The Direct Answer: Wire and Breaker Sizing for a 50A Subpanel
For a standard 240V feeder, use 6 AWG copper wire protected by a 50-amp double-pole breaker. This assumes copper conductors with THHN/THWN-2 insulation rated at 75°C, installed in conduit at a 30°C (86°F) ambient temperature. If your run exceeds 140 feet, upsize to 4 AWG copper to manage voltage drop.
- Material: Copper (Aluminum requires different sizing)
- Insulation: THHN/THWN-2 in conduit, or NM-B (Romex)
- Temperature Column: 75°C (standard for modern panel terminations)
- Ambient Temperature: 30°C (86°F) or lower
- System Voltage: 240V split-phase (2 hots, 1 neutral, 1 ground)
Sizing a subpanel feeder is not just about matching the breaker to the wire's absolute melting point; it is about navigating the intersection of thermal limits, voltage drop physics, and strict National Electrical Code (NEC) overcurrent rules. Below is the exact framework to select your materials without failing inspection or starving your downstream circuits of voltage.
The Ampacity Table: Why 6 AWG and Not 8 AWG?
A common mistake on the jobsite is looking at the 75°C column of the ampacity table, seeing that 8 AWG copper is rated for 50 amps, and pulling 8 AWG wire. This is a code violation.
| AWG Size | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) | Max Breaker Size (NEC 240.4) |
|---|---|---|---|---|
| 8 AWG | 40A | 50A | 55A | 40A* |
| 6 AWG | 55A | 65A | 75A | 60A |
| 4 AWG | 70A | 85A | 95A | 80A |
Under NEC Article 240.4(D), there is a specific restriction for small conductors: 8 AWG copper is strictly limited to a maximum 40A overcurrent protective device for standard installations. Even if your terminations are rated for 75°C and the wire's 90°C insulation could theoretically handle more heat, the breaker cannot exceed 40A. Therefore, to feed a 50A subpanel, 6 AWG copper (rated 65A at 75°C) is the absolute legal minimum.
When using NM-B cable (commonly known as Romex) instead of individual THHN wires in conduit, you are legally bound to the 60°C column of NEC Table 310.16, regardless of the fact that the panel lugs are rated for 75°C. Fortunately, 6 AWG NM-B is rated for 55A in the 60°C column, which still safely clears the 50A breaker threshold. However, NM-B is limited to indoor, dry locations and cannot be pulled through underground conduit. For any outdoor run or trench, you must use individual THHN/THWN-2 conductors inside PVC or EMT conduit.
Voltage Drop: When Distance Forces an Upsize
Ampacity dictates the minimum wire size to prevent a fire; voltage drop dictates the maximum distance you can run that wire before your equipment malfunctions. The NEC recommends a maximum 3% voltage drop on feeders. When a 240V welder or air compressor receives only 228V due to excessive line drop, the motor draws higher amperage to compensate for the missing wattage. This excess current generates heat in the windings, prematurely killing the motor and potentially tripping the branch breaker inside the subpanel.
Let's run the math for a 50A load at 240V using 6 AWG copper (Circular Mils = 26,240; K-factor for copper = 12.9) using the standard voltage drop formula:
- At 100 feet: Voltage drop is ~4.9V (2.0%). Result: Pass.
- At 140 feet: Voltage drop is ~6.9V (2.8%). Result: Pass (barely).
- At 150 feet: Voltage drop is ~7.4V (3.1%). Result: Fail.
If your trench or conduit run measures 150 feet or more from the main panel to the subpanel, 6 AWG copper will starve your 240V tools and HVAC equipment. You must upsize to 4 AWG copper for runs up to 200 feet.
