Switch – Feeding single circuit from two different subpanels (with interlock)

power-backupsubpaneltransfer-switch

I have a main panel and a critical loads sub-panel fed by a PV inverter + battery backup. I need to avoid overloading the normal capacity in the critical loads panel (50A), and I'm already at a comfortable max on that for normal use.

However I'd like to be able to run my garage door opener off of that panel selectively, so that I don't have any concerns about tripping the breaker backfeeding that sub-panel, but also be able to easily switch over to backup temporarily to still run the garage door opener during an extended outage.

Is there a standard/easy way to accomplish this? I suppose I could use something like the Square D QO1DM10030TRBR 30-Amp Generator Main Breaker Outdoor Manual Transfer Switch (and just remove the generator twist-lock receptacle from the bottom and instead feed directly from the critical loads sub-panel), but I wanted to see if there was a better way of accomplishing the same thing.

Edit: In an ideal world there'd be a way to a setup a "transfer switch" like this without require breakers in both the panels and in the switch itself.

Best Answer

You have to switch the neutral in your application. Fortunately your garage door load is only 120V, so you can use 2-pole interlocks.

Now, you're overspending on that particular panel. What you're looking at is a common "QO" 8-space panel ($35) + a QO2DTI interlock ($30) + a pricey 100A breaker you don't need ($65) + a common 30A breaker ($16 because QO) + a socket you don't need.

You can buy just the components and avoid the expense of socket and large breaker: just a "QO" 4-space panel + the QO2DTI interlock and 2 2-pole breakers. Don't leave room for anything else in the panel; it's an oddball 120V switched-neutral panel.

An even cheaper way to do this is to bring a receptacle fed from each power source... and right next to that, have an inlet. From the inlet proceed with normal house wiring to the garage door opener (so this section of permanent wiring is isolated from all others). Then have a short 1' extension cord that you plug into the inlet and move to whichever outlet is relevant.

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