How should I connect the left light switch?

Question

I am replacing three smart light switches in the following wire box with new smart light switches that don't need the neutral wire:

The wire box has 4 sets of cables for only 3 light switches and I forgot to take a picture of how they were connected before. I remember that the two left white wires were connected together though.
I was able to connect the two right switches and they worked correctly, but the way I connected the left one fried when I turned it on.
How should I be connecting the left one correctly?

ThreePhaseEel · Accepted Answer

Easy enough here
The good news is that the two left cables aren't doing anything complicated.  One of them is an incoming power feed, and the other is the onward feed to the light, providing both hot and neutral.  Furthermore, since the white wires there were connected to each other as expected, we know the black wires are our line and load connections.  So, I'd figure out which of those two black wires is the always-hot (line) connection, and wire that to the line-in terminal on the smart switch.  Then, the other black wire is the switched-hot, which gets wired to the load-out terminal on the smart switch.  The two neutrals get nutted to each other, and last but not least, the ground for the smart switch gets added to the existing grounding bundle in the box.  Then you can button everything back up, turn the breaker on, and enjoy your new smartswitches!

Harper - Reinstate Monica · Answer

Some cleanup necessary with grounds.  All grounds need to connect to each other -- all ground wires, the metal box, and all switch grounds.
As you have it wired, you are taking cable 3 and 4's ground straight to the respective switch.   That is not how grounds are done; the cable onward to the lamp can't ground the switch as it has no connection back to the panel.
I see what look like at many as six "ground screws" in the back of the metal box.  You are free to terminate any ground wires there.
The switches themselves can ground through their mounting screws.  If you want to run a ground wire from the switch to the box, you can do that too, but it's not necessary given the grounded metal boxes.

How should I connect the left light switch?

2 Answers

Easy enough here

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