What Wire Size for 80 Amp Sub Panel Feeder? (Copper & Aluminum)

The correct wire size for an 80 amp sub panel is 4 AWG copper or 2 AWG aluminum, paired with an 80A double-pole breaker. This baseline assumes standard 75°C terminations, a 30°C ambient temperature, and a run under 100 feet. Always verify voltage drop and local AHJ requirements before pulling wire.

The Core Sizing Table: Copper vs. Aluminum Feeder Wire

When sizing a feeder for an 80A subpanel, you must look at the NFPA 70 (National Electrical Code) Table 310.16. The most common mistake DIYers make is reading the 90°C column because THHN wire is rated for 90°C. However, nearly all residential breakers and panel lugs are only rated for 75°C. You must size the wire based on the 75°C column to prevent the termination points from overheating.

Why 4 AWG Copper and Not 6 AWG?

It is tempting to use 6 AWG copper wire for an 80A subpanel because 6 AWG is commonly used for 50A and 60A circuits (like electric ranges or hot tubs). However, using 6 AWG on an 80A breaker is a direct violation of NEC 240.4 and a severe fire hazard.

Here is the physics and code breakdown of why 6 AWG fails and 4 AWG succeeds:

• The 6 AWG Limit: In the 75°C column, 6 AWG copper has an ampacity of 65A. Even if you look at the 90°C column, it only reaches 75A. Neither value meets the 80A minimum requirement for the breaker.
• The 4 AWG Solution: 4 AWG copper in the 75°C column is rated for 85A. Because 85A is greater than the 80A load and the 80A breaker rating, it complies with code.
• The Termination Bottleneck (NEC 110.14(C)): You cannot use the 90°C ampacity of 4 AWG (95A) to size your breaker because the mechanical lugs inside standard residential load centers (like Square D Homeline or Siemens EQ) are only tested and rated to 75°C. If you push 90°C heat into a 75°C lug, the lug will degrade, oxidize, and eventually arc.

Voltage Drop: When Distance Forces an Upsize

Ampacity tables assume the wire can handle the heat, but they do not account for the resistance of the wire over long distances. If your subpanel is far from the main panel, voltage drop will cause your 240V tools to run hot and your 120V lighting to flicker. The NEC recommends a maximum 3% voltage drop for feeders.

Let's run the math for 4 AWG Copper carrying a full 80A load at 240V:

Formula used: VD = (2 × K × I × D) / Circular Mils. (K=12.9 for Copper, I=80A, 4 AWG CM = 41,740).

If your run exceeds 120 feet, 4 AWG copper will push you past the 3% threshold at full load. You must step up to 3 AWG copper (which has 52,620 circular mils) to keep the voltage drop under 3% at 150 feet. If you are using aluminum for a long run, you will likely need to jump from 2 AWG to 1/0 AWG aluminum to compensate for aluminum's higher resistance (K=21.2).

Derating, Bundling, and AHJ Inspections

The baseline sizes (4 AWG Cu / 2 AWG Al) assume you are pulling three wires (two hots, one neutral) plus a ground through a single conduit. But real-world jobsite conditions often change the math. According to Electrical Contractor Magazine and NEC Table 310.15(C)(1), when you bundle multiple circuits in the same raceway, the heat cannot dissipate, forcing you to "derate" the wire's ampacity.

What Changes the Wire Size Answer?

• More than 3 Current-Carrying Conductors (CCCs): If you pull two separate 240V circuits (4 hots) in the same PVC conduit, you have 4 CCCs. NEC requires an 80% derating factor. 4 AWG THHN (95A at 90°C) derated to 80% is 76A. Because 76A is less than your 80A breaker, you must upsize to 3 AWG copper if bundling multiple circuits.
• High Ambient Temperatures: If your conduit runs through an unventilated attic in a southern climate where ambient temperatures hit 50°C (122°F), you must apply an ambient temperature correction factor. At 50°C, the correction factor for 90°C wire is 0.82. (95A × 0.82 = 77.9A). Again, 4 AWG fails, and you must upsize.
• Aluminum Terminations: If you choose 2 AWG aluminum to save money on the wire, you must account for the termination process. Aluminum expands and contracts more than copper and oxidizes rapidly. You must wire-brush the aluminum strands, apply a UL-listed anti-oxidant compound (like Noalox or Penetrox), and torque the lugs to the exact inch-pound specification printed on the breaker label using a calibrated torque screwdriver. Failure to do so will result in a high-resistance connection that will melt the breaker lug within a year.

When an Engineer or AHJ Must Confirm

While this guide covers 95% of standard residential and light-commercial subpanel feeder runs, you must pull a permit and consult your local Authority Having Jurisdiction (AHJ) or a licensed Professional Engineer (PE) in the following scenarios:

1. Continuous Loads: If the subpanel will supply a continuous load (defined by the NEC as a load expected to run for 3 hours or more, like a commercial grow room or continuous-duty HVAC), the wire and breaker must be sized at 125% of the load. An 80A continuous load requires 100A wire (3 AWG Cu) and a 100A breaker.
2. Service Entrance vs. Feeder: If this 80A panel is not a subpanel fed from an existing main, but rather a new service drop from the utility pole, entirely different rules apply (NEC Article 230). Utility companies have their own strict service entrance manuals that override standard feeder rules.
3. Local Amendments: Some municipalities (notably in Chicago, New York, and parts of Canada under the CEC) require rigid metal conduit (RMC) or have banned aluminum branch/feeder wiring entirely inside residential structures. Always check your local municipal code portal before buying materials.

By sticking to 4 AWG copper or 2 AWG aluminum for standard runs under 100 feet, respecting the 75°C termination limit, and keeping your neutral and ground isolated at the subpanel, you will build a feeder that is safe, code-compliant, and ready to pass inspection on the first try.