What Size Wire for an 80 Amp Breaker? The Complete 80A Wire Size Guide

An 80A wire size is the minimum conductor cross-section required to safely carry 80 amps of current without exceeding the thermal limits of the wire insulation or the connected equipment terminals. For a standard 80-amp breaker, you need 4 AWG copper or 2 AWG aluminum wire when using the 75°C column of NEC Table 310.16. If you are using NM-B (Romex) cable, which is restricted to the 60°C column, you must step up to 3 AWG copper.

Sizing wire isn't just about matching a number on a breaker handle. It requires navigating termination temperature limits, voltage drop over distance, and continuous load derating. Below is the exact data you need to pull the right wire from the spool without failing inspection or creating a fire hazard.

The NEC Ampacity Table for 80A Circuits

The National Electrical Code (NEC) dictates wire sizing based on the insulation temperature rating and the terminal temperature rating of your equipment. Most modern breakers and lugs are rated for 75°C, meaning you must use the 75°C ampacity column, even if your wire insulation (like THHN) is rated for 90°C.

Worked Example: Sizing an 80A Subpanel Feeder

Ampacity tables assume ideal conditions and short runs. In the real world, voltage drop dictates your final wire size. NEC 210.19(A) recommends keeping voltage drop under 3% for branch circuits and feeders to ensure equipment operates efficiently.

The Scenario: You are running a 240V feeder to a garage subpanel protected by an 80A breaker. The one-way distance from the main panel to the subpanel is 120 feet. You plan to use copper THHN in PVC conduit.

Step 1: Check base ampacity.
4 AWG Copper at 75°C is rated for 85A. This is legally sufficient to terminate on an 80A breaker.

Step 2: Calculate Voltage Drop (VD).
The formula for single-phase voltage drop is: VD = (2 × K × I × L) / CM

• K (Copper resistivity) = 12.9
• I (Current) = 80A
• L (Length) = 120 ft
• CM (Circular Mils for 4 AWG) = 41,740

VD = (2 × 12.9 × 80 × 120) / 41,740 = 5.93V

Step 3: Evaluate the percentage.
5.93V / 240V = 2.47%. This is under the 3% threshold. 4 AWG Copper is approved for this run.

The Edge Case: What if the garage is 180 feet away?
Recalculating with L=180 yields a voltage drop of 8.9V (3.7%). This exceeds 3%. You must bump the wire size to 3 AWG Copper (CM = 52,620), which drops the VD to 7.07V (2.94%), bringing it back into code compliance. Always run the math on long runs; the NEC table only gets you halfway there.

Where You Meet 80A Loads in Practice

You won't find 80A circuits powering standard wall receptacles. This gauge of wire and breaker size is reserved for heavy, dedicated infrastructure. Here is where you will actually pull this wire, and what it changes in your physical installation:

• EV Level 2 Chargers: High-speed residential chargers (like the Tesla Wall Connector or ChargePoint Home Flex) can be hardwired for 48A to 64A continuous draws. This requires 60A to 80A breakers. What it changes: Because EV charging is a "continuous load" (over 3 hours), NEC 210.20(A) requires the breaker to be sized at 125% of the load. A 64A charger requires an 80A breaker and wire sized for 80A.
• Subpanels: Feeding a detached garage, workshop, or barn. What it changes: You must pull four wires (two hots, one neutral, one ground). Four 4 AWG THHN wires require a minimum of 1-inch EMT or PVC conduit to satisfy NEC Chapter 9 conduit fill tables.
• Tankless Water Heaters: Large whole-home electric tankless units often require dual 40A breakers or a single massive 80A to 120A feed. What it changes: The physical stiffness of 4 AWG and 2 AWG wire makes routing difficult in tight spaces. You must account for bending radius and use a torque screwdriver to tighten the lugs (typically 35-45 in-lbs, verify with the breaker manufacturer) to prevent thermal loosening over time.

Common Sizing Mistakes and Code Traps

When sizing wire for an 80A circuit, DIYers and junior electricians frequently fall into three specific traps that result in failed inspections or overheated panels.

Trap 1: The 90°C Column Illusion

THHN wire is rated for 90°C. Looking at the table above, 4 AWG copper at 90°C is rated for 95A. Many builders assume they can protect 4 AWG THHN with a 90A breaker. This violates NEC 110.14(C). The breaker lugs are almost certainly rated for 75°C. The weakest link in the thermal chain dictates the ampacity. You must use the 75°C column for termination sizing, capping 4 AWG at 85A.

Trap 2: Confusing the Load with the Breaker

If your appliance draws 80 amps continuously, you cannot use an 80A breaker. Continuous loads must be derated by 125%. An 80A continuous load requires a 100A breaker and wire sized for 100A (which means stepping up to 3 AWG Copper or 1 AWG Aluminum at 75°C). Only use 4 AWG Copper if the breaker itself is 80A, protecting a non-continuous load or a continuous load no larger than 64A.

Trap 3: Ignoring Aluminum Oxide

Aluminum wire (like 2 AWG Al for an 80A feeder) is significantly cheaper than copper, making it popular for long subpanel runs. However, aluminum oxidizes and creeps under pressure. If you use aluminum, you must apply an anti-oxidant compound (like Noalox) to the stripped conductor before torquing it into the lug, and you must use lugs explicitly rated for aluminum (marked AL or CU/AL). Skipping this step leads to high-resistance connections and melted breaker terminals.