The builder went to the effort of wiring up the panel for a generator backup that included a safety interlock for the circuit breaker. Almost there but not quite, for some reason the Romex wire terminated at a wall outlet box in the garage and sans plug socket. No way to connect anything. The unconnected cable lay inside the box behind a blanking plate — unused.

Switching OFF the main breaker to the grid before powering the household with a genset is an actionable item. Leaving the grid connected would be fatal. The interlock is necessary by code and it prevents the main breaker and backup CB from both being on at the same time. That way you don’t have the undesired state of back feeding the grid or current to your backup when the power comes back on. Idiot proof.

Part: NEMA L14-30P shore plug. Tools: hand tools, drill bit and a hole cutter.

Result!

A Genset was mentioned but the astute will recognize that the source for energy seen in the video is an EV. Gas powered generators while worthy, have added considerations. The garage is not a suitable space for a genset although it might help to mitigate the noise or prevent theft. Exhaust fumes and gasoline must be kept on the outside. A very long extension cord would be required. Shown is a prefab 10′ cord with a NEMA 14-50 plug for the truck and NEMA L14-30R on the other end. Caution – do not use a Suicide Cord

I suspect that the builder had difficulty sourcing a socket plug and gave it up. Most socket plugs come mounted in an enclosure for outdoor use with the generator. In my case I’m glad he hit the dead end. The indoor location is ideal for my purpose but did require the DIY fabrication of a mounting point and now 23 years after the original job was begun, it’s finished.