Chris Dzombak

Automated generator load-shedding for the house

Another recent house-related problem, and a solution.

My house has an old 7 kW natural gas generator. During a power outage, it makes sure the house still has heat and running water, and it also powers some lights and outlets in the bedroom, office, and kitchen.

The “office” part of that sentence is the problem. There’s a little bit of stuff in the office that I want the generator to run (like the cable modem and network router) but a lot of stuff that I definitely don’t want on the generator (in particular, a laser printer).

When the power goes out and the generator kicks on, the laser printer begins to power up, adding a sudden and entirely unnecessary ~1 kW load that you could hear the generator struggling with. I wanted a device that could drop the printer from the circuit as soon as the power went out and would require manual intervention to “plug the printer back in.”

Smart plugs didn’t quite work for this use case, since all the smart plugs I have restore their previous power state when plugged in.

These magnetic switches fit the bill perfectly. Once one of these switches is turned on, it stays on; but when it’s first connected to power, it’s off regardless of its previous state.

I designed and 3D-printed a custom enclosure for this use case. I chose PETG for resilience and its flame resistance when compared to PLA.[1] Notable features of this enclosure are the multiple cable placement options (via printed-in-place pop-out hole covers) and the dedicated space on the switch panel for a label.

(This embedded 3D viewer is very much a work in progress; please forgive its flakiness!)

With the enclosure designed and printed, and some short 3-prong extension cords acquired, wiring and assembling a few of these switch boxes was simple (taking care to follow the wiring diagram included with the switches).

I also placed one of these switch boxes on a small fridge used only for drinks, and a third on the space heater that lives in my office in the winter, ensuring that everything noncritical drops offline before the generator starts up.

The end product, mounted on the wall near the laser printer: