3 way light wiring question

[biscuit141]

So I recently purchased a new house and I’m in the process of replacing the light fixtures and switches throughout. I encountered two similar strange wiring situations on two different fixtures which I assume is normal but can’t figure out. The first was a hallway light on a 3 way switch, I was replacing the light fixture and had the breaker off for the light but the laundry room light was still on in the next room. The box was not hot in the hallway, but when I disconnected the group of neutral wires in the box (two whites plus the light fixture neutral all wire nutted together) the laundry room light went out and one of the white wire now had voltage to it. Second one was similar, it was a 4-way light circuit, breaker was off. I disconnected the neutral wires in the box because I wanted to put a new wire nut on them but they were not connected to the switch. When I disconnected them the entry light next to me that was on another breaker went off and one of the wires now had voltage. The first time I thought something was wrong, but since I had this happen twice, I assume this is not something wrong but how this is wired, I just don’t understand it.

 

[wyliesdiesels]

Sounds like several light fixtures on 2 or more circuits are sharing a neutral.

Are the breakers for these lights handle-tied? If not then this is a code violation and it is wrong

 

[biscuit141]

Sounds like several light fixtures are sharing a neutral.

Are the breakers for these lights handle-tied? If not then this is a code violation and it is wrong

Handle tied? Like connected at the breaker? No, individual.

 

[exranger06]

These lights are probably part of a multi-wire branch circuit (MWBC). That's where you basically have two circuits that share the same neutral wire. MWBCs should be hooked up to a double pole breaker, or to two single pole breakers that are handle-tied together. This way, you have to turn off both parts of the circuit to work on it, which prevents this very problem from happening.

I have several MWBCs in my house and I had to add handle ties to my single-pole breakers, since whoever installed them never did.

Does the voltage on the neutral go away if you turn off a breaker adjacent to the one you switched off?

 

[Terry D]

Agreed, most likely a shared neutral. Be very careful disconnecting the neutrals on a MWBC. In this case you were ok because one of the two circuits was de energized. If you would do this with both circuits still on, and the homerun neutral was in this group, and there was loads connected to these circuits, and depending on the resistance of the loads. You have the potential of putting up to 240 volts to certain loads. If it is just lights on the circuits, and they were on, you would see some lights get real bright and some get real dim. Unless you have two perfectly balanced circuits, nothing would happen. But that is almost impossible. And yes, a 2-pole breaker or handle ties should be used. At least in my area, this is something that is now been enforced in the few years

 

[Wrench97]

How old is the house?

It was pretty common around here in the 50's and 60's track housing to use 14 /2 romex on the 3 way circuit and grab a neutral from another circuit