For an 80 amp breaker, you need 4 AWG copper wire (THHN/THWN-2) or 2 AWG aluminum wire. This assumes a standard 75°C termination rating, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway. Always pair this with an 80A double-pole breaker for 240V loads.
The Baseline Assumptions for 80A Sizing
Wire sizing is never a single universal number; it is the result of specific environmental and material variables. Before pulling any wire through conduit, you must verify that your installation matches the baseline assumptions used to select 4 AWG copper. If your conditions differ, the required wire gauge will change.
- Material: Copper (Cu) or Aluminum (Al) — never interchange their ampacities.
- Insulation: THHN/THWN-2 (rated for 90°C, but terminated at 75°C).
- Temperature Column: 75°C (per NEC 110.14(C), modern breakers and lugs >100A are 75°C rated, and 80A equipment is universally treated as 75°C for termination limits).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway (no derating required).
Under these exact conditions, the National Electrical Code (NEC) Table 310.16 dictates that 4 AWG copper has an allowable ampacity of 85 amps in the 75°C column. Because 85A exceeds the 80A breaker rating, the wire is protected from overcurrent, satisfying NEC 240.4.
Ampacity Specifications and Voltage Drop Check
Selecting a wire that won't melt under the breaker's trip curve is only half the job. The other half is ensuring the voltage at the far end of the run remains within acceptable limits. The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently and motors don't overheat.
| Wire Gauge | Material | Insulation Type | Ampacity (75°C) | Circular Mils (CM) |
|---|---|---|---|---|
| 4 AWG | Copper | THHN/THWN-2 | 85 Amps | 41,740 |
| 3 AWG | Copper | THHN/THWN-2 | 100 Amps | 52,620 |
| 2 AWG | Aluminum | XHHW-2 | 90 Amps | 66,360 |
Voltage Drop Calculation at 120 Feet
Let's run a voltage drop check for a standard 240V circuit using 4 AWG copper, pulling a full 80A load over a distance of 120 feet. Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM.
- K (Copper resistance constant) = 12.9
- I (Current) = 80A
- D (Distance) = 120 ft
- CM (Circular mils for 4 AWG) = 41,740
VD = (2 × 12.9 × 80 × 120) / 41,740 = 5.93 Volts.
A 5.93V drop on a 240V circuit is a 2.47% drop. This is well under the 3% maximum threshold. However, if your run extends to 160 feet, the drop hits 3.3%, meaning you must upsize to 3 AWG copper to maintain code-recommended efficiency. You can verify your specific run using the Southwire Voltage Drop Calculator.
Decision Tree: When to Upsize Your Wire
Real-world installations rarely match textbook baselines perfectly. Use this decision path to determine your exact wire pick based on your specific routing constraints.
| Installation Condition | Required Action | Final Wire Pick |
|---|---|---|
| Standard run < 145 feet, Copper | Use baseline sizing. | 4 AWG Copper |
| Standard run > 145 feet, Copper | Upsize to mitigate >3% voltage drop. | 3 AWG Copper |
| Standard run < 115 feet, Aluminum | Use baseline aluminum sizing. | 2 AWG Aluminum |
| 4 to 6 current-carrying conductors in one conduit | Apply 80% derating factor (NEC Table 310.15(C)(1)). 85A × 0.80 = 68A (Fails 80A requirement). | 3 AWG Copper (100A × 0.80 = 80A) |
| Ambient temperature is 40°C (104°F) or higher | Apply temperature correction factor. 4 AWG at 40°C drops below 80A. | 3 AWG Copper |
If your 80A load will run continuously for 3 hours or more (e.g., a large commercial heater, or an EV charger configured to pull 64A+ continuously), NEC 210.20(A) requires the branch circuit to be rated at 125% of the continuous load. An 80A continuous load requires a 100A breaker and 3 AWG copper wire. If you are locked into an 80A breaker, your maximum continuous load cannot exceed 64A (80A / 1.25). Do not ignore this rule; thermal buildup over time will degrade 4 AWG insulation if pushed to 80A continuously.
Why Not One Size Smaller? (The 6 AWG Trap)
A common and dangerous mistake on the jobsite is attempting to use 6 AWG copper wire on an 80A breaker, usually because 4 AWG is stiff, difficult to pull through conduit, and expensive.
Looking at NEC Table 310.16, 6 AWG copper in the 75°C column is rated for only 65 amps (and 55 amps in the 60°C column). If you connect 6 AWG wire to an 80A breaker, you have created a severe fire hazard. The breaker will not trip until the current exceeds 80 amps. If the circuit pulls a sustained 75-amp load, the breaker stays closed, but the 6 AWG wire is operating 10 amps above its thermal limit. The insulation will soften, melt, and eventually short out or ignite inside the walls. The overcurrent protective device must always be sized to protect the weakest link in the circuit; therefore, the wire's ampacity must always meet or exceed the breaker's rating.
When an Engineer or the AHJ Must Confirm
While the guidelines above cover 95% of residential and light-commercial 80A subpanel feeds or heavy appliance circuits, certain scenarios require formal sign-off from a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ).
- High-Temperature Environments: If the conduit runs through an attic that routinely exceeds 50°C (122°F) in the summer, or near a boiler room, standard derating tables may not suffice. An engineer must calculate the exact thermal envelope.
- Complex Harmonic Loads: If the 80A circuit feeds heavy variable-frequency drives (VFDs) or large banks of LED drivers, neutral currents can exceed phase currents due to triplen harmonics. The AHJ may require an oversized neutral or a different derating schedule.
- Utility Interconnection: If this 80A circuit is part of a solar inverter output or a backup generator transfer switch, the utility company's specific interconnection standards will override general NEC baseline sizing.
For standard runs to a subpanel, a workshop welder, or a heavy-duty tankless water heater, stick to the decision tree above. Buy high-quality THHN/THWN-2 copper, torque your lugs to the manufacturer's exact inch-pound specifications using a calibrated torque screwdriver, and your 80A circuit will operate safely for decades.
