Adding a smart switch to a box originally containing two single-pole switches

Question

I currently have two single-pole switches in one box, with the first/upper controlling an interior light and the second/lower controlling an exterior light. I want to replace the second/lower with a smart switch while keeping the first/upper as a single pole. The smart switch calls for a neutral wire and a ground wire in addition to the load and line. However, my box does not appear to have a ground, or the ground was somehow incorporated into a bypass.

If I understand correctly, the neutral wire goes to the three white wires that are bundled under a wire cap at the back of the box and pass from top to bottom without going through a device. The load wire is the red wire that comes from the bottom of the first/upper switch (and currently attaches to the bottom of the second/lower switch). The line wire goes from the top of the second/lower switch through the bottom of the box to the rest of the circuit. However, the wire running from the bottom of the first/upper switch to the bottom of the second/lower switch is bundled via wire cap with two other wires that come from the top and the bottom of the box. Is that supposed to be the ground?
Also, what is that other yellow wire that feeds into the top of the first/upper switch rather than being screwed onto the side?

FreeMan · Accepted Answer

It looks very much like your wiring is run in conduit and that's very handy because the conduit itself should be carrying your ground for you (unless something was done incorrectly).
To install your new smart switch:

Unscrew the wire nut from the white bundle in the back and add the white from the new switch to it. That nut looks to be pretty small, so the addition of a 4th wire may be too much for it. You may need to purchase a larger wire nut (or there might possibly be a big enough one in the package with your new switch, but those tend to be pretty small, too).
Connect one of the hots from the new switch to the red wire where the red wire nut is.*
Connect the other hot to the black wire on your switch.
Connect the green/bare ground wire from the switch to the metal box itself. There should be a green grounding screw somewhere in the box for that purpose, though it's not readily apparent. There also appears to be a bare wire along the left-hand side of the box, and that could be a ground wire. Track that to see where it leads.

As to the 2nd question about the other yellow wire. That's called a "backstab" and they're rather unpopular. It's a way of attaching wiring to the switches that's a bit easier and faster because you just "stab" the wire into the hole in the back of the switch. The bad part is that it's held by a spring on the inside of the switch and it can work loose over time. It's likely that the wire runs to a 2nd light that's controlled by this switch.

*Note that I suggest you do these steps one wire at a time (i.e. never have 2 wires disconnected from the switch). This will help prevent the dreaded, "uh oh... which wire was supposed to go under this wire nut?" panic. Even when I'm replacing a light switch/dimmer (as I just did in my own house over the weekend) that I previously installed, I replace one wire at a time to ensure that I'm not confusing myself as to which wire is which. It's not critical on a plain toggle switch which wire goes to which terminal, but it can be very important on smart switches, and when you're disconnecting wire nuts from pig tails in the back of the box.

Ed Beal · Answer

You do have a ground or a bare copper on the left side, if your wiring is run in conduit (it looks like it is) the box itself should be ground even if the bare ground is not. So installing a green 10-32 grounding screw on the back right small hole will be a NEC compliant way if your smart switch has a green wire. You are correct the whites look to be neutrals. The incoming hot will still need to be jumpered to both switches. The 2 yellow wires are on the same terminal one back stabbed and the other screwed down I like pig tails but this method is code compliant. I can’t tell on my computer monitor but it looks like the red is the hot , jumpered to both switches then the other side black would be the switched hot Similarly the red to the other switch hot to 2 yellows switched hots. Without testing that’s how I would call it and wire it. The model of the smart switch would have helped us give a connection list usually if the smart switch has red it is the switched conductor black is the hot white you have figured out to the other 3 and a green wire to the box would wire it up from what I see.

Answered by Ed Beal on May 7, 2021

Harper - Reinstate Monica · Answer

Legal colors in conduit wiring:
ALWAYS-HOT: black, brown, red, orange, yellow, pink, blue, or purple
SWITCHED-HOT: black, brown, red, orange, yellow, pink, blue, or purple
There's a convention of black for always and red for switched, but that's purely optional.
Now, these 2 switches have independent control, right? Neither switch is dependent on the other to be on, right?   So... that tells us that whichever wires go to BOTH switches must therefore be always-hot.  Therefore, red is always-hot.
Black is on the other switch terminal, so it must be switched-hot.
On the other switch, I see 2 yellow wires on the same terminal.  One uses the screw and the other uses the backstab.  Sloppy using the backstab but somebody really, really wanted to use the 50 cent switches instead of the $3.00 ones.  Anyway, those must be switched-hot, even though there are two of them.
Neutral, you've met. Make sure those neutrals are part of the same cable or conduit as the hots you are intending to switch, as that's a Code requirement. But they are, here.
I have no idea why there are 2 wires going to the switched-hot. Normally that indicates more than one lamp.  However that's how it was done, so that's how it should continue.  They'll be pigtailed to the new switch unless it has provision for 2 wires (most better switches do).
As for grounds... this hole is already tapped for a machine screw (bolt) of size  #10-32.  Readily available in North America, they even sell green ones just for ground screws - but any will do.

Adding a smart switch to a box originally containing two single-pole switches

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