What Wire Size for 30 Amps 220 Volts: 10 AWG Rule & Long Runs

Wire size for a 30-amp 220-volt (nominal 240V) circuit is determined strictly by the breaker rating and the conductor's ampacity, meaning 10 AWG copper is the correct baseline size for runs under 50 feet. While colloquially called "220V," modern US residential split-phase systems deliver a nominal 240V across the two hot legs. The voltage itself does not dictate the wire thickness; the amperage does. Below, we break down the exact National Electrical Code (NEC) requirements, where this configuration is used, and the voltage drop math that forces you to upsize the wire on longer runs.

The Direct Answer: Sizing Wire for a 30-Amp 240V Circuit

For a standard 30-amp double-pole breaker, the NEC mandates that the wire's ampacity must meet or exceed the breaker rating. According to NEC Table 310.16 and termination rules under 110.14(C), 10 AWG copper wire is rated for exactly 30 amps in the 60°C column, which is the default column for most residential branch circuits and receptacles.

Wire Gauge (AWG)	Material	60°C Ampacity	75°C Ampacity	Standard Use Case
12 AWG	Copper	20A	25A	Too small (Max 20A breaker)
10 AWG	Copper	30A	35A	Standard 30A 240V circuits
8 AWG	Copper	40A	50A	Long runs / Voltage drop mitigation
8 AWG	Aluminum	30A	40A	Feeders (Rare for 30A branch)

Where You Meet This in Practice

You will typically encounter a 30-amp 240V requirement in specific heavy-duty residential and light-commercial applications. The physical receptacle dictates the exact cable assembly you need (specifically, the number of conductors).

• NEMA 6-30R (3-wire, no neutral): Common for older welders, heavy-duty air compressors, and some EV chargers. Requires 10/2 NM-B cable with a ground (two hots, one ground).
• NEMA 14-30R (4-wire, with neutral): The standard for modern electric clothes dryers. Requires 10/3 NM-B cable (two hots, one neutral, one ground) because the dryer's control board and drum motor run on 120V, while the heating element runs on 240V.
• NEMA L6-30R (Twist-lock, 3-wire): Found in server rooms, commercial workshops, and RV parks for 240V equipment. Uses individual THHN wires in conduit or a 10/2 SOOW flexible cord.

Worked Scenario: The 150-Foot Workshop Welder Run

Amperage dictates wire size for heat dissipation, but distance introduces voltage drop. Here is a real-world bench scenario where the baseline 10 AWG rule fails.

Setup: A hobbyist is installing a NEMA 6-30R receptacle for a 240V MIG welder at the back of a 150-foot deep property, fed from the main panel. They pull standard 10/2 NM-B copper cable and terminate it on a 30-amp double-pole breaker.

Numbers: The welder draws a sustained 26 amps during heavy bead laying. We calculate the voltage drop using the standard single-phase formula: VD = (2 × K × I × L) / CM.
- K (Copper constant) = 12.9
- I (Current) = 26A
- L (One-way length) = 150 ft
- CM (Circular mils for 10 AWG per NEC Chapter 9, Table 8) = 10,380
VD = (2 × 12.9 × 26 × 150) / 10,380 = 9.69 Volts.
Percentage drop = 9.69V / 240V = 4.03%.

Outcome: The NEC recommends a maximum 3% voltage drop for branch circuits. At 4.03%, the voltage at the receptacle sags to roughly 230V under load. The welder's arc becomes unstable, spattering excessively, and the 10 AWG wire inside the walls runs noticeably warm to the touch.

What went wrong: The builder sized the wire solely for the breaker's thermal trip limit (ampacity) and ignored the distance. The Fix: Upsize to 8 AWG copper (CM = 16,510). Recalculating the drop: (2 × 12.9 × 26 × 150) / 16,510 = 6.09V. The new drop is 2.53%, well under the 3% threshold, resulting in a stable arc and cool wires. You can verify these thresholds using the Southwire Voltage Drop Calculator or similar industry tools.

What Changes When You Mis-size the Wire

A common point of confusion among DIYers is the assumption that higher voltage requires thicker wire. Voltage does not dictate wire thickness; amperage does. Think of wire gauge as the diameter of a plumbing pipe: it only cares about the physical volume of water (amps) flowing through it, not the pressure (volts) pushing it. A 30-amp load at 12V DC generates the exact same amount of resistive heat in a wire as a 30-amp load at 240V AC.

However, what does change with 240V is the insulation requirement and the breaker configuration. While 10 AWG wire handles 30 amps regardless of voltage, a 240V circuit requires a double-pole breaker that simultaneously disconnects both ungrounded (hot) conductors. Furthermore, the outer jacket of the cable (like NM-B) must be rated for the potential difference between the two hots (240V), which standard residential cables inherently are.

If you undersize the wire (e.g., using 12 AWG on a 30A breaker), the breaker will not trip immediately because it is designed to hold 30 amps. However, the 12 AWG wire is only rated for 20 amps. The wire will overheat, the insulation will soften and melt, and it can cause a concealed structure fire long before the breaker's thermal overload mechanism trips.

Common Confusions: Termination Limits and Aluminum

When buying materials for a 30-amp 220V circuit, keep these specific hardware constraints in mind:

1. The 75°C vs 60°C Termination Trap: Modern 10 AWG THHN wire is rated for 40 amps at 90°C. However, NEC 110.14(C) states you must size the wire based on the lowest temperature rating of any connected device. Most 30A breakers and NEMA receptacles are rated for 75°C or 60°C. Fortunately, 10 AWG copper is exactly 30A in the 60°C column, so you are safe. If you were trying to use the 90°C column to downsize the wire, you would violate code.
2. Aluminum Wire Sizing: If you are running SER cable or THWN-2 aluminum feeders to a subpanel that will supply a 30A 240V circuit, you must step up to 8 AWG aluminum. 10 AWG aluminum is technically rated for 30A at 60°C, but it is mechanically fragile and rarely stocked for branch circuits; 8 AWG is the standard, safe choice for aluminum at this amperage.
3. NEMA TT-30 is NOT 240V: Do not confuse a 30-amp 240V setup with a NEMA TT-30 RV receptacle. A TT-30 is 30 amps at 120V. Wiring a TT-30 with 240V will instantly destroy an RV's appliances and poses a severe shock hazard.

FAQ: 30-Amp 220V Wiring Questions