How to Wire for a Generator: AWG and Breaker Sizing for 20kW Standby Units

For a 20kW (83.3A at 240V) standby generator, use 2 AWG copper THWN-2 conductors protected by a 110A double-pole breaker. This satisfies the 125% continuous load rule (104A minimum) assuming 75°C terminations, 30°C ambient, and three current-carrying conductors in PVC conduit.

The Core Sizing Matrix: Generator kW to AWG & Breaker

Before pulling wire, you must establish your baseline assumptions. Sizing changes drastically if you alter the insulation type, ambient temperature, or conductor material. The matrix below is the quick-reference spec sheet for standard residential split-phase (120/240V) standby generators.

Source: Sizing derived from NFPA 70 (NEC) Table 310.16 and Table 250.122.

Why 2 AWG and Not 3 AWG? The 125% Continuous Load Rule

A common mistake when figuring out how to wire for a generator is sizing the wire strictly to the nameplate continuous amperage. A 20kW generator pushing 83.3A at 240V seems like a perfect match for 3 AWG copper, which is rated for 100A in the 75°C column. This is a code violation.

Under NEC Article 210.20(A) and 215.2, continuous loads (those expected to run for 3 hours or more, which a standby generator absolutely will during a multi-day outage) must be multiplied by 125% for conductor sizing.

• Base Load: 83.3A
• 125% Multiplier: 83.3A × 1.25 = 104.1A

Your conductor must have an ampacity of at least 104.1A. Looking at NEC Table 310.16 (75°C column), 3 AWG is only good for 100A. It fails. 2 AWG is rated for 115A, making it the minimum legal copper size.

For the breaker, NEC 240.4(B) allows you to round up to the next standard size if the exact ampacity doesn't match a standard breaker. The next standard size above 104.1A is 110A. (Note: If your specific Automatic Transfer Switch (ATS) is rated for 100% continuous duty—a rare and expensive specification for residential gear—you could use a 100A breaker, but 110A is the standard off-the-shelf requirement).

Voltage Drop Verification at 150 Feet

Ampacity keeps the wire from melting; voltage drop keeps your HVAC compressors and well pumps from burning out. The NEC recommends a maximum 3% voltage drop on branch circuits and feeders.

Let's verify our 2 AWG copper run from the generator pad to the ATS at a realistic distance of 150 feet.

At 2.02%, we are well under the 3% threshold. However, if your generator pad is located 250 feet away from the ATS, the drop jumps to 3.37%. At that distance, you must upsize to 1/0 AWG copper to maintain power quality, regardless of the thermal ampacity rules.

Variables That Force an Upsize: Length, Bundling, and Aluminum

The baseline matrix assumes ideal conditions. Jobsite realities often force you to upsize your conductors. Here is what changes the answer.

1. Conduit Bundling and Derating

NEC Table 310.15(C)(1) requires you to derate conductor ampacity when you bundle more than three current-carrying conductors in a single raceway.

Bench Tip: In a standard 120/240V single-phase split-phase system, the neutral only carries the unbalanced load. Under NEC 310.15(E)(1), it is not counted as a current-carrying conductor for derating purposes. Therefore, your two hot legs (L1, L2) count as two conductors. You can safely pull a 4-wire feed (L1, L2, Neutral, Ground) without triggering the 80% derating penalty. If you are wiring a 3-phase commercial generator, the neutral often counts, forcing you to apply the 80% derating factor and upsize your wire.

2. Switching to Aluminum (XHHW-2)

Many installers prefer aluminum for long outdoor runs to save money, but you cannot swap copper and aluminum interchangeably. Aluminum has higher resistance and lower thermal tolerance. If you switch to aluminum, you must consult the 75°C column for aluminum and typically jump up two AWG sizes.

Note: When terminating aluminum wire, you must use an antioxidant compound (like Noalox) and torque the lugs to the manufacturer's exact inch-pound specification to prevent cold creep and subsequent arcing.

3. Wet vs. Dry Location Insulation

THHN is strictly rated for dry locations. Once that wire enters an exterior PVC conduit, condensation turns the inside of the raceway into a wet location. You must use wire marked THWN-2 or XHHW-2. Most modern building wire is dual-rated (THHN/THWN-2), but always check the printing on the jacket before pulling it into outdoor conduit.

When the AHJ or a Licensed Engineer Must Confirm

While sizing a feeder from a generator to an ATS is straightforward, certain scenarios push the project out of DIY territory and require a licensed electrical engineer or explicit AHJ sign-off.

1. Service Entrance Ratings: If your ATS is acting as the main service disconnect (a "Service Rated" ATS), the conductors between the utility meter and the ATS are Service Entrance Conductors. These are governed by NEC Article 230, not standard feeder rules, and require utility coordination.
2. Paralleling Conductors: If you are wiring a massive 60kW+ liquid-cooled unit requiring 400A+ feeders, you will likely need to parallel multiple sets of 350 kcmil or 500 kcmil wires. Paralleling requires exact matching of length, material, and routing, and must be explicitly approved by the local inspector.
3. Utility Interconnect Agreements: Even with an ATS that physically prevents backfeeding, many utilities require a formal interconnect application and a visible, lockable exterior disconnect switch before they will approve the transfer switch installation.

Getting the wire size right on the first pull saves you from digging up a buried PVC trench because the inspector flagged a 125% continuous load violation. Stick to the 75°C column, respect the wet-location ratings, and always torque your lugs with a calibrated inch-pound torque screwdriver.