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What happens when there is 120v hot wire connected to one side of a light switch and an identical 120v hot wire connected to the other side of the light switch? Light switch is then switched ON and OFF.

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    That would be a wiring mistake, because when your switch is on, you are paralleling. Not allowed. – Harper - Reinstate Monica Sep 3 '18 at 23:52
  • @Harper - I agree it is wrong, and a bit of a problem if they are from two separate circuits (same leg, hopefully...). But two short wires coming out of nutted bunch of blacks from one circuit and then going to the two sides of a switch will effectively do nothing. So not allowed, but nothing gained either - might as well just connect nothing at all to the switch. – manassehkatz-Reinstate Monica Sep 4 '18 at 0:46
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Nothing

Two hots of same circuit - nothing should happen.

Two hots of same leg but different circuits (i.e., 2 different fuses or breakers) - nothing should happen in most cases, but if you have GFCI protection either at the panel or upstream of at least one of the wires and any devices on (besides this nonfunctional switch) then the GFCI may detect that the hot (now coming from two different circuits) doesn't balance the neutral (on the circuit the GFCI is protecting) and trip. I'm not 100% certain of that, and I certainly wouldn't do such a thing deliberately (or any other mode of "wire up the switch with two hots"). This can also lead to imbalance between pairs of hot/neutral (even though the total of all hots == total of all neutrals) which is why this is something you should never do.

However:

  • Hot to neutral or hot to ground - short circuit which should blow fuse or trip breaker
  • Hots from two different legs - not sure but I wouldn't try it.
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    Hots from different legs = flash boom and time to buy a new switch. – Ed Beal Sep 4 '18 at 1:16

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