What Wire Gauge for a 50A 120/240V Generator at a 125 ft Distance?

When setting up a 50A manual transfer switch or an interlock kit for a portable generator, distance is your biggest enemy. While a standard 50-amp circuit might use #6 gauge wire for short runs inside a house, a 125-foot run introduces significant voltage drop. If your wire is too thin, your appliances will struggle to start, motors may overheat, and your generator will work harder than necessary. Here is the technical breakdown for the correct wire gauge at this distance.

1. The 3% Voltage Drop Rule

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits. At 240V, a 3% drop allows for a loss of only 7.2 volts.

• The Problem: A standard #6 AWG copper wire at 50 amps over 125 feet results in a voltage drop of approximately 3.3% to 3.5%.
• The Recommendation: While #6 copper is technically "rated" for 50A (per the 75°C column), many electricians recommend "upsizing" to #4 AWG for a 125-foot run to ensure sensitive electronics and HVAC compressors receive full power.

2. Copper vs. Aluminum Wire Gauge

Because copper is expensive, many homeowners choose Aluminum (SER or USE-2) for long generator runs. However, aluminum is less conductive, so you must use a larger gauge.

• Copper (#4 AWG): This is the premium choice. It handles the 50A load with minimal voltage drop (approx 2.1%) at 125 feet.
• Aluminum (#2 AWG): To match the performance of copper at this distance, you should use #2 AWG Aluminum. Aluminum requires anti-oxidant paste (Noalox) at the terminals and must be rated for the lugs on your inlet box.

3. Selecting the Correct Wire Type

The environment where the wire is placed dictates the insulation type:

• THHN/THWN-2: Best for pulling through PVC Conduit. You will need four individual strands (Black, Red, White, and Green).
• UF-B (Underground Feeder): Rated for direct burial, though #4 or #2 UF-B cable is very stiff and difficult to work with.
• USE-2 or Mobile Home Feeder: Common for long-distance underground aluminum runs.

4. Conduit Size Requirements

If you are pulling #4 Copper or #2 Aluminum, a standard 1-inch conduit will be a very difficult pull due to "conduit fill" limits and the stiffness of the wire.

1. For #4 Copper (4 wires): Use at least 1-1/4" Schedule 40 PVC.
2. For #2 Aluminum: Use 1-1/2" Schedule 40 PVC to allow for easier bends and heat dissipation.

5. The Grounding Requirement

Even though the generator is 125 feet away, you must run a four-wire system (two hots, one neutral, and one equipment ground).

• Do not rely on a local ground rod at the generator to act as the return path.
• The ground wire for a 50A circuit should be #10 Copper or #8 Aluminum, regardless of the "upsizing" of the hot conductors.

Summary Table: 50A 125ft Wire Selection

Conclusion

For a 50A generator run at 125 feet, you should move past standard #6 wire. To stay well within safety margins and ensure your home's appliances run cool, #4 AWG Copper or #2 AWG Aluminum is the professional choice. This "upsizing" accounts for the voltage drop over the distance and provides a more robust electrical connection that can handle the surge currents of starting a well pump or a central A/C unit during a power outage.