My worry is that most of the outer insulation on those wires has worn down over the years with such a high load it could cause bad things to happen even though the old wiring may be capable of handling it?
I cannot speak for all old wiring. I recently did some wiring for bitcoin miners in a new place. And had some grey wire from the 90's. It was in a box about 3/4 of it still there.
This grey 12-2 i a LOT thicker insulation then the yellow stuff. I used both and either worked fine. But granted this grey wire was not being activly used since 90's. But it had set in a garage. It is pretty amazing in you ever get the chance to compare the grey 12-2 and yellow 12-2. The grey is FAR more substantial on all insulation (and a heck of a pain to strip). Needless to say it is good wire.
But yes I would be worried about unknown wire when running a S4+ on 110.
*Edited post as chain was very long read above for back story.
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