I'm a diy'er installing a ceiling fan in a bedroom that had no existing fixtures in the ceiling. Only a few outlets and a single pole switch that controls one of the outlets. I have access via the attic and was scoping out the situation before I went shopping for supplies. There was 14/2 romex in the attic going straight across the room that I figured would be perfect to tap into. I tested the [insulated] wire with a voltage detector to make sure it was hot and it started beeping so I thought I was good. I had a live/hot wire right where I needed it that I could use.
Fast forward… I cut the cable I found in the ceiling, fed it through the new ceiling fan box, and tied it all back together with the ceiling fan wiring. Turned power on and tested the new connections again with the voltage detector. Had constant beeping so should be all good, right? Finished assembly of the ceiling fan and went to pull the pull chains for the light and fan and NOTHING! No light, no fan, nothing.
Held my voltage detector all around the ceiling fan and was still getting constant beeping so I was pretty sure I had power and made all my connections correctly. As a last ditch effort we started trying all the light switches in the area. Dun dun dun.
There is a switch in the master bedroom that we never had any idea what it went to. Turns out that switch is part of a 3-way switch that controls our backyard flood light. (Master bedroom is on front of the house, flood light is in the back, that's why we never figured it out). So what I have successfully done is wired my daughter's new ceiling fan into the 3-way switch circuit that controls the outdoor flood light. In order for the ceiling fan to work, the outdoor flood light also has to be on. And we can essentially control my daughters ceiling fan from the master bedroom or the downstairs mudroom. Hahaha.
I know how to fix my mistake. I need to go back into the attic, fix the 3-way switch cable I spliced into, and find a new circuit for the ceiling fan. My question is… How could I have prevented this mistake in the first place? I used a simple voltage detector to check the wire and got beeps so I assumed it was a constant live wire. If it was a switched wire I wouldn't have expected beeps. (the flood light was off at the time, it's almost always off). How was I to know that I was tapping into a switched circuit? Are 3-way circuits always hot?
Best Answer
To answer your question are 3 way switched always hot? YES, there are a pair of travelers and a common what a 3 way switch does is flip the power from 1 traveler to the other, so if the breaker is on there will be power in that cable.
The other possibility is phantom voltage as others have mentioned.
I used to commonly wire hot to the switch location then run x3 wire, this is still allowed if both switches are in sight of each other code will allow only 1 to have the common for the 3 way.
If I am putting in a new circuit with only a light fixture I still feed the switch location and run x3 wire so a light and a fan can be installed later the wires are there. In this case only one of the hot wires are used red or black. With the light on the other wire having been capped at both ends will indicate a live conductor with a non contact voltage detector or even a voltage meter (unless low Z meter or setting).
The only way to know for sure is to put a signal on the wire and trace it but as mentioned this could be a traveler on a 3 way.