Field tested: wiring an automatic standby generator

Sizing the generator and the transfer

Start with the load calc, not the sales sheet. Article 220 gives you the math, and Article 702 covers optional standby systems. For whole-house coverage on a single dwelling, run a standard 220.82 calc and pad 25% for motor starting and future loads. Partial-load jobs lean on 702.4(B)(2), where the transfer equipment must be rated for the load it actually serves.

Automatic transfer switches fall under 702.5 for capacity and 702.12 for outdoor installations. Service-entrance rated ATS units simplify the meter-main arrangement and satisfy 230.70 disconnect rules in one box. If the generator is a separately derived system, you are bonding the neutral at the genset and floating it at the ATS. If it is not separately derived, neutral stays solid through the switch and you do not re-bond. Get this wrong and you trip GFCI, confuse ground-fault sensing, and create parallel neutral paths.

Verify the ATS neutral configuration before you ever open the genset door. Three-pole vs four-pole switches dictate your bonding scheme, and retrofitting after the slab is poured is a bad afternoon.

Siting, clearances, and the pad

Manufacturer spec sheets drive placement, but 110.26 working space still applies around any serviceable enclosure. Most air-cooled residential units want 18 inches from the structure on the exhaust side, 36 inches on the service side, and 60 inches clear from any openable window or fresh-air intake. Fuel line routing has to respect NFPA 54 and local AHJ overrides, so confirm before you trench.

The pad itself is often overlooked. Composite pads are fine for air-cooled units under 22 kW, but anything liquid-cooled wants a poured slab with rebar and a gravel base for drainage. Check frost depth and soil bearing. A settled pad pulls on the flex conduit, cracks the fuel line bushing, and voids the warranty.

• 18 in. from combustible walls on exhaust side (verify with listing)
• 60 in. from windows, doors, soffit vents, and outside air intakes
• 36 in. clear front for service access per 110.26(A)
• Drip loops on all flexible conduit entries
• Grounding electrode within 6 ft of the pad when required

Feeders, conduit, and conductor selection

Size the feeder from the genset output to the ATS per 310.16 with the genset nameplate current, not the breaker. Apply the 125% continuous factor from 215.2(A)(1) where the manufacturer flags the output as continuous duty. For most 22 kW air-cooled units at 240V single-phase, that lands you at 2/0 copper or 4/0 aluminum in a 1-1/4 in. PVC run, but do the math for your distance and ambient.

Voltage drop is the silent killer on long runs. Keep it under 3% on the feeder and under 5% total. A 22 kW unit sitting 150 feet from the ATS on undersized conductors will drop enough voltage that the ATS logic hunts, especially under motor start loads like well pumps and AC compressors.

Use liquidtight flexible metal conduit, not nonmetallic, for the final whip into the genset. Vibration chews through LFNC over a few seasons. Bond both ends and keep the whip length short enough to avoid derating under 348.22.

Grounding, bonding, and the neutral question

Treat this as its own job, not an afterthought. Per 250.30, a separately derived system needs a system bonding jumper at the generator and a grounding electrode conductor to a local electrode. A driven rod at the pad, bonded back to the service grounding electrode system, is the clean approach.

For non-separately derived installs, you are relying on the existing service ground and neutral. The ATS passes the neutral through unswitched, and the genset frame bonds to the equipment grounding conductor only. Pull the factory bonding jumper at the generator. Label it. Take a photo for the file.

If the AHJ or POCO asks, you want to point at a single sheet showing ATS type, bonding scheme, and electrode path. Draw it before you wire it.

Commissioning and the cold start

Before utility power comes back on, run the full test sequence. Exercise the unit under load with a portable load bank if you have one, or stage the house loads manually. Verify transfer timing falls inside the manufacturer window, typically 10 to 30 seconds from utility loss. Check retransfer delay, cooldown cycle, and weekly exercise schedule.

Measure voltage and frequency at the ATS line side during generator run. You want 240V +/- 5% and 60 Hz +/- 0.5 Hz under load. Anything outside that points to governor, AVR, or load imbalance issues. Document readings on the job card.

1. Utility off at main, confirm ATS senses loss
2. Crank delay expires, genset starts, warm-up completes
3. Transfer to emergency, measure voltage and frequency under load
4. Restore utility, verify retransfer delay and cooldown
5. Set exercise schedule, hand owner the run log

Documentation and handoff

Leave the homeowner a one-page reference: generator model and serial, ATS model, exercise day and time, low-oil shutdown behavior, and your callback number. Attach a printed copy of the load calc and the permit card. The next electrician who touches this system, maybe you in five years, will thank you.

Photograph the inside of the ATS with the deadfront off, the genset connection box, and the grounding electrode landing. Keep these with the job file. When a warranty claim or an insurance question comes up, you have the evidence in hand instead of scrambling three years later.