Decision Tree: Choose Your Exact Wire and Breaker
Use this matrix to lock in your shopping list. Find your distance and material, and buy exactly what is listed.
| Scenario / Run Length | Wire Size (Copper) | Wire Size (Aluminum) | Breaker Size |
|---|---|---|---|
| Run < 100 ft, standard loads | 6 AWG THHN or NM-B | 4 AWG XHHW | 50A Double-Pole |
| Run 100–140 ft | 6 AWG THHN or NM-B | 4 AWG XHHW | 50A Double-Pole |
| Run 140–200 ft | 4 AWG THHN | 2 AWG XHHW | 50A Double-Pole |
| Run > 200 ft | 3 AWG or 2 AWG THHN | 1 AWG or 1/0 AWG | 50A Double-Pole |
What Changes the Baseline? (Aluminum, Bundling, and Continuous Loads)
The 6 AWG copper baseline holds true for 90% of residential garage and workshop subpanels. However, three specific variables will force you to alter this pick.
1. Switching to Aluminum Conductors
Copper and aluminum are not interchangeable. Aluminum has higher resistance and expands/contracts more under thermal cycling. If you choose aluminum to save money on long runs, you must use the 75°C column. 4 AWG aluminum is rated for 65A at 75°C, making it the legal minimum for a 50A breaker. Many DIYers are tempted by the lower cost of aluminum XHHW, but it requires meticulous termination prep. You must wire-brush the aluminum conductor to remove the invisible oxide layer immediately before applying an anti-oxidant paste (like Noalox). Furthermore, if your panel lugs are not explicitly marked 'AL' or 'AL/CU', you cannot use aluminum.
2. Conduit Bundling and Derating
If you are pulling this 4-wire feeder (2 hots, 1 neutral, 1 ground) through a conduit by itself, no derating is required. The ground does not count as a current-carrying conductor (CCC), and the neutral only carries unbalanced return current, which NEC Table 310.15(C)(1) excludes from derating calculations. However, if you share that conduit with another circuit, pushing your total CCC count to 4 or more, you must apply an 80% derating factor. In that scenario, 6 AWG THHN (90°C base rating of 75A × 0.8 = 60A) still passes, but you are cutting it close. Keep subpanel feeders in their own dedicated raceway.
3. The Continuous Load Trap
A 50A breaker is only legally allowed to supply 40A of continuous load (current expected to run for 3 hours or more). If your subpanel is feeding a kiln, a continuous-duty air compressor, or an EV charger that pulls 48A constantly, a 50A breaker will nuisance-trip or overheat. For continuous loads exceeding 40A, you must upsize the breaker to 60A or 70A, which mandates stepping up to 4 AWG copper wire.
When to Call the AHJ or an Engineer
While the physics and the NEC provide a clear baseline, local Authority Having Jurisdiction (AHJ) amendments always hold final authority. You must pull a permit and have an inspector verify your work in the following scenarios:
- Detached Structures: If the subpanel is in a detached garage or barn, NEC 250.32 requires a separate Grounding Electrode System (typically two ground rods) at the detached building, and the neutral and ground bars in the subpanel must remain isolated. The grounding electrode conductor (GEC) from the subpanel to the ground rods must be sized per NEC Table 250.66. For a 50A feeder using 6 AWG copper, a bare 8 AWG copper GEC is typically the minimum requirement. Getting the bonding strap wrong here is a leading cause of inspection failures and shock hazards.
- Local Minimums: Some municipal codes mandate a minimum of 4 AWG copper or 2 AWG aluminum for any subpanel feeder, regardless of the breaker size, to ensure mechanical strength and future-proof the infrastructure.
- Outdoor Disconnects: If your main panel is indoors and the subpanel is fed via an underground trench, recent NEC editions require an exterior emergency disconnect. Sizing and grounding this exterior enclosure requires site-specific evaluation.
Stick to 6 AWG copper for runs under 140 feet, torque your lugs to the manufacturer's spec sheet using a calibrated inch-pound torque screwdriver (typically 35 to 50 inch-pounds for these sizes), and keep your neutral and ground isolated in the subpanel. That is the definitive path to a safe, code-compliant 50-amp feeder.
