How to add a kill switch to a 3-way light switch

Question

I'm not good with electrical terms, so please keep it dumbed down for me. The ones I use here are probably the only ones I know.

I have an existing 3-way switch setup (2 light switches that both work with a single light). This works as expected - both switches control the light. This is old style wiring with the wires in a metal tube exposed on the wall. The gang sticks out from the drywall. I've never worked with that type of gang before.

What I need is a kill switch for this setup at only one of the switches. The idea is that when the kill switch is off, then neither 3-way switch will control the light. When the kill switch is on, then it works like it does now. The kill switch and one light switch would be on the second floor while the other switch is on the first floor. The electrical panel is in the basement. My assumption is that the second floor light switch is at the end of this circuit.

Assuming everything is currently wired correctly, how would I go about this?

My thoughts of possibilities:

1. Replace the light switch with one of those dimmer switches that have an on/off toggle under it. The dimmer would act as the kill switch. Pros: Same gang box can be used. Cons: Could be confusing as a user.

2. Install a second switch next to the light switch. Pros: Very obvious that the second switch is kill switch. Cons: Need 2-gang box.

Regardless of which option above is done (or if there is a better option you all know of), I wouldn't know how to wire it.

Thanks for any help. Let me know if you need any clarifications.

Answer 1

You want to maintain your 3 way switch function but want to have main switch to turn it off.

For that the single gang box could have two 3 way switches in single gang box like shown.

You wire it as is for 3 way by choosing the upper or lover switch.

The other 3 way switch is just wired as two way switch and be your main power cut off switch and the 3 way circuit will not work any more, if that is what you want.

If you can find single switch and 3 way combination it will be better, but this will work.

https://www.homedepot.com