How to Size Wire for a Generator Connection

Getting the wire size right for a generator connection matters more than most people realize. Undersized cable overheats, trips breakers, and can damage both the generator and the appliances connected to it. This guide walks through the full sizing process, from reading your generator's nameplate to pulling the correct gauge through conduit to your transfer switch.

Converting Generator Output to Amps

Generator capacity is listed in kilowatts (kW) or kilovolt-amperes (kVA), but wire sizing depends on amps. The formula for a 240 V single-phase generator is straightforward:

Amps = (kW × 1000) ÷ Voltage

For a 10 kW generator at 240 V: (10,000 ÷ 240) = 41.7 A

For a 22 kW generator at 240 V: (22,000 ÷ 240) = 91.7 A

Those are the continuous output amperes at full rated load. Most residential standby generators run at unity power factor, so kW and kVA are the same number. If your generator specs show kVA instead, multiply by the power factor (often 0.8) to get kW first.

Ampacity rules require the feeder to handle 125% of continuous loads. So the 10 kW generator's feeder must be rated for at least 52 A (41.7 × 1.25), and the 22 kW feeder needs at least 115 A (91.7 × 1.25).

Feeder Wire Size from Generator to Transfer Switch

Once you have the required ampacity, you select wire from an ampacity table. For copper conductors in conduit at 75 °C terminations, common reference values are:

These are starting points based on 75 °C copper ampacity. Derating for conduit fill, high ambient temperature, or a long outdoor run can push you to the next size up. Always verify against the current edition of the NEC and have your installation reviewed by a licensed electrician.

For a deeper look at how ampacity tables work, see the ampacity explained guide.

Portable Generator Inlet Box Wiring

A portable generator connects to the house through a generator inlet box mounted on an exterior wall. The inlet box gets a dedicated circuit from a manual transfer switch or interlock kit inside the panel.

The feeder from the inlet box to the transfer switch is sized exactly as above, based on the largest generator you plan to connect. Most homeowners use a 30 A inlet (NEMA L14-30) for a 5,000 to 8,000 W portable, which requires 10 AWG copper minimum. A 50 A inlet (NEMA 14-50 or L14-50) suits larger portables and needs 6 AWG copper.

The cord set connecting the generator to the inlet box must match or exceed the inlet rating. Undersized extension cords are a common failure point, especially with longer lengths. A 25-foot run at 30 A gets away with 10 AWG, but 50 feet at the same load should step up to 8 AWG to keep voltage drop under 3%.

Note that inlet boxes do not provide automatic transfer. The homeowner manually disconnects utility power, connects the generator, and starts it. That manual step is what prevents backfeed onto the utility line.

Standby Generator with Automatic Transfer Switch

A whole-home standby generator connects to an automatic transfer switch (ATS) or a load-center transfer switch that monitors utility voltage and switches over within seconds of a power failure. The wiring runs from the generator's output terminals through conduit to the ATS, then from the ATS to the main panel (or a subpanel).

Because the ATS feeds the entire panel, the feeder must be sized for the generator's full rated output, not just the expected load. A 22 kW standby feeding a 200 A main panel uses 2 AWG copper for the generator-to-ATS feeder. The ATS-to-panel side reuses whatever conductors are already in place.

Larger whole-home systems in the 30 to 48 kW range are often wired with 1/0 or 2/0 AWG, or may use aluminum feeders where the longer run justifies the weight and cost savings. If you go aluminum, bump up one size compared to copper and use anti-oxidant compound at all terminations.

The how to size wire for a subpanel article covers the panel-side wiring in more detail if you are adding a subpanel downstream of the transfer switch.

Voltage Drop on Long Outdoor Runs

Generator installations often involve a long run from the generator pad to the house, sometimes 50 to 150 feet across a yard. Voltage drop on that run reduces the voltage available at the transfer switch and can cause appliances to run inefficiently or motors to overheat on startup.

The guideline is to keep voltage drop below 3% on the feeder, though 2% is better for sensitive electronics and motor loads. On a 240 V circuit, 3% equals 7.2 V of allowable drop.

For the 10 kW generator (41.7 A) over a 100-foot run (200 feet of conductor for the round trip), 6 AWG copper drops about 6.7 V, which is just inside the 3% limit. At 150 feet, that same 6 AWG drops over 10 V, exceeding the limit. Stepping up to 4 AWG brings it down to around 6.5 V and restores acceptable performance.

The sizing cable for a long run article has voltage-drop tables and the full calculation method, including a worked example at several common distances.

For a systematic approach to any cable sizing job, how to size a cable step by step covers the full process from load calculation through derating.

Worked Examples

10 kW portable generator, 75-foot run to manual transfer switch:

• Output: 41.7 A at 240 V
• Required ampacity: 41.7 × 1.25 = 52 A
• Wire selected: 6 AWG copper (65 A rated at 75 °C)
• Voltage drop check at 75 ft (150 ft conductor): approximately 5 V, or 2.1% at 240 V. Acceptable.
• Conduit: 3/4-inch EMT, with two hots, one neutral, one ground.

22 kW standby generator, 50-foot run to ATS:

• Output: 91.7 A at 240 V
• Required ampacity: 91.7 × 1.25 = 114.6 A
• Wire selected: 2 AWG copper (130 A rated at 75 °C)
• Voltage drop check at 50 ft (100 ft conductor): approximately 2.3 V, or under 1% at 240 V. Well within limits.
• Conduit: 1-1/4-inch PVC Schedule 40, run underground with appropriate burial depth for the conduit type.

Frequently Asked Questions

What size wire do I need for a 7500-watt generator?

A 7,500 W generator at 240 V outputs 31.3 A. After the 125% continuous load factor, you need a feeder rated for at least 39 A. That points to 8 AWG copper (50 A rated), which gives adequate headroom. If the run is longer than 75 feet or conduit fill is tight, step up to 6 AWG.

Can I use aluminum wire for generator feeder?

Aluminum is acceptable for larger feeders (typically 1 AWG and up) where the cost and weight savings are significant. Use aluminum-rated lugs on both ends, apply anti-oxidant compound, and size up one gauge compared to the copper equivalent. Do not use aluminum for branch circuits or smaller runs where connection integrity is harder to maintain.

Does the wire from the generator to the inlet box need to be in conduit?

That depends on the installation. A fixed wiring method (conduit, MC cable, or UF cable buried at the correct depth) is required for permanent inlet box installations. The flexible cord connecting the generator to the inlet is a cord-and-plug connection, not permanent wiring, so it is not in conduit. Check your local authority having jurisdiction for specific requirements on exterior conduit type and burial depth.

How many circuits can a transfer switch support?

Manual transfer switches range from 6-circuit to 10-circuit models designed for partial-home backup. A whole-home ATS switches all circuits in the main panel. The transfer switch rating must match or exceed the generator's output amperage. A 10 kW generator with a 50 A outlet can feed a 6 to 10 circuit manual transfer switch rated at 50 A or higher.