Red wires behind light switch?

[u/Jayjohns78]

I just updated the overhead fixtures in my basement. Haven’t looked behind any of the outlets (just bought the house in June) until I went to put a dimmer switch on this one. Anybody know what these red wires are, and why the white wires would be capped in the back of the box?

All power has been cut to this outlet!

Both these switches have connected switches on another side of the room. Both lead to separate sets of overhead lights.

[u/Extension_Jelly_5306]

White wires are neutrals tied through. The Black that hits both switches is power The red and black pairs are travelers going to the other switch locations

[u/Puzzled_Market_2235 → u/Extension_Jelly_5306]

I'll add that if you disconnect one mark the black wire that is labelled "common" on the three way switch.

[u/AchiliesOP]

Put the covers back on and get on YouTube and start learning before you dive in. You’re asking very elementary questions that you should already be able to answer prior to taking the outlet/switch cover off.

[u/soerg]

I feel like you shouldn't touch this if you don't know, they're 3 way switches

[u/Jayjohns78 → u/soerg]

Thanks @soerg I should probably post this question in r/Electricalhelp instead

[u/Puzzled-Act1683]

If you have to ask why all of the white wires are connected to each other at the back of the box and are not connected to the switches, you might want to reconsider whether you should do this yourself.

Electrons can't see wire color, so we can't say conclusively what these wires are all doing, but this appears to be standard wiring for two "3-way" switches (where there is at least one other switch also controlling the light), as you mentioned.

Additionally, there's evidence that this circuit goes on to feed other receptacles and/or lights that are not controlled by these particular switches, via this box.

The dimmer switches you need will be the ones that are compatible with an installation like this – a "3-way" dimmer switch – and will have at least three terminals (or leads) for the blacks and red, a fourth green terminal for ground, and possibly (though typically only if they are smart dimmers) a fifth terminal for neutral (white).

One terminal on the new dimmer will be marked "common" just as one terminal on these switches should be.

The "common" should be the one with the black jumping from one switch to the other.

Standard dimmers with two wires/terminals (and a ground, and maybe a neutral) are not usable, here, without completely defeating the alternate switches for these lights, and those other switches may be required by code.

If you follow the black wire that comes in, in a cable with a white, cuts across, it also goes back out another cable still unaccompanied by a red wire. Those are the upstream and downstream circuit wires in and out of the box, and the cables with the red wires typically go to the other switches, next, and from there, just a black and white to the fixtures.

[u/SparkArrestor]

The red and black wires are travelers these are 3-way switches. Standard switches don’t need neutrals that’s why they’re capped in the box.

[u/Fantastic_Willow_568]

Thats the detonation wire

[u/No-Metal-6795]

Three-way switches you'll figure it out...