The correct wire gauge for a 30 amp circuit is 10 AWG copper or 8 AWG aluminum, paired with a 30-amp breaker. This assumes standard THHN/THWN-2 insulation in a raceway at 30°C ambient temperature. If your run exceeds 120 feet, you must step up to 8 AWG copper to mitigate voltage drop.

The Baseline Assumptions (And Why They Matter)

Before pulling any wire from the spool, we must lock in the environmental and material variables. Wire ampacity is not a fixed number; it is a sliding scale based on heat dissipation. If you size a wire without stating your assumptions, you are guessing, and guessing with mains voltage causes fires.

Safety & Code Caveat: The following guidance is based on NEC-style rules (specifically NFPA 70). Your local Authority Having Jurisdiction (AHJ) or municipal inspector has the final legal authority on all installations. Always de-energize the panel, lock out the main breaker, and verify dead with a tested CAT III or CAT IV multimeter before touching any bus bars or lugs.

For the baseline recommendation of 10 AWG copper, we are operating under the following strict assumptions:

  • Material: Solid or stranded copper (aluminum requires different sizing, covered below).
  • Insulation Type: THHN or THWN-2 (the standard 90°C rated building wire found in conduit).
  • Termination Temperature: 75°C. While the wire itself is rated for 90°C, standard residential and light-commercial breakers and lugs are rated for 75°C. Per NEC 110.14(C), we must use the 75°C column for our final ampacity limit.
  • Ambient Temperature: 30°C (86°F) or lower. Standard NEC tables assume this baseline.
  • Conduit Fill: No more than three current-carrying conductors in a single raceway.
  • Load Type: Non-continuous load (operates for less than 3 hours continuously).

NEC Ampacity Tables: 10 AWG Copper vs. 8 AWG Aluminum

Why 10 AWG and not one size smaller? If you look at NEC Table 310.16, 12 AWG copper in the 75°C column is rated for exactly 25 amps. Because standard breaker sizes (per NEC 240.6) jump from 20A to 25A to 30A, a 12 AWG wire cannot be protected by a 30A breaker. The breaker would allow 30 amps to flow, but the wire would overheat before the breaker tripped. 10 AWG copper, however, is rated for 35 amps in the 75°C column, making it perfectly safe to protect with a 30A breaker.

NEC Table 310.16 Ampacity Excerpt (Copper vs. Aluminum)
Wire Gauge (AWG) Material 60°C Column (Amps) 75°C Column (Amps) 90°C Column (Amps)
12 AWG Copper 20A 25A 30A
10 AWG Copper 30A 35A 40A
8 AWG Copper 40A 50A 55A
10 AWG Aluminum 25A 30A 35A
8 AWG Aluminum 30A 40A 45A

Notice the aluminum rows. Aluminum has lower conductivity and higher thermal expansion than copper. To safely carry the same current and terminate on a 30A breaker, you must step up to 8 AWG aluminum. Never use 10 AWG aluminum for a 30A circuit; its 75°C rating is exactly 30A, which leaves zero margin for termination heating and violates the spirit of the next-size-up breaker rule.

Voltage Drop: When to Upsize for Long Runs

Ampacity tells you if the wire will melt. Voltage drop tells you if your equipment will actually run. The NEC recommends (in informational note 310.15(B)) keeping voltage drop under 3% for branch circuits and 5% overall. For a 240V circuit, a 3% drop is 7.2 volts.

Let's run the math for a 30-amp load on 10 AWG copper at a distance of 100 feet (one way):

  • Formula (Single Phase): VD = (2 × K × I × D) / CM
  • K (Copper constant): 12.9 ohms
  • I (Current): 30 amps
  • D (Distance): 100 feet
  • CM (Circular Mils for 10 AWG): 10,380 (per NEC Chapter 9, Table 8)

VD = (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts.

The 120-Foot Cutoff: At 100 feet, 7.45V is a 3.1% drop on a 240V circuit—just barely over the 3% recommendation, but perfectly legal and functional for most 240V appliances (like a dryer or water heater) which can tolerate down to 208V. However, if your run hits 150 feet, the drop becomes 11.18V (4.6%). At that point, you must upsize to 8 AWG copper (CM = 16,510), which drops the loss to 4.69V (1.9%).

Decision Tree: Picking Your Exact Wire and Breaker

Use this decision path to lock in your exact materials. Do not skip steps; continuous loads and long distances are where DIYers make costly mistakes.

Condition / Question If YES If NO
Is the load continuous (runs 3+ hours, e.g., EV charger, kiln, heater)? Multiply load by 1.25. A 30A continuous load requires a 40A breaker and 8 AWG Copper. Stop here. Proceed to next question.
Is the one-way wire run longer than 120 feet? Upsize to 8 AWG Copper (or 6 AWG Aluminum) to maintain voltage drop under 3%. Stop here. Proceed to next question.
Are you using Aluminum wire instead of Copper? Use 8 AWG Aluminum and ensure lugs are rated for AL and treated with antioxidant paste. Stop here. Proceed to final pick.
Standard run, non-continuous load, copper wire? FINAL PICK: Buy 10 AWG Copper THHN/THWN-2 and a 30-Amp Breaker.

Derating Factors: Bundling, Heat, and Continuous Loads

The baseline 35A rating for 10 AWG copper (75°C column) assumes ideal heat dissipation. Real-world jobsites rarely offer ideal conditions. According to NEC conductor sizing principles, you must apply derating factors that can drastically reduce a wire's effective ampacity.

1. Conduit Bundling (NEC 310.15(C)(1))

If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors in a pipe, you must multiply the wire's ampacity by 80%. For 10 AWG copper in the 90°C column (40A), 40 × 0.80 = 32A. This is still above 30A, so 10 AWG survives. But if you bundle 7 to 9 conductors, the derating factor drops to 70%. 40 × 0.70 = 28A. Your 10 AWG wire is now only good for 28 amps, and you must upsize to 8 AWG.

2. High Ambient Temperature (NEC 310.15(B)(1))

If you are routing conduit through an unventilated attic in the summer, ambient temperatures can easily exceed 110°F (43°C). At 41-45°C ambient, the correction factor for 90°C wire is 0.87. If the attic hits 50°C (122°F), the factor drops to 0.82. Always check the 90°C column for ambient temperature derating, then verify the final derated number is still high enough to terminate on your 75°C breaker lugs.

When to Call an Engineer or the AHJ

While 10 AWG copper on a 30A breaker covers 90% of residential branch circuits (like standard dryers, RV receptacles, and small air compressors), certain scenarios cross the line from DIY into professional engineering territory:

  • Motor Circuits: If your 30A load is a large motor (like a well pump or heavy-duty compressor), NEC Article 430 governs. Motors draw massive inrush currents (Locked Rotor Amps). The breaker sizing and overload protection calculations for motors do not follow standard branch circuit rules.
  • Feeder Taps and Subpanels: If this 30A circuit is actually a feeder to a subpanel, you must calculate the total connected load using NEC Article 220 demand factors. A 30A feeder is rarely sufficient for a modern subpanel.
  • Solar and Battery Systems: If the 30A circuit is connecting an inverter or charge controller, NEC Article 690/706 applies. DC voltage drop calculations and continuous inverter loads require strict 125% sizing margins that often force an upsize to 8 AWG even on short runs.

For standard appliance circuits under the baseline assumptions, stick to the 10 AWG copper rule. For anything involving complex load calculations, high-heat environments, or motor inrush, pull a permit and have a licensed electrical engineer or your local AHJ review the single-line diagram before you cut a single wire.