What Wire Size for 80 Amps? Copper, Aluminum, and NEC Rules

The correct wire size for 80 amps is 4 AWG copper or 2 AWG aluminum when protected by an 80-amp breaker for non-continuous use, governed by the 75°C ampacity column in NEC Table 310.16. Choosing the right gauge isn't just about preventing a fire; it fundamentally changes how your circuit handles heat dissipation, limits voltage drop over long runs, and dictates the mechanical torque required at your termination lugs. The most common mistake DIYers make is confusing the physical breaker size (80A) with the continuous load limit (64A), or falsely assuming that because THHN wire is rated for 90°C, they can use the 90°C ampacity column to downsize their wire. They cannot. Termination limits almost always force you back to the 75°C column.

The Core Rule: Sizing Wire for an 80-Amp Circuit

When sizing conductors for an 80-amp overcurrent protective device, you must look at the 75°C column of the National Electrical Code (NEC) Table 310.16. Even if you pull 90°C THHN wire through your conduit, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component. Since standard breakers and lugs are typically rated for 75°C, your wire ampacity is capped at that column.

Where You Meet This in Practice

You won't find 80-amp circuits powering standard wall receptacles. This gauge of wire and breaker size is reserved for heavy-duty, high-draw equipment in residential and light commercial settings:

• Subpanels: Feeding a 100-amp or 125-amp subpanel in a detached garage or workshop often utilizes an 80-amp breaker at the main panel to limit the feeder load, requiring 4 AWG copper or 2 AWG aluminum.
• EV Level 2 Chargers: While most home electric vehicle chargers run on 40A to 60A circuits, hardwired commercial dual-chargers or high-speed 19.2kW chargers pull continuous loads that push into the 80A breaker territory.
• Large Tankless Water Heaters: Whole-home electric tankless units frequently require multiple 40A breakers, but some high-capacity single-point commercial units require a dedicated 80A feed.
• Welders: Industrial MIG or TIG welders with high duty cycles often require 80A overcurrent protection, though NEC Article 630 allows specific conductor sizing based on the welder's nameplate duty cycle.

The Continuous Load Trap: A Real-World Scenario Walkthrough

The "continuous load" rule (NEC 210.20) states that if a load runs at its maximum current for three hours or more, the circuit must be sized at 125% of the load. Here is how ignoring this destroys installations.

1. The Setup: A hobbyist builds a home woodworking shop and installs a 65-amp continuous-load dust collection system that runs for 4-hour sessions. They install an 80-amp double-pole breaker and pull 4 AWG copper THHN (rated 85A at 75°C).
2. The Numbers: The wire is rated for 85A. The breaker is rated for 80A. The load pulls 65A. On paper, 65A is less than 80A, so it looks safe.
3. The Outcome: After about two and a half hours of continuous runtime, the 80-amp breaker trips violently. The woodworker resets it, and it trips again 20 minutes later. The neutral lug at the subpanel shows distinct blue/black heat discoloration.
4. What Went Wrong: Because the 65A load runs for more than three hours, it is a continuous load. NEC requires the circuit to be rated for 125% of 65A, which equals 81.25A. An 80-amp breaker is only legally allowed to carry a continuous load of 64A (80A × 0.80). The breaker's internal bimetallic strip slowly accumulated heat from the 65A draw and eventually tripped. The heat discoloration on the lug was caused by thermal creep from the breaker running at 101% of its continuous capacity.

The Fix: For a 65A continuous load, the woodworker needed a 90A or 100A breaker, and the wire should have been upgraded to 3 AWG copper (100A at 75°C) to match the new breaker size.

Worked Numeric Example: Voltage Drop and Temperature Derating

Ampacity tables assume an ambient temperature of 30°C (86°F). If your wire runs through a hot attic, or if the run is exceptionally long, you must apply math to verify your 4 AWG choice.

Scenario A: Voltage Drop on a 100-Foot Run
You are feeding an 80A load at 240V, 100 feet away, using 4 AWG copper. The formula for single-phase voltage drop is VD = (2 × K × I × L) / CM.

• K (Copper resistivity) = 12.9
• I (Current) = 80A
• L (Length) = 100 ft
• CM (Circular mils for 4 AWG) = 41,740

VD = (2 × 12.9 × 80 × 100) / 41,740 = 4.94V
4.94V on a 240V circuit is a 2.05% drop. This is well under the NEC's recommended 3% maximum for branch circuits. 4 AWG is perfectly fine here.

Scenario B: Temperature Derating in a Hot Attic
Now imagine that same 100-foot run passes through an attic that reaches 113°F (45°C) in the summer. You must apply the NEC Table 310.15(B)(1) correction factors. For the 75°C column at 45°C ambient, the derating factor is 0.82.

Derated Ampacity = 85A (base) × 0.82 = 69.7A
Your 4 AWG wire is now only legally allowed to carry 69.7 amps. If your load is 80A, the wire will overheat, the THHN insulation will soften, and you risk a short circuit. You must step up to 3 AWG copper (100A × 0.82 = 82A) to safely handle the 80A load in that environment.

Common Confusions and Mistakes to Avoid

Aluminum Oxidation: If you choose 2 AWG aluminum to save money on a long subpanel feeder, you must use a wire brush to clean the conductor and apply an anti-oxidant compound (like Noalox) before termination. Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates intense heat at the lug. Furthermore, aluminum creeps under pressure; you must torque the lugs to the manufacturer's exact specification (usually between 35 and 45 in-lbs for this size) using a calibrated torque screwdriver.

Frequently Asked Questions