What Size Wire for 220V 30 Amp: AWG, Breaker & Voltage Drop Guide

For a standard 220V (240V nominal) 30-amp circuit, use 10 AWG copper wire protected by a 30-amp double-pole breaker. This assumes copper conductors, THHN/THWN-2 or NM-B insulation, a maximum of three current-carrying conductors in a raceway, and an ambient temperature of 30°C (86°F).

The Baseline Sizing Matrix

When determining what size wire for 220v 30 amp applications, you must look at the intersection of wire material, insulation temperature rating, and the breaker's terminal limits. While 220V is the common colloquial term, US residential split-phase systems deliver a nominal 240V. The wire size does not change based on this 20V difference, but the voltage drop math does.

*Note: NM-B (Romex) cable is strictly limited to the 60°C ampacity column per NEC 334.80, regardless of the breaker's 75°C rating. Fortunately, 10 AWG copper is rated 30A in the 60°C column, making it fully compliant for both conduit and NM-B runs.

Why 10 AWG and Not 12 AWG?

A common mistake on the bench or jobsite is looking at the 75°C column of NEC Table 310.16, seeing that 12 AWG THHN is rated for 25 amps, and assuming it can safely handle a 30-amp load if the breaker is swapped. This is a severe fire hazard and a direct violation of NEC 240.4(D), the "Small Conductor Rule."

NEC 240.4(D) explicitly overrides the standard ampacity tables for small copper conductors. It dictates that the overcurrent protection (breaker) for 12 AWG copper cannot exceed 20 amps, and for 14 AWG, it cannot exceed 15 amps. Even if your 12 AWG THHN wire is theoretically capable of dissipating the heat of a 25A load in free air, the breaker terminals, the receptacle contacts, and the branch circuit as a whole are not rated to safely interrupt a fault on a 30A breaker with 12 AWG wire. If you push 30 amps through a 12 AWG wire protected by a 30A breaker, the wire will overheat and melt its insulation long before the breaker trips.

Therefore, 10 AWG is the absolute minimum copper size for a 30A breaker. If you are using aluminum wire (such as SER cable for a subpanel feed), you must step up to 8 AWG, as 10 AWG aluminum is only rated for 25A at 60°C and is not manufactured with standard 75°C building wire insulation ratings that would allow a 30A breaker.

Voltage Drop: When Distance Forces an Upsize

Ampacity tables only tell you if the wire will melt. They do not tell you if the voltage at the far end of the run will be sufficient to operate the equipment. The NEC recommends (via Informational Notes in Article 210.19) that branch circuit voltage drop should not exceed 3%. For a 240V nominal circuit, a 3% drop is 7.2 volts.

Let's run the math on 10 AWG copper, which has an approximate resistance of 1.24 ohms per 1,000 feet at 75°C. Using the standard single-phase voltage drop formula: VD = (2 × Length × Current × Resistance) / 1000.

As shown in the matrix, if your run from the panel to the 220V receptacle (like a dryer outlet or EV charger) exceeds roughly 95 feet, 10 AWG copper will result in a voltage drop greater than 3%. At 100 feet, you must upsize to 8 AWG copper. You can verify your specific run parameters using the Southwire Voltage Drop Calculator, which accounts for exact AC impedance and power factor.

Pro-Tip: When upsizing wire for voltage drop, you must also ensure the breaker terminals can physically accept the larger wire. Most standard 30A double-pole breakers (like the Square D QO230 or Eaton BR230) are rated to accept up to 8 AWG or 4 AWG, but always check the breaker datasheet before terminating.

Derating Factors and Continuous Loads

The baseline 10 AWG answer assumes a standard, non-continuous load in a normal temperature environment. Real-world jobsite conditions frequently invalidate these assumptions. Here is when you must deviate from 10 AWG and when to bring in an engineer or the local Authority Having Jurisdiction (AHJ).

1. Continuous Loads (The 125% Rule)

NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include baseboard heaters, kilns, and Level 2 EV chargers. Per NEC 210.20(A), the branch circuit overcurrent device must be sized at 125% of the continuous load.

• The Math: 30A continuous load × 1.25 = 37.5A.
• The Fix: You must step up to a 40-amp breaker and use 8 AWG copper wire. A 30A breaker on a 30A continuous load will eventually nuisance-trip as its internal thermal element heat-soaks over time.

2. Ambient Temperature and Bundling Derating

If you are routing your conduit through a hot attic (e.g., 110°F / 43°C ambient) or bundling more than three current-carrying conductors in a single raceway, you must apply derating factors from NEC Table 310.15(B)(1)(1) and 310.15(C)(1).

• High Heat: At 41-45°C ambient, THHN (90°C rated) wire must be derated to 82% of its base ampacity. 10 AWG THHN base is 40A. 40A × 0.82 = 32.8A. This still covers a 30A breaker, but leaves almost no margin. If the attic hits 50°C (derated to 75%), 40A × 0.75 = 30A exactly, which is too tight for safe termination. Upsize to 8 AWG.
• Bundling: If you pull four current-carrying conductors (e.g., two 240V circuits sharing a neutral or multi-wire branch circuit configurations) in one conduit, you must derate to 80%. Again, 10 AWG THHN drops to 32A. Upsizing to 8 AWG is the standard professional practice here to maintain a safe thermal buffer.

When to Confirm with the AHJ

Always submit your wire and breaker sizing plan to your local electrical inspector (AHJ) if you are installing equipment with specific manufacturer-mandated minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) values. HVAC compressors and large shop machinery often list an MCA of 24A but an MOCP of 35A or 40A. In these cases, the manufacturer's nameplate overrides standard NEC branch circuit rules under NEC 440. Never guess on motor-compressor circuits; follow the nameplate exactly and let the inspector verify the termination ratings.