Quick reference for wiring an automatic standby generator

Sizing the generator and the transfer equipment

Start with the load calc per NEC Article 220. Optional standby systems under 702 do not require the full dwelling load, but the listed transfer switch must be rated for the total connected load it serves. For a whole-house automatic system, run the standard or optional method calc (220.82 for dwellings) and match the genset output to it, not to the service size.

Transfer equipment must be identified for the application per 702.5(B). Service-rated automatic transfer switches (ATS) simplify the install by combining the service disconnect and transfer in one enclosure, which is the cleanest path for a 200A residential service. Verify the ATS short-circuit current rating (SCCR) meets the available fault current at the line side.

• Confirm genset kW against calculated demand load, not nameplate service amps.
• Check ATS poles: 3-pole switches the neutral, 2-pole solidly bonds it. Pick based on whether the genset is a separately derived system.
• Verify SCCR of the ATS against utility available fault current.

Separately derived or not: the neutral question

This is where most installs go sideways. Per 250.30, a generator is a separately derived system (SDS) only if the transfer switch opens the grounded (neutral) conductor. If the ATS is a 2-pole switch on a single-phase 120/240V system, the neutral stays bonded at the service and the genset neutral must NOT be bonded at the generator frame. Pull the bonding jumper.

If the ATS is a 3-pole (switched neutral) unit, the generator IS a separately derived system. You need a system bonding jumper at the genset, a grounding electrode conductor sized per 250.66, and a local grounding electrode (250.30(A)(4)) within 6 feet of the genset where practical.

Field tip: before energizing, ohm from the genset neutral to the frame with the bonding jumper installed or removed per your ATS configuration. Two N-G bonds on a non-SDS install will trip GFCI loads and cause nuisance breaker trips that take days to chase.

Conductors, conduit, and the supply circuit

Size conductors per 310.16 for the genset output ampacity at the highest ambient temperature seen in the run. For most residential air-cooled units (14kW to 26kW), you are pulling 4 AWG to 2/0 copper or aluminum between the genset and the ATS. Check the manufacturer instructions, they often require larger conductors than ampacity tables suggest because of voltage drop over the run.

Outdoor conduit between the genset and the structure is typically PVC or liquidtight flexible nonmetallic conduit (LFNC) for the final connection to the vibration-isolated genset enclosure. Use a sealing fitting where the conduit transitions from the cold outdoor environment into a heated interior space (300.7(A)) to prevent condensation from migrating into the ATS.

• Apply the 60C column for terminations on most residential ATS units unless listed otherwise (110.14(C)).
• Allow flex at the genset to absorb vibration and frame movement.
• Derate for ambient and conduit fill if multiple current-carrying conductors share the raceway.

Control wiring, communication, and start signal

Modern automatic systems use a 2-wire start circuit (often labeled 23 and 194 on Generac, or similar on Kohler and Cummins) plus a battery charger circuit. Run control conductors in a separate raceway from the power conductors, or use a multiconductor cable rated for the application. Class 1 control wiring follows Article 725.

The ATS senses utility loss, waits the programmed time delay (usually 10 seconds), signals the generator to start, monitors for stable voltage and frequency, then transfers the load. On utility return, it transfers back and runs the genset through a cooldown cycle. Verify all timers during commissioning, never assume factory defaults match what the homeowner expects.

Grounding, bonding, and the GEC

For a non-SDS install (2-pole ATS), the generator frame still requires an equipment grounding conductor (EGC) run with the supply circuit, sized per 250.122. No separate grounding electrode is required by Code, though many manufacturers recommend one for surge and lightning protection. If you drive a rod, bond it to the frame, do not run a separate GEC back to the service.

For an SDS install (3-pole ATS), the system bonding jumper goes at the first disconnect on the load side of the transfer point, which is usually inside the genset enclosure. Size the GEC per 250.66 based on the largest ungrounded supply conductor. Connect to a grounding electrode per 250.50, typically a driven rod near the genset.

Commissioning and handoff

Run a full transfer test under load, not just a no-load exercise. Pull the main utility disconnect, time the start sequence, verify voltage and frequency at the panel, and check rotation and phasing if it is a three-phase install. Cycle utility back on and confirm retransfer plus cooldown.

Field tip: leave a laminated one-page sequence of operation inside the ATS door. Include the exercise schedule, the manual transfer procedure, and the service contact. The next tech to open that cabinet, possibly you in five years, will thank you.

Document the install per 702.7: a permanent plaque or directory at the service indicating the location of the standby source and the disconnecting means. Label the ATS, the genset disconnect, and the load side feeder. File the load calc and the commissioning sheet with the job folder.