For an 80-amp breaker, use 4 AWG copper wire or 2 AWG aluminum wire. This assumes copper THHN/THWN-2 in conduit, 75°C rated terminals, and a 30°C ambient temperature. The breaker size is exactly 80 amps. If your run exceeds 145 feet at 240V, upsize to 3 AWG copper to mitigate voltage drop.

The Core Sizing Decision and Baseline Assumptions

When pulling wire for an 80-amp subpanel feeder, a heavy-duty welder outlet, or a large EV charger, guessing the wire gauge is how you end up with melted terminal lugs or a breaker that trips under load. The National Electrical Code (NEC) does not allow you to simply match the wire's absolute maximum thermal rating to the breaker; you must size the conductor based on the weakest link in the termination chain.

Why 4 AWG copper and not 6 AWG? In the 75°C column of the NEC ampacity tables, 6 AWG copper is only rated for 65 amps. Installing 6 AWG on an 80-amp breaker violates NEC 240.4, which requires the conductor ampacity to be equal to or greater than the overcurrent device rating. 4 AWG copper, however, is rated for 85 amps at 75°C. Because 85A is greater than 80A, it is the minimum legal and safe size for this circuit.

Baseline Assumptions for This Guide:
  • Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2/THHN)
  • Temperature Column: 75°C (Standard for residential/commercial breaker and panel lugs)
  • Ambient Temperature: 30°C (86°F) or lower
  • Installation Method: Single circuit in a raceway (conduit) or cable assembly, no more than 3 current-carrying conductors bundled together
  • System Voltage: 240V (Split-phase or 2-pole)

Ampacity Table and the 75°C Terminal Rule

A common trap for DIYers is looking at the 90°C column on the wire's spec sheet. A 4 AWG THHN wire is indeed rated for 95 amps at 90°C. However, NEC 110.14(C) mandates that you must size the wire based on the temperature rating of the terminations (the breaker lugs and the subpanel busbar lugs). Almost all standard residential and light-commercial breakers and panels are rated for 75°C. Therefore, you must use the 75°C column for your final ampacity check.

NEC Table 310.16 Ampacity Lookup (75°C Column)
Wire Size (AWG/kcmil) Copper (75°C) Aluminum (75°C) Verdict for 80A Breaker
6 AWG 65A 50A Too Small (Fire Hazard)
4 AWG 85A 65A Copper Passes / Al Fails
3 AWG 100A 75A Copper Oversized / Al Fails
2 AWG 115A 90A Copper Oversized / Aluminum Passes

As shown in the table, if you choose aluminum to save on material costs, 4 AWG is insufficient. You must step up to 2 AWG aluminum, which provides 90 amps of ampacity at 75°C, safely clearing the 80-amp threshold.

Voltage Drop: When to Upsize the Wire

Ampacity tells you what size wire will prevent a fire. Voltage drop tells you what size wire will actually deliver usable power to the other end of the run. The NEC recommends a maximum 3% voltage drop for branch circuits and feeders. On a 240V system, a 3% drop equates to 7.2 volts.

Let's run the math for 4 AWG copper carrying 80 amps. The resistance of 4 AWG copper is approximately 0.308 ohms per 1,000 feet. Using the single-phase voltage drop formula VD = (2 × Length × Current × Resistance) / 1000:

7.2V = (2 × L × 80A × 0.308) / 1000
7.2 = 0.04928 × L
L = 146 feet

This means 4 AWG copper is perfectly adequate for runs up to roughly 145 feet. If your conduit run is 150 feet or longer, the voltage drop exceeds 3%, and you should upsize to 3 AWG copper (good up to ~180 feet) or 2 AWG copper (good up to ~225 feet).

Wire Sizing Decision Path by Run Distance (240V, 80A Load)
One-Way Run Distance Copper Pick Aluminum Pick Action Required
Under 145 feet 4 AWG 2 AWG Standard install. Terminate and torque to spec.
145 to 180 feet 3 AWG 1/0 AWG Upsize one gauge to maintain <3% voltage drop.
180 to 225 feet 2 AWG 2/0 AWG Upsize two gauges. Verify conduit fill capacity.
Over 225 feet 1/0 AWG or larger 3/0 AWG or larger Consult an engineer; consider stepping up voltage.

Copper vs. Aluminum for 80A Feeders

Choosing between copper and aluminum isn't just about the wire gauge; it changes your entire installation workflow. According to the Copper Development Association, copper remains the benchmark for conductivity and ease of termination, but aluminum has made a massive comeback for feeder circuits due to material costs.

Choose 4 AWG Copper when: You are pulling wire through tight bends, working in existing conduit with limited fill space, or terminating into older panels where lug compatibility is questionable. Copper is also mandatory if you are using NM-B (Romex) cable, though finding 4 AWG NM-B is difficult and expensive; most installers use THHN in conduit or SER cable.

Choose 2 AWG Aluminum when: You are running a long, straight subpanel feeder where material cost savings outweigh the labor of pulling a thicker, stiffer wire. Aluminum is significantly cheaper per foot. However, you must apply an anti-oxidant compound (like Noalox) to the stripped aluminum strands before termination to prevent galvanic corrosion and high-resistance heating over time.

Torque It Right: NEC 110.14(D) requires that all terminations be tightened to the manufacturer's specified torque. You cannot 'guess' tightness by feel. Use a calibrated inch-pound torque screwdriver or torque wrench. An under-torqued 80A lug will arc and melt; an over-torqued lug will snap the screw or deform the wire strands, creating a hot spot.

Continuous Loads and When the AHJ Must Confirm

The most critical distinction in electrical sizing is the difference between an 80-amp breaker and an 80-amp continuous load. The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more (NEC Article 100). EV chargers, server racks, and commercial lighting are continuous loads.

If your breaker is 80 amps, the maximum continuous load you can legally attach to it is 64 amps (80A ÷ 1.25). The 4 AWG copper wire we selected is perfectly fine for this scenario.

However, if your actual equipment load is 80 amps continuous (for example, a massive 19.2kW EV charger), the math flips. You must multiply the load by 1.25. 80A × 1.25 = 100A. In this scenario, you need a 100-amp breaker and wire rated for 100 amps (which requires stepping up to 1 AWG copper or 1/0 AWG aluminum).

You must involve a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) to confirm your sizing if:

  • You are bundling more than three current-carrying conductors in a single conduit, which triggers NEC 310.15(C)(1) ampacity derating factors.
  • The ambient temperature in your attic, crawlspace, or rooftop conduit exceeds 30°C (86°F), requiring temperature correction factors.
  • You are sizing for a continuous load that lands exactly on the boundary of standard breaker sizes (NEC 240.6), requiring formal approval to round up to the next standard size.

For the vast majority of standard residential 80-amp subpanel feeders or non-continuous 80-amp equipment circuits under 145 feet, 4 AWG copper THHN/THWN-2 pulled in conduit with an 80-amp double-pole breaker is the definitive, code-compliant default choice.