For a standard 50 amp subpanel, use 6 AWG copper wire or 4 AWG aluminum wire with a 50A double-pole breaker. This assumes THHN/THWN-2 conductors in conduit, a 75°C temperature rating, and a run under 100 feet. Let's break down the exact code rules and edge cases that dictate this pick.
The Baseline Assumptions
Wire sizing is never universal; it is strictly bound by installation conditions. Before pulling any cable, verify your project matches the baseline assumptions below. If your install deviates from these parameters, you must adjust your wire size accordingly.
- Material: Copper (default recommendation) or Aluminum (cost-saving alternative).
- Insulation Type: THHN/THWN-2 (standard for conduit) or XHHW-2.
- Temperature Column: 75°C (dictated by NEC 110.14(C) for standard residential terminals rated 100A or less).
- Ambient Temperature: 30°C (86°F) or less. Higher temperatures require derating.
- Installation Method: Raceway (PVC or EMT conduit) or direct burial (UF/USE-2 cable).
- Conductor Count: Maximum of 3 current-carrying conductors in the raceway (derating applies if you exceed this).
Ampacity and the 75°C Column Rule
A frequent and dangerous mistake DIYers make is looking at the 90°C column of NEC Table 310.16 and assuming 8 AWG copper THHN (rated 55A at 90°C) is sufficient for a 50A breaker. It is not.
While the wire insulation can handle 90°C, the physical brass and steel lugs inside your main panel, subpanel, and the 50A breaker itself are almost universally rated for a maximum of 75°C. The NEC requires you to size the wire based on the lowest temperature rating of any connected component. Therefore, we must use the 75°C column.
| Wire Size (AWG) | Copper (75°C) | Aluminum (75°C) | Sufficient for 50A? |
|---|---|---|---|
| 8 AWG | 40A | 30A | No (Undersized) |
| 6 AWG | 65A | 50A | Yes (Copper only) |
| 4 AWG | 85A | 65A | Yes (Both) |
In the 75°C column, 8 AWG copper is only rated for 40A. To safely carry a 50A load and terminate on a 50A breaker, you must step up to 6 AWG copper, which provides 65A of ampacity. If you choose aluminum, 6 AWG is only rated for 50A exactly, which leaves no margin for continuous load calculations; therefore, 4 AWG aluminum (65A) is the correct minimum pick.
Voltage Drop: When Distance Changes the Wire Size
Ampacity tells you what the wire can handle without melting. Voltage drop tells you what the wire will deliver to the load. The NEC recommends keeping voltage drop under 3% for feeders to ensure equipment operates efficiently. While often an informational note rather than a strict enforceable rule in all jurisdictions, many local inspectors enforce it rigorously.
Let's run a concrete voltage drop check for a 50A load at a distance of 150 feet using 240V single-phase power.
K = 12.9 (Copper), I = 50A, D = 150 ft, CM = 26,240 (Circular Mils for 6 AWG)
Plugging in the numbers for 6 AWG copper:
VD = (2 × 12.9 × 50 × 150) / 26,240 = 7.37 Volts.
Percentage: (7.37 / 240) × 100 = 3.07%.
At a full 50A continuous draw over 150 feet, 6 AWG copper slightly exceeds the 3% recommendation. If your subpanel will actually pull a continuous 50A (e.g., running a large EV charger and a welder simultaneously), you must bump the wire up to 4 AWG copper for a 150-foot run. However, if the 50A panel is for a detached garage with LED lighting, a few standard receptacles, and a 30A RV plug, the actual calculated load will rarely exceed 30A. At 30A, 6 AWG copper yields a highly acceptable 1.84% drop at 150 feet. Use the Southwire Voltage Drop Calculator to verify your specific continuous load.
Copper vs. Aluminum Feeder Decisions
Material choice drastically changes both the physical wire size and your termination procedure. Aluminum is significantly cheaper per foot than copper, but it requires a larger gauge due to higher electrical resistance and different thermal properties.
For a 50A feeder, the cost savings of aluminum over copper is relatively small (often less than $40 total for a 100-foot run). Because 6 AWG copper is much easier to bend in conduit, strips cleaner, and doesn't require anti-oxidant compounds, copper is the default recommendation for runs under 100 feet.
If you do choose 4 AWG aluminum (often sold as 4-4-4-6 AL SER cable for direct burial or indoor use), you must adhere to strict termination protocols:
- Apply an anti-oxidant paste (like Noalox) to the stripped aluminum conductors before inserting them into the lugs.
- Use a calibrated torque screwdriver to tighten the lugs to the exact inch-pound specification printed on the breaker or panel label. Aluminum undergoes thermal expansion and contraction at a different rate than brass terminals; under-torqued aluminum connections will loosen over time, arc, and cause a fire.
Decision Tree: Picking Your Exact Feeder Cable
Use this decision matrix to finalize your materials list. Follow the path that matches your installation conditions to arrive at your concrete wire pick.
| Condition / Scenario | Material | Required Wire Size | Breaker Size |
|---|---|---|---|
| Run < 100 ft, Standard Load | Copper | 6 AWG | 50A Double-Pole |
| Run < 100 ft, Standard Load | Aluminum | 4 AWG | 50A Double-Pole |
| Run 100–150 ft, High Continuous Load | Copper | 4 AWG | 50A Double-Pole |
| Run 100–150 ft, High Continuous Load | Aluminum | 2 AWG | 50A Double-Pole |
| Run > 150 ft | Either | Calculate VD; likely 3 AWG Cu / 1 AWG Al | 50A Double-Pole |
| Bundled with 4-6 other current-carrying wires | Copper | 4 AWG (Derating applied) | 50A Double-Pole |
When to Call an Engineer or the AHJ
While the guidelines above cover 95% of residential detached garage, shed, and workshop subpanel installs, certain edge cases require formal derating calculations or approval from your local Authority Having Jurisdiction (AHJ). You must consult a licensed electrical engineer or your local inspector if:
- Ambient Temperature Exceeds 30°C (86°F): If your conduit runs through an unventilated attic in a hot climate, or adjacent to a boiler, you must apply the temperature correction factors in NEC Table 310.15(B)(1). A 6 AWG wire in a 45°C attic loses nearly 30% of its ampacity.
- Conduit Fill Exceeds 3 Current-Carrying Conductors: If you are pulling two separate 240V circuits through the same PVC conduit (totaling 4 hot wires and 2 neutrals), you hit the adjustment factors in NEC 310.15(C)(1). The ampacity of 6 AWG copper drops to 52A (80% of 65A), which is dangerously close to the 50A breaker limit, necessitating a bump to 4 AWG.
- Continuous Load Dominance: If the primary purpose of the subpanel is to supply a single continuous load (running 3 hours or more, like commercial lighting or a continuous-duty compressor), NEC 210.20(A) requires the breaker to be sized at 125% of the load. A true 50A continuous load requires a 60A breaker and 4 AWG copper wire, not a 50A breaker.
For standard residential applications matching the baseline assumptions, purchase your 6 AWG copper THHN, a 50A double-pole breaker, and a 4-wire subpanel with an isolated neutral bar. Torque the lugs to spec, and you will have a safe, code-compliant feeder that passes inspection on the first try.
