped at 30 amps for residential branch circuits.

Decision Path: Choosing Your Exact Cable Type

Knowing you need 10 AWG copper is only half the battle; you must select the correct jacket and insulation type for your specific installation environment. Use this decision tree to lock in your materials list.

Installation Environment Required Cable Type Why This Type?
Concealed inside standard residential walls (studs/drywall) 10/2 or 10/3 NM-B Non-metallic sheath is approved for concealed dry locations and staples directly to studs.
Exposed along basement ceilings or masonry walls 10 AWG THHN in EMT conduit NM-B cannot be used where subject to physical damage; THHN in metal conduit provides impact protection.
Underground direct burial to a detached workshop 10/2 UF-B Underground Feeder cable has a solid moisture-resistant jacket that does not require conduit when buried at 24 inches.
Wet locations or outdoor exposed conduit 10 AWG THWN-2 THWN-2 is rated for wet locations, unlike standard THHN which is strictly for dry indoor use.
The Default Concrete Pick: For 90% of indoor residential retrofits (running a new dryer or welder outlet through finished or unfinished walls), buy Southwire 10/2 NM-B with ground (if no neutral is needed) or 10/3 NM-B (if a NEMA 14-30 neutral is required). It is universally available, easy to strip, and strictly compliant with NEC Article 334 for dry, concealed indoor locations.

FAQ: 240V 30-Amp Wiring Questions

Do I need a neutral wire for a 220V 30-amp outlet?
It depends entirely on the receptacle type. A NEMA 6-30 (used for welders and compressors) is a pure 240V circuit and only requires two hot wires and a ground (10/2 cable). A NEMA 14-30 (used for modern electric dryers) requires two hots, a neutral, and a ground (10/3 cable) because the appliance uses 120V for internal electronics.

Can I use aluminum wire instead of copper for a 30-amp circuit?
Yes, but you must increase the wire size. Aluminum has higher resistance than copper. To safely carry 30 amps, you must use 8 AWG aluminum (rated 30A in the 60°C column). Additionally, you must apply an anti-oxidant compound (like Noalox) to the terminations to prevent galvanic corrosion and high-resistance heating at the breaker lugs.

What happens if my run is longer than 50 feet?
At 50 feet, 10 AWG copper yields a voltage drop of roughly 2.44V (about 1% of 240V), which is well within the NEC recommended 3% maximum. However, if your run exceeds 100 feet, the voltage drop approaches 5V (2%). For runs over 100 feet, step up to 8 AWG copper to maintain optimal voltage at the appliance terminals and prevent motor burnout in compressors or welders.