Sizing Your ATS: The Decision Path

An Automatic Transfer Switch (ATS) must match or exceed the rating of the utility service entrance conductors feeding it. If you undersize the ATS, it becomes a bottleneck and a fire hazard; if you oversize it unnecessarily, you waste money and struggle with stiff, oversized cables in tight enclosures. Use the decision table below to match your main breaker size to the correct ATS and wire gauge.

Main Breaker SizeRequired ATS RatingMinimum Wire Gauge (Aluminum XHHW-2)Minimum Wire Gauge (Copper THHN)
100 Amp100A ATS1/0 AWG3 AWG
150 Amp150A ATS2/0 AWG1 AWG
200 Amp200A ATS4/0 AWG2/0 AWG
Concrete Pick: Over 90% of modern US residential builds utilize a 200-amp service. Unless your meter explicitly states 150A or 100A, default to the Generac RXSW200A3 (200A Service Entrance Rated ATS). It includes a built-in 200A main breaker, a pre-configured neutral bus, and integrated generator exerciser logic, making it the benchmark for residential standby installs.

Tools, Materials, and Wire Sizing

Before pulling a single wire, stage your materials. This guide assumes a 200A service using aluminum conductors, which is the industry standard for residential feeders due to cost and weight.

Materials List

  • ATS Unit: Generac RXSW200A3 (200A, 120/240V, 1-phase, Service Entrance Rated)
  • Feeder Wire (Utility & Load): 4/0 AWG Aluminum XHHW-2 (Black, Red, White) + 2 AWG Aluminum (Green/Bare for ground). Note: 4/0 AL is rated for 180A at 75°C, but NEC 240.4(B) allows the next standard breaker size (200A) for overcurrent protection.
  • Feeder Wire (Generator): Sized to the generator's maximum output. For a 22kW generator (approx 91A at 240V), 2 AWG Aluminum XHHW-2 is sufficient.
  • Control Wires: 14 AWG THHN (Red, White, Green) pulled in a dedicated 1/2-inch EMT conduit.
  • Lugs & Connectors: ILSCO 4/0-14 AWG dual-rated mechanical lugs (if not pre-installed on ATS), plus Polaris-insulated tap connectors if splicing is required.

Tool List

  • Calibrated digital torque wrench (inch-pounds)
  • CAT III or CAT IV digital multimeter (DMM) and non-contact voltage tester (NCVT)
  • Wire strippers and cable cutters rated for 4/0 AWG
  • Insulated screwdrivers and 1/2-inch drive socket set
  • Fish tape and wire pulling lubricant (Polywater J or equivalent)

CRITICAL SAFETY: De-Energize and Verify

DANGER: LETHAL VOLTAGE. This procedure involves the service entrance conductors, which carry unlimited fault current directly from the utility transformer. A dropped wrench across these lugs will cause an arc flash capable of severe burns or death.

1. Contact your utility provider to de-energize the meter, OR have them pull the meter seal and physically remove the meter if local regulations permit.
2. Shut off the main breaker inside the existing main panel.
3. Verify dead: Use a tested CAT III/IV multimeter to measure Line-to-Line (L1 to L2) and Line-to-Ground (L1 to N, L2 to N) at the main lugs. All readings must be exactly 0.0V.
4. Lock out and tag out the meter base or main disconnect.
Disclaimer: NEC-style guidance provided here; your local Authority Having Jurisdiction (AHJ) or utility may require a licensed electrician to perform the meter-side termination.

Step-by-Step Wiring Procedure

With the system verified dead, mount the ATS enclosure between the meter base and the main distribution panel. Route your conduits and pull the conductors. Follow this exact termination sequence to prevent crossed phases.

1. Utility Feed (Source 1) Terminations

These conductors run from the utility meter base to the ATS. Strip exactly 7/8-inch of insulation from the 4/0 AWG wires.

  1. Land the Black utility wire onto the Source 1 L1 lug. Torque to the manufacturer's spec (typically 250 in-lbs for 4/0 mechanical lugs).
  2. Land the Red utility wire onto the Source 1 L2 lug. Torque to spec.
  3. Land the White utility neutral wire onto the Source 1 N (Neutral) bus bar. Torque to spec.
  4. Land the Green/Bare utility ground wire onto the Equipment Ground Bus. Torque to spec.

