It has a line cord attached, so it can be plugged into an outlet. This would have been one of my dad's generator service tools, from his days with Kohler and Onan.
I think I may, just for the heck of it, install this in the furnace room of my home as a monitoring instrument. Any time I I want to check the voltage level we're being supplied, I can look in on it. I have what I need to make a mounting bracket for it. I think I'll do that. I'll show you when I'm done.
This can go on my 'work in progress' shelf.
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