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

For a standard 30 amp 220V (240V nominal) circuit, you need 10 AWG copper wire and a 30-amp double-pole breaker. This assumes copper conductors, a 75°C termination rating, 30°C ambient temperature, and a run under 50 feet. If your run exceeds 75 feet, upsize to 8 AWG copper to prevent excessive voltage drop.

Core Wire and Breaker Sizing Matrix

Before pulling any wire, verify your scenario against the table below. This data is derived from the National Fire Protection Association (NFPA) NEC Table 310.16 and standard voltage drop calculations for a 240V system drawing a full 30A load.

Why 10 AWG and Not 12 AWG? (The NEC Rules)

A common mistake on the bench or jobsite is looking at the 90°C column of NEC Table 310.16, seeing that 12 AWG THHN is rated for 30A at 90°C, and assuming it can be used on a 30A breaker. This is a code violation and a fire hazard.

Two specific NEC articles prevent you from using 12 AWG for a 30A circuit:

1. Termination Temperature Limits (NEC 110.14(C)): Most standard 30A breakers and receptacles are rated for 75°C terminations. In the 75°C column, 12 AWG copper is only rated for 25A. You must size the wire based on the weakest link in the chain, which is the terminal.
2. Small Conductor Rule (NEC 240.4(D)): This article strictly overrides standard ampacity tables for small conductors. It explicitly states that the overcurrent protection for 12 AWG copper shall not exceed 20A, and for 10 AWG copper shall not exceed 30A, regardless of the insulation's 90°C rating.

Variables That Force You to Upsize to 8 AWG

While 10 AWG is the baseline, real-world conditions frequently force an upsize to 8 AWG copper. Here is the decision framework for when 10 AWG is no longer sufficient.

1. Voltage Drop Over Distance

The NEC recommends (via Informational Note to 210.19) a maximum voltage drop of 3% for branch circuits. For a 240V system, 3% is 7.2 volts. Let's run the math using the standard voltage drop formula: VD = (2 × K × I × L) / CM.

• K (Copper constant): 12.9
• I (Current): 30A
• CM (Circular Mils for 10 AWG): 10,380

If your one-way wire length (L) is 50 feet, the voltage drop is 3.72V (1.55%). You are perfectly fine with 10 AWG.
If your length is 75 feet, the drop is 5.59V (2.3%). Still acceptable.
If your length is 100 feet, the drop is 7.45V (3.1%). You have crossed the 3% threshold. Upsize to 8 AWG copper to bring the resistance down and maintain equipment efficiency, especially for motor-driven loads like well pumps or air compressors which can overheat on low voltage.

2. Conductor Bundling and Derating

If you are pulling multiple circuits through the same conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to derate the ampacity based on the number of current-carrying conductors (grounds do not count).

• 1 to 3 conductors: 100% of 90°C rating (40A for 10 AWG). No derating needed.
• 4 to 6 conductors: 80% adjustment factor. 40A × 0.80 = 32A. Since 32A is greater than your 30A load, 10 AWG is still legal.
• 7 to 9 conductors: 70% adjustment factor. 40A × 0.70 = 28A. Your wire is now only rated for 28A, but your breaker is 30A. You must upsize to 8 AWG.

3. Continuous Loads (The 125% Rule)

Is your 220V load running for 3 hours or more continuously? This applies to EV chargers, kilns, baseboard heaters, and commercial lighting. Under NEC 210.20(A), the overcurrent device must be rated at 125% of the continuous load.

If your equipment draws a continuous 26A, 26A × 1.25 = 32.5A. You must step up to a 40A breaker. Once you install a 40A breaker, NEC 240.4(D) mandates a minimum of 8 AWG copper wire. Always check the equipment nameplate for the "Minimum Circuit Ampacity" (MCA) and "Maximum Overcurrent Protection" (MOCP) values before buying wire.

Aluminum Wire and AHJ Involvement

Copper is the standard for branch circuits, but aluminum is sometimes used for longer runs to save money. Do not treat them interchangeably. 10 AWG aluminum building wire is practically non-existent in standard supply houses; the smallest you will typically find is 8 AWG.

8 AWG aluminum (XHHW-2 or THWN-2) is rated 40A in the 75°C column, making it perfectly safe for a 30A breaker. However, aluminum requires strict installation practices:

• Apply an anti-oxidant compound (like Noalox) to the conductor ends before termination to prevent galvanic corrosion and high-resistance heating.
• Use a calibrated torque screwdriver to tighten lugs to the manufacturer's exact inch-pound specification. Aluminum creeps under pressure over time, and loose connections are a primary cause of panel fires.
• Ensure the breaker and receptacle terminals are explicitly rated "AL/CU" (Aluminum/Copper).

For comprehensive guidance on torque specifications and termination practices, refer to ECMWeb's NEC coverage and manufacturer datasheets.

When to Call an Engineer or the AHJ

While this guide covers 95% of residential and light-commercial 30A 220V scenarios, you must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) if:

• The ambient temperature in the conduit or attic exceeds 110°F (43°C), requiring additional temperature correction factors from NEC Table 310.15(B)(1).
• You are installing equipment with high inrush currents (like large industrial motors) that require specialized time-delay breakers or adjusted wire sizing to prevent nuisance tripping.
• Local municipal amendments override the baseline NEC rules. Some jurisdictions mandate 8 AWG as the absolute minimum for all 240V branch circuits regardless of distance, purely for mechanical durability.

Always de-energize the panel, verify the bus bars are dead with a tested CAT III or CAT IV multimeter, and use a torque screwdriver for all terminal connections. If you are not comfortable working inside a live or recently de-energized load center, hire a licensed electrician.