2. Generator Feed (Source 2) Terminations

These conductors run from the generator's main breaker to the ATS.

  1. Land the Black generator wire onto the Source 2 L1 lug. Torque to spec.
  2. Land the Red generator wire onto the Source 2 L2 lug. Torque to spec.
  3. Land the White generator neutral wire onto the Source 2 N bus bar. Torque to spec.
  4. Land the Green/Bare generator ground wire onto the Equipment Ground Bus. Torque to spec.

3. Load Feed (To Main Panel) Terminations

These conductors feed your home's existing breaker panel.

  1. Land the Black load wire onto the Load L1 lug (or the load side of the internal 200A breaker). Torque to spec.
  2. Land the Red load wire onto the Load L2 lug. Torque to spec.
  3. Land the White load neutral wire onto the Load N bus bar. Torque to spec.
  4. Land the Green/Bare load ground wire onto the Equipment Ground Bus. Torque to spec.

4. Control Wiring (ATS to Generator)

The control wires tell the generator when the utility fails. Do not run these in the same conduit as the 240V AC feeders to prevent inductive interference.

  1. Pull 14 AWG THHN through a dedicated 1/2-inch EMT conduit.
  2. Connect the Red control wire to the T1 terminal on the ATS controller, and to terminal T1 on the generator.
  3. Connect the White control wire to the T2 terminal on the ATS controller, and to terminal T2 on the generator.
  4. Connect the Green control wire to the 0 (Zero/Ground) terminal on the ATS, and to the corresponding ground/0 terminal on the generator.

The Most Common Botch: The Neutral-Ground Bond

The single most frequent mistake when installing a Service Entrance (SE) rated ATS is failing to relocate the neutral-ground bond. According to NEC Article 250.24, the neutral and ground must be bonded at the first point of disconnect. When you install an SE-rated ATS, the ATS becomes the new service disconnect.

The Botch: Leaving the green bonding screw or bonding strap in place inside the home's main distribution panel.
The Symptom: When the ATS transfers to generator power, neutral return current splits and flows along the bare copper equipment grounding conductors. This causes stray voltage on appliance chassis, immediate nuisance tripping of AFCI/GFCI breakers, and potential overheating of ground wires.

The Fix: Inside the ATS, ensure the factory-installed neutral-ground bonding strap or screw is INTACT. Then, open your home's main distribution panel, locate the main neutral bar, and REMOVE the green bonding screw or copper strap that connects the neutral bar to the panel enclosure. Your main panel is now legally a subpanel; neutrals and grounds must remain strictly isolated from this point downstream.

Verify and Test: Expected Meter Readings

Never assume the wiring is correct without testing under both power sources. Set your DMM to AC Voltage (V~) and follow this sequence.

Phase 1: Utility Power Verification

  1. Re-energize the utility meter. Turn ON the utility disconnect.
  2. Measure Source 1 L1 to N: Expected reading is 120V (±5%).
  3. Measure Source 1 L2 to N: Expected reading is 120V (±5%).
  4. Measure Source 1 L1 to L2: Expected reading is 240V (±5%).
  5. Measure Load N to Equipment Ground: Expected reading is < 1.0V. (If you read >2V, you have a loose neutral or a double-bonding issue).

Phase 2: Generator Transfer Verification

  1. Switch the ATS controller to 'OFF' and then to 'AUTO'.
  2. Turn OFF the utility main breaker (or use the ATS test switch to simulate a utility drop).
  3. Wait 10-15 seconds. The generator should crank and start.
  4. Once the ATS clicks over to Source 2, measure Load L1 to N: Expected 120V.
  5. Measure Load L2 to N: Expected 120V.
  6. Measure Load L1 to L2: Expected 240V.
  7. Restore utility power. The ATS should wait the programmed cool-down delay (usually 1 minute), transfer back to Source 1, and shut the generator down.

For deeper technical specifications on transfer equipment grounding and bonding requirements, refer to the Generac Installation Support Documentation for your specific model. If all meter readings match the expected values and the neutral-ground bond is correctly isolated to the ATS enclosure, your installation is complete, safe, and ready for inspection.