For an 80 amp sub panel, use a minimum of 4 AWG copper or 2 AWG aluminum wire, protected by an 80A double-pole breaker. This assumes 75°C terminations and a short run. For practical installations exceeding 50 feet, upsize to 3 AWG copper to mitigate voltage drop.
- Conductor Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2).
- Temperature Column: 75°C column per NEC 110.14(C) for termination ratings.
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single circuit in conduit (Raceway) or NM-B cable, not bundled with other current-carrying conductors.
Note: This is NEC-style guidance; your local AHJ (Authority Having Jurisdiction) has final authority on all installations.
The Core Sizing Matrix: Copper vs. Aluminum for 80A
Sizing a feeder for an 80A subpanel requires matching the wire's ampacity to the breaker rating while respecting the weakest link in the thermal chain: the termination lugs. Even if you use 90°C rated THHN wire, NEC 110.14(C) mandates that you must use the 75°C ampacity column for sizing unless the equipment is explicitly rated for 90°C (which almost no residential panels are).
Below is the exact data from NEC Table 310.16 for the conductors typically considered for an 80A feeder.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Derating Base) | Max Standard Breaker | Passes 80A Minimum? |
|---|---|---|---|---|---|
| 6 AWG | Copper | 65A | 75A | 70A | No (Undersized) |
| 4 AWG | Copper | 85A | 95A | 90A | Yes (Minimum Legal) |
| 3 AWG | Copper | 100A | 110A | 100A | Yes (Recommended for VD) |
| 2 AWG | Aluminum | 90A | 100A | 90A | Yes (Minimum Legal Al) |
| 1 AWG | Aluminum | 100A | 115A | 100A | Yes (Recommended for VD) |
Don't forget the Equipment Grounding Conductor (EGC): Per NEC 250.122, an 80A breaker requires a minimum 6 AWG copper or 4 AWG aluminum ground wire.
Why 4 AWG Copper (And Why 6 AWG Fails the NEC Test)
A common mistake on DIY forums is recommending 6 AWG copper for an 80A subpanel, confusing it with the 60A requirement. Let's look at the math. Under the 75°C column, 6 AWG copper is rated for exactly 65A. Because 65A is less than the 80A breaker protecting it, the wire will overheat before the breaker trips during a sustained fault. This is a direct violation of NEC 240.4.
Stepping up to 4 AWG copper gives you an 85A rating at 75°C. Since 85A is greater than the 80A load/breaker requirement, it passes the baseline ampacity test. Furthermore, under NEC 240.4(B), you are allowed to round up to the next standard breaker size if the ampacity doesn't perfectly match a standard breaker. If you had a calculated load of exactly 80A, 4 AWG (85A) protects it perfectly on an 80A breaker.
When would you need 3 AWG or 2 AWG?
If your subpanel is feeding a continuous load (defined by the NEC as a load expected to run for 3 hours or more, like a heavy EV charger or large server rack) that draws a full 80A, you must multiply the load by 125%. 80A × 1.25 = 100A. In that specific scenario, 4 AWG (85A) fails, and you must step up to 3 AWG copper (100A) or 2 AWG copper (115A) to satisfy the continuous load rule.
Voltage Drop: The 50-Foot Tipping Point
Ampacity keeps the wire from melting; voltage drop keeps your appliances from browning out. The NEC recommends a maximum 3% voltage drop on a feeder (NEC Informational Note 310.15(B)). For a 240V system, 3% equals a 7.2V drop.
Let's run the voltage drop formula: VD = (2 × K × I × L) / CM
- K (Copper constant) = 12.9
- I (Current) = 80A
- L (One-way length in feet)
- CM (Circular Mils for 4 AWG) = 41,740
If you run 4 AWG copper at a full 80A load for 100 feet, the drop is 4.94V (2.05%). You are safe. But if that run stretches to 150 feet, the drop hits 7.41V (3.08%), pushing you over the 3% threshold.
| One-Way Distance | Copper Size Required | Aluminum Size Required | Voltage Drop at Full 80A |
|---|---|---|---|
| 0 - 50 feet | 4 AWG | 2 AWG | < 1.5% (Excellent) |
| 51 - 140 feet | 4 AWG | 2 AWG | 1.5% - 2.9% (Acceptable) |
| 141 - 180 feet | 3 AWG | 1 AWG | 2.3% - 2.9% (Acceptable) |
| 181 - 250 feet | 2 AWG | 1/0 AWG | 2.2% - 2.9% (Acceptable) |
Derating, Bundling, and When to Call the AHJ
The baseline sizes above assume your conduit contains only one feeder (two hot wires and one neutral). If you change the physical installation method, the ampacity changes drastically.
The Bundling Trap (NEC 310.15(C)(1))
Suppose you are running feeders to two separate detached garages and pull both through the same 1.5-inch PVC conduit. You now have four current-carrying conductors (the two hots from each feeder; neutrals carrying only unbalanced 120V return current do not count). Four to six conductors in a single raceway triggers an 80% derating factor.
Here is where the 90°C column saves you. You must apply the derating factor to the 90°C ampacity of the wire:
- 4 AWG THHN (90°C rating): 95A × 0.80 = 76A. This is now less than your 80A breaker. It fails.
- 3 AWG THHN (90°C rating): 110A × 0.80 = 88A. This is greater than 80A. It passes.
If you bundle multiple 80A feeders in one conduit, you must upsize to a minimum of 3 AWG copper, regardless of the run length.
Ambient Temperature Derating
If your conduit runs across an unventilated attic in a hot climate where ambient temperatures exceed 30°C (86°F), you must apply temperature correction factors. At 41-45°C (105-113°F), the correction factor for 90°C wire is 0.87. Again, calculate against the 90°C column to ensure the final derated ampacity remains above 80A.
When an Engineer or AHJ Must Confirm
While a standard 80A detached garage or workshop subpanel is well within the scope of standard NEC rules and typical DIY/homeowner permitting, you must involve a licensed electrical engineer or consult your local AHJ directly if:
- You are paralleling conductors (only legal for 1/0 AWG and larger, which doesn't apply to standard 80A feeds but is a common point of confusion).
- The subpanel is part of a complex solar/battery backup system with bidirectional power flow, which alters busbar rating calculations under NEC 705.12.
- Your local municipality has strict amendments enforcing a hard 2% or 3% voltage drop limit that requires signed engineering calculations rather than the standard NEC informational notes.
Always torque your panel lugs to the exact specification printed on the equipment label (typically 40-50 in-lbs for these smaller residential lugs) using a calibrated inch-pound torque screwdriver. Loose connections on 80A feeders are the leading cause of thermal degradation and panel fires, far outstripping wire sizing errors.
