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few questions re switch loop (power from light) setup

527 Views 6 Replies 4 Participants Last post by  AllanJ
I'm wanting to install a smart/programmable light switch, to replace the existing switch. I removed the cover to find that it is a switch loop system (the white is hot 100% time and the black hot when the switch is on - there is no neutral). Oddly though inside the box is a 2nd feed of white/black which I also confirmed is another feed of a switched loop setup which I also confirmed is on a different circuit (breaker) than the one currently connected to the switch - this 2nd feed was not connected (was miret'ed). Well kind of - the black from the 2 feeds were tied together (remember, blacks are only hot when the switch is on), but the white (which is hot 100%) is the one that was disconnected on this 2nd feed.

So my question - is it possible that the 1 switch in the past (previous owners) was connected to both sets of switch looped circuits, such that when the one switch was turned it, it would complete the loop of both circuits and basically light-up (power) whatever they were connected to? Or is connected 2 different sweitch loops on different circuits, one one switch not possible (or if possible is it dangerous).

Second question - being the 2nd switch loop was disconnected, I'm curious why and what it powers? I assume there would be no harm in connecting a switch to this 2nd switch loop and seeing what it does when turned on - or does anyone think this may be a bad idea (perhaps it was disconnected for a reason)?

Lastly, the new smart switch I bought needs a neutral, but as mentioned there is not one in my box. However about 3' below the switch box is an outlet which has hot and neutral. If I can fish a wire between the 2 boxes, can I use the neutral from that outlet box? Note that the circuit that the outlet is on is the same as the disconnected switch loop circuit (not the circuit of the switch loop which was currently connected), but as stated the blacks of these 2 switch loops was connected, so they kind of were tied together already (well, it the white was connected, which it wasn't).

I did a little goggling about using neutral from different circuits - in my case I think it would be OK, cause the only reason the smart switch needs it is so it can keep memory power on at all times (in a switch loop system power is only obtaoined when the light/switch is on) - the new switch needs constant power for its memory and programming - so to me it sounds like a little switch like this would not take much power for memory, so using the neutral from below sounds OK to me?

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There must be exactly one possible path for neutral current to get from any outlet (light or receptacle or appliance, etc) back to the panel neutral. The corresponding hot path for current must follow that same physical path from the panel breaker, with the exception that (in a switch loop) the hot path may diverge from its neutal to go to a switch and return back to (e.g. light fixture box) where it diverged.

You may not "grab" a neutral in a different cable, even on the same branch circuit, to go with a given hot conductor.

The most modern code requires a neutral going to the switch box. Switch loops for power arriving first at a light box can still be created. Here a one way neutral accompanies the raw hot, e.g. black and white wires in a 3 conductor cable. Raw hot and switched hot properly accompany each other, e.g. black and red in the 3 conductor cable. But neutral for the light goes directly to the light, not coming back from the switch on a second white wire accompanying the switched hot conductor.

Yes you can repurpose the second switch loop to become a new raw hot and matching neutral entering the switch box, provided you can prove that this is what you accomplished (may require some wire tracing).

Then yes you can repurpose the first switch loop be be a new continuation of switched hot and neutral to the light, decommissioning the original raw hot and neutral going first to that light.

Then finally install a new programmable switch, discarding the new switch you fried.
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