For an 80-amp circuit, use 4 AWG copper wire or 2 AWG aluminum wire paired with an 80-amp double-pole breaker. This assumes standard residential conditions: copper conductors with 75°C rated insulation (like THHN/THWN-2), an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a single conduit.
- Material: Copper (unless aluminum is explicitly specified)
- Temperature Column: 75°C (standard for most modern residential breakers and terminals)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors (no derating required)
- Load Type: Non-continuous (operates for less than 3 hours at a time)
The Core Sizing Rules and Ampacity Math
When sizing wire for 80 amps, the National Electrical Code (NEC) dictates that the conductor's ampacity must meet or exceed the load, and the overcurrent protective device (breaker) must protect the wire. According to NEC Table 310.16, we look at the 75°C column for standard residential terminations.
| Wire Size (AWG) | Material | 75°C Ampacity | 90°C Ampacity (THHN) | Suitable for 80A Load? |
|---|---|---|---|---|
| 6 AWG | Copper | 65A | 75A | No (Too Small) |
| 4 AWG | Copper | 85A | 95A | Yes (Standard Choice) |
| 3 AWG | Copper | 100A | 115A | Yes (Oversized) |
| 2 AWG | Aluminum | 90A | 100A | Yes (Aluminum Choice) |
Why 4 AWG and Not One Size Smaller?
A common and dangerous mistake is using the 90°C column because the wire jacket (like THHN) is rated for 90°C. In the 90°C column, 6 AWG copper shows 75A. Under the NEC 'next size up' rule (240.4(B)), some DIYers assume they can protect 75A wire with an 80A breaker. However, NEC 110.14(C) requires you to use the lowest temperature rating of any connected component. Since almost all residential breaker terminals are rated for 75°C, you are legally and physically bound to the 75°C column. In that column, 6 AWG is only good for 65A. An 80A breaker will not adequately protect 65A wire under a full 80A load, risking a thermal meltdown at the lug before the breaker trips. I once inspected a DIY subpanel feed where the builder used 6 AWG THHN on an 80A breaker, citing the 90°C column on the wire spool. The breaker terminals weren't rated for 90°C, the lug overheated, and the insulation melted back an inch. Always terminate based on the equipment's lowest temperature rating.
Voltage Drop: When to Upsize for Long Runs
Ampacity tells you what the wire can handle thermally; voltage drop tells you what the load will actually receive. The NEC recommends a maximum 3% voltage drop for branch circuits and feeders to ensure equipment operates efficiently. For a 240V circuit, a 3% drop means you can lose a maximum of 7.2V.
Let's run the math for 4 AWG copper carrying 80A. The formula is VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is 80A, D is distance in feet, and CM is the circular mil area of 4 AWG (41,740).
| One-Way Distance | Calculated Voltage Drop | Percentage (at 240V) | Action Required |
|---|---|---|---|
| 50 feet | 2.47V | 1.03% | Use 4 AWG Copper |
| 100 feet | 4.94V | 2.06% | Use 4 AWG Copper |
| 150 feet | 7.41V | 3.08% | Upsize to 3 AWG Copper |
| 200 feet | 9.88V | 4.11% | Upsize to 2 AWG Copper |
If your run from the main panel to an 80-amp subpanel exceeds 130 feet, 4 AWG copper will result in a voltage drop greater than 3%. At 150 feet, you must step up to 3 AWG copper (CM = 52,620) to bring the drop back down to a safe 2.45%.
Derating and Environmental Factors That Change the Math
The baseline 4 AWG answer assumes ideal conditions. Real-world jobsites rarely cooperate. Here is what changes the answer and forces you to upsize:
- Conductor Bundling (Derating): If you pull more than three current-carrying conductors through a single conduit, they heat each other up. According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% adjustment factor. This is the one time you use the 90°C column: 4 AWG THHN at 90°C is 95A. Multiply 95A by 0.80, and your derated ampacity is 76A. Because 76A is less than your 80A load, 4 AWG fails when bundled. You must upsize to 3 AWG (115A × 0.80 = 92A).
- High Ambient Temperature: If your conduit runs through an unventilated attic in a hot climate where ambient temperatures routinely exceed 30°C (86°F), you must apply a temperature correction factor. At 40°C (104°F), the 75°C column correction factor is 0.88. 85A × 0.88 = 74.8A. Again, 4 AWG is now too small, and you must move to 3 AWG.
- Aluminum Conductors: If you are using aluminum wire (common for long feeder runs due to cost), the 75°C ampacity for 2 AWG aluminum is 90A, which safely covers the 80A requirement. Never mix copper and aluminum without proper bi-metallic lugs and antioxidant paste.
When an Engineer or AHJ Must Confirm
While this guide covers standard residential feeder and branch circuit sizing, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer confirm your sizing under the following conditions:
- Continuous Loads: If the 80-amp load will run for 3 hours or more continuously (e.g., heavy commercial HVAC, continuous-duty EV charging), NEC Article 210.20 requires you to multiply the load by 125%. An 80A continuous load becomes a 100A sizing requirement, mandating 3 AWG copper wire and a 100A breaker.
- Service Entrance Conductors: If this 80-amp feed is acting as the primary service drop from the utility meter to the main disconnect, utility specifications and local service entrance codes override standard feeder rules.
- High Fault Current: If your utility transformer supplies exceptionally high available fault current, the breaker's AIC (Ampere Interrupting Capacity) rating and the wire's bracing must be verified by an engineer to ensure the system won't explode during a dead short.
Frequently Asked Questions (Wire for 80 Amps)
Can I use 6 AWG wire on an 80-amp breaker for a short run?
No. Even if the run is only 10 feet long, 6 AWG copper is only rated for 65A in the 75°C termination column. While the 'next size up' breaker rule (NEC 240.4(B)) allows you to protect a wire with the next standard breaker size if the exact ampacity doesn't match a breaker, the wire's ampacity must still be greater than or equal to the actual calculated load. If your actual load is only 60A, you can use 6 AWG wire and an 80A breaker. But if you are wiring specifically to deliver a full 80 amps to a subpanel or appliance, 6 AWG will overheat and is a code violation.
What size wire do I need for an 80-amp subpanel 100 feet away?
For a 100-foot run to an 80-amp subpanel, 4 AWG copper or 2 AWG aluminum is sufficient for both ampacity and voltage drop (the drop will be roughly 2.06%, well under the 3% recommendation). However, remember that a subpanel feeder requires four wires: two hots, one neutral, and one equipment grounding conductor. If you are pulling all four through a single conduit, the neutral carries unbalanced current and counts as a current-carrying conductor in certain non-linear load scenarios, though for standard residential 240V/120V subpanels, the grounding wire does not count toward the derating bundle limit. You will have 3 current-carrying conductors (Hot, Hot, Neutral), meaning no derating is required, and 4 AWG copper remains the correct choice.
Does an 80-amp continuous load change the wire size?
Yes, drastically. The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. For continuous loads, you must size the conductors and the overcurrent device at 125% of the load. Therefore, an 80-amp continuous load requires wire rated for 100 amps (80 × 1.25 = 100). You would need to upsize to 3 AWG copper (100A at 75°C) and install a 100-amp breaker. Never put an 80A continuous load on a wire sized exactly for 80A; the thermal buildup over time will degrade the insulation and nuisance-trip the breaker.
