Checking a Generator Interlock & Inlet Before First Start

General field practice for qualified electricians. The equipment manufacturer's instructions, your adopted code edition, and the AHJ govern where they differ.

⚠ Backfeed kills line workers. The utility main and the generator breaker must never be ON at the same time. Run portable generators outdoors only, far from windows, doors, and vents. CDC guidance is at least 20 ft from the home.^[1]

1 · Paperwork & parts

The interlock kit is listed for that exact panel make, model, and cover. It is either the panel manufacturer's kit or a listed third-party kit, installed exactly as its instructions say, including any required breaker location.
The permit and inspection are done where required. The adopted code edition's Article 702 (optional standby) and the generator manufacturer's instructions apply.
A sign at the service shows the type and location of the standby source (702.7).

2 · Mechanical interlock check (power off)

With the main ON, try to turn the generator breaker ON. The interlock must block it.
With the generator breaker ON, try to turn the main ON. It must be blocked.
The backfed generator breaker is secured with a hold-down or retainer. 408.36(D) requires a backfed plug-in breaker to be secured by an additional fastener.
The cover is installed with every screw and the interlock plate slides freely.

3 · Inlet & wiring

The inlet rating matches the cord and the generator outlet, for example L14-30 (30 A, 125/250 V, 4-wire) or 50 A CS6375 / 14-50. Wiring: X and Y = hots, W = neutral, G = ground.
The generator breaker is no larger than the inlet and cord rating, and the conductors are sized for that breaker (see Generator Hookup).
The inlet box is weatherproof and rated for its location. The cover closes over the cord if it's in use while wet.
Neutral bonding: an interlock doesn't switch the neutral, so the generator is not a separately derived system. Follow the generator manufacturer's instructions for this connection. Many portable generators have a bonded neutral, and some must be converted to a floating neutral. A bonded neutral sends neutral current onto the ground path and can trip the generator's GFCI outlets.

4 · Pre-start electrical checks

Utility ON, generator breaker OFF: measure across the inlet pins (X–Y, X–W, Y–W, X–G, Y–G). Every reading must be 0 V. Any voltage means a wiring error or a failed interlock. Stop and fix it.
With everything off, check continuity between inlet G and the panel ground bar, and between inlet W and the neutral bar. Check that the hots are not shorted to each other, the neutral, or ground.

5 · First start

Turn the main OFF and lock it if possible. Turn every branch breaker OFF. Slide the interlock and turn the generator breaker ON.
Start the generator with no load connected. Check its output at the generator outlet: ≈120/120/240 V and about 60 Hz.
Shut the generator down, connect the cord to the inlet, and restart.
At the panel, measure L1–N ≈ 120 V, L2–N ≈ 120 V, and L1–L2 ≈ 240 V.
Turn on loads one at a time, largest first, staying within the generator's running watts. Watch the voltage.
Return to utility: turn the generator breaker OFF, slide the interlock, turn the main ON, then shut down the generator.

Sources

CDC, carbon monoxide poisoning basics (generators outdoors, at least 20 ft from windows, doors and vents) — https://www.cdc.gov/carbon-monoxide/about/index.html

Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC