Master electrician guide to wiring an automatic standby generator

Scope and load calculation first

Before you touch a conduit, define what the generator must carry. Optional standby systems fall under NEC Article 702, legally required standby under 701, and emergency systems under 700. Most residential and light commercial jobs land in 702. Size the unit off a proper load calc per Article 220, not the homeowner's wish list.

For a whole-house automatic standby, calculate the total connected load and apply demand factors. If the generator cannot carry the full load, you are installing a managed system and load shedding becomes part of the design. NEC 702.4(B)(2) permits automatic load management equipment to reduce the calculated load to match generator capacity.

• Run a dwelling load calc per 220.82 or 220.83 for existing service upgrades.
• Document the largest motor starting load, typically the AC compressor or well pump.
• Confirm fuel supply sizing with the gas fitter, LP or natural gas BTU delivery kills more installs than wire sizing.

Transfer equipment and the separately derived question

The transfer switch is the heart of the install. Article 702.5 requires transfer equipment to be listed, and 702.5(B) sets the minimum capacity. You have two choices, a service-rated ATS that replaces the main disconnect, or a non-service-rated ATS downstream of the existing service. Service-rated units simplify the wiring and often the inspection.

Decide early whether the generator is a separately derived system. If the transfer switch opens the grounded conductor, it is separately derived and you bond the neutral at the generator with a grounding electrode per 250.30. If the neutral is solidly connected through the transfer switch, it is not separately derived, and you must remove any neutral-to-ground bond at the generator. Getting this backward creates objectionable current on the equipment grounding conductor and nuisance GFCI tripping.

Check the generator nameplate and the ATS listing before you pull wire. A three pole switched-neutral ATS with a bonded genset means pulling the bond screw. A solid neutral ATS means leaving it alone. Verify with a megger and a clamp on the EGC after commissioning.

Conductor sizing, overcurrent, and conduit

Size conductors per 310.16 based on the overcurrent device at the generator, not just the ATS rating. NEC 445.13 requires generator output conductors to be rated at least 115 percent of the nameplate current, unless the OCPD is sized at 100 percent of the nameplate. For a 22 kW air cooled unit at 200 amp, that typically means 2/0 copper or 4/0 aluminum in a wet location raceway.

Outdoor runs from the genset to the ATS are wet locations per 300.5 and 300.9. Use conductors listed for wet locations, THWN-2 or XHHW-2, and treat the interior of any underground raceway as wet even above grade inside the building. Provide an expansion fitting on long PVC runs and maintain 24 inch cover for nonmetallic conduit under 600 volts.

• Flex whip at the generator, liquidtight flexible metal conduit or LFNC, sized per 348 or 356.
• Bonding jumper around the flex whip if required by 250.102 and 348.60.
• Separate equipment grounding conductor in every run, do not rely on the raceway alone for a genset EGC.

Grounding, bonding, and the GEC

If the system is separately derived, 250.30(A) applies. Establish a system bonding jumper at the generator or at the first disconnect, and run a grounding electrode conductor to a nearby electrode, usually a driven rod or the structure's grounding electrode system. Size the GEC per Table 250.66 off the largest ungrounded conductor.

If the system is not separately derived, the existing service grounding electrode system serves the generator. Do not drive a second rod and bond it separately to the genset frame, you create a parallel path. The generator frame is bonded through the EGC back to the service.

On a typical 200 amp residential install with a service-rated ATS and a solid neutral, the genset is not separately derived. Pull the bond screw at the generator, land the EGC on the frame, and let the service handle the electrode system.

Disconnect, working space, and commissioning

NEC 445.18 requires a disconnecting means for the generator output. Many packaged units include an integral output breaker that satisfies this, confirm it is within sight of the generator or capable of being locked in the open position. Working clearances per 110.26 still apply, 36 inches in front, 30 inches wide, 78 inches high, and you need that space on day one and after the landscaper plants shrubs.

Commissioning is not optional. Exercise the unit under load, verify transfer and retransfer timing, and confirm the neutral switching behavior with a clamp meter on the EGC during a utility outage simulation. Document the exercise schedule and hand the owner the manual with the ATS settings written on the inside cover.

• Verify voltage and frequency at steady state and during the largest motor start.
• Time the transfer and retransfer, default is usually 10 seconds to transfer, 5 minutes cooldown.
• Check battery charger output and block heater operation, cold starts kill unmaintained units.
• Label the ATS, service disconnect, and generator per 702.7 with the type and location of each power source.