The 30 amp 240 wire size refers to the minimum American Wire Gauge (AWG) cross-sectional area required to safely carry 30 amps of current across two 120V hot legs without exceeding the thermal limits of the insulation or terminations. For standard residential and light commercial installations, the direct answer is 10 AWG copper wire, protected by a 30-amp double-pole breaker. However, simply matching the breaker to the wire's base ampacity ignores the reality of voltage drop over distance and the specific temperature ratings of your terminations.

Mains Safety Warning: Working inside a panel to install a 240V circuit involves lethal voltage. De-energize the main breaker, use a non-contact voltage tester and a multimeter to verify the bus bars are dead, and wear arc-flash rated PPE. If you are not comfortable with panel work, hire a licensed electrician. NEC-style guidance provided here does not override your local Authority Having Jurisdiction (AHJ).

The Core Sizing Table for 30A 240V Circuits

Wire sizing is not a single number; it depends on the insulation type, the ambient temperature, and the temperature rating of the lugs you are terminating on. According to Cerrowire's NEC Table 310.16 ampacity charts, 10 AWG copper is the baseline, but you must choose the correct temperature column.

NEC Table 310.16 Ampacity & Voltage Drop Reference for 30A 240V Circuits
Wire Size & Material 60°C Column (NM-B) 75°C Column (THHN/THWN) Resistance (Ω/kft) Application Notes
10 AWG Copper 30A 35A 1.24 Standard minimum for runs under 100 ft.
8 AWG Copper 40A 50A 0.778 Required for runs 100-150 ft to limit voltage drop.
6 AWG Copper 55A 65A 0.491 Used for runs over 150 ft or high-ambient heat.
8 AWG Aluminum 30A 40A 1.28 Acceptable for feeders, rarely used for 30A branch.
6 AWG Aluminum 40A 50A 0.808 Alternative to 8 AWG Cu for long underground runs.
The 60°C vs 75°C Trap: Even if you use 90°C THHN wire in conduit, NEC 110.14(C) requires you to size the wire based on the temperature rating of the terminations. Most modern residential breakers are rated 75°C, but if you are using NM-B (Romex) cable, NEC 334.80 forces you to use the 60°C column regardless of the breaker rating. Fortunately, 10 AWG at 60°C is exactly 30A.

What Changes in a Real Circuit: Voltage Drop and the 100-Foot Rule

While 10 AWG copper satisfies the thermal requirements for a 30-amp breaker, it does not guarantee that your appliance will receive adequate voltage at the end of the run. Voltage drop is the loss of electrical pressure due to the inherent resistance of the wire. Think of it like water pressure dropping across a long, narrow garden hose; the pump (breaker) is pushing 30 amps, but friction in the wire steals some of the voltage before it reaches the load.

The NEC recommends a maximum voltage drop of 3% for branch circuits. On a 240V circuit, 3% equals 7.2 volts. Let's look at a worked numeric example to see when 10 AWG fails and 8 AWG becomes mandatory.

Worked Numeric Example: EV Charger Installation

You are installing a 24-amp continuous Level 2 EV charger (which requires a 30-amp breaker per NEC 210.20(A) for continuous loads). The panel is 120 feet away from the garage outlet.

  • Scenario A: 10 AWG Copper
    Formula: VD = (2 × K × I × L) / Circular Mils
    Using the simplified resistance method: 10 AWG is 1.24 Ω per 1,000 ft. A 120 ft run means 240 ft of total wire length (Hot 1 + Hot 2).
    Resistance = 0.248 Ω.
    Voltage Drop = 30A × 0.248 Ω = 7.44V.
    Percentage = (7.44 / 240) × 100 = 3.1%.
    Result: Fails the 3% recommendation. The EV charger may throttle charging speed or throw a low-voltage fault.
  • Scenario B: Upsizing to 8 AWG Copper
    8 AWG resistance is 0.778 Ω per 1,000 ft.
    Resistance for 240 ft = 0.186 Ω.
    Voltage Drop = 30A × 0.186 Ω = 5.58V.
    Percentage = (5.58 / 240) × 100 = 2.32%.
    Result: Passes. You must pull 8 AWG copper for this 120-foot run, even though the breaker is only 30 amps.

Where You Meet This in Practice

You will encounter the 30-amp 240V requirement in several specific residential and workshop scenarios. Identifying the correct receptacle and wire configuration is critical for passing inspection and preventing fires.

  • Electric Dryers (NEMA 14-30R): Modern dryers require a 4-wire setup (two hots, one neutral, one ground). You must run 10/3 NM-B with ground (or four individual 10 AWG THHN wires in conduit). The neutral carries the 120V load for the dryer's timer and electronics, while the hots carry the 240V heating element load.
  • Welders and Plasma Cutters (NEMA 6-30R): These are pure 240V loads requiring no neutral. You run two 10 AWG hot wires and a 10 AWG ground. Because welders have a specific duty cycle, NEC Article 630 sometimes allows smaller wire for the branch circuit based on the nameplate, but 10 AWG remains the standard practical choice for a 30A breaker.
  • Transfer Switches and Generators: A 30-amp 240V generator inlet (like a Reliance Controls PB30) uses a 4-wire L14-30 configuration. The feed from the inlet to the transfer switch or interlock kit must be 10 AWG 4-wire.

Common Confusions and Mistakes to Avoid

Does 240V require thicker wire than 120V for the same amperage?

No. Wire size is dictated by amperage (current), which generates heat, not voltage. A 30-amp 120V circuit and a 30-amp 240V circuit both require 10 AWG copper wire to prevent the wire from melting. The difference is that the 240V circuit uses two hot legs sharing the load, while the 120V circuit pushes the full 30 amps down a single hot leg.

Is a 30-amp RV outlet (TT-30) a 240V circuit?

This is one of the most dangerous confusions in DIY electrical work. A NEMA TT-30R receptacle looks similar to a 240V dryer outlet, but it is strictly 120V. It uses one hot, one neutral, and one ground. If you wire a TT-30 with two 120V hot legs (240V) because you assumed '30 amps means 240V', you will instantly destroy the RV's electronics and create a severe shock hazard. Always verify the receptacle type before terminating wires.

Can I use 10 AWG aluminum wire for a 30-amp 240V circuit?

While 8 AWG aluminum is rated for 30A at 60°C, the NEC generally prohibits the use of aluminum wire smaller than 8 AWG for branch circuits, and many local AHJs ban aluminum for branch circuits entirely due to historical issues with oxidation and creep at terminations. Stick to copper for branch circuits under 60 amps. Aluminum is best reserved for large feeder cables (like a 100A subpanel feed using 2 AWG SER cable).

Do I need to derate the wire if I bundle it with other circuits?

Yes. If you pull multiple 240V circuits through the same conduit, NEC Table 310.15(C)(1) requires ampacity derating. If you have 4 to 6 current-carrying conductors in a single conduit, you must derate the ampacity to 80%. In this case, 10 AWG THHN (base 35A at 75°C) derates to 28A, which is insufficient for a 30A breaker. You would be forced to upsize to 8 AWG copper simply because the wires are sharing a pipe.