Two Breakers on One Circuit

Question

I was adding an outlet to a circuit this weekend and had a slightly scary moment. Any chance anyone could point me in the right direction in understanding what might be going on here?

I turned off circuit breaker 14, voltage-tested (tester pen) both hot wires and both neutrals on an electrical outlet (no voltage) and took apart the connections. I proceeded to do what I needed to do to prepare to install the new outlet (remove the box, cut a new whole in the wall, etc.). After dinner, I returned, flipped a switch in the foyer outside the room although the lights did not work (that is on a different circuit breaker 15), and voltage-tested the 4 wires again to confirm no voltage before resuming work.

To my great surprise, one hot and one neutral wire were now showing voltage! After some searching, I realized that these two wires appear to be controlled by the 3-way switch in the foyer (that is on circuit 15).

Thus it seems that the outlet has 2 hot wires (1 from circuit 14 and 1 from circuit 15) and 2 neutral wires (again 1 from circuit 14 and 1 from circuit 15) and the tabs are not broken to separate the top and bottom outlet. A few odd properties / discoveries below - curious if anyone can help me make heads or tails.

• The 3-way switches outside the room in the foyer controls the two circuit 15 wires. Note that when "on", comparing both the hot / ground and neutral / ground yields 120v (and 0v differential between hot and neutral). Note that this neutral (that reads 120v) has been connected to a neutral (on circuit 14) that reads 0v for years! What could this be?
• When all 4 wires are connected and circuit 14 / 15 are both on, all lights work on circuits 14 / 15.
• When circuit 14 is attached to the outlet and on but circuit 15 is disconnected , some lights work on circuit 14 but others do not. The outlet works
• When circuit 14 is attached to the outlet and on and circuit 15 is connected but off, all lights work on circuit 14 but none work on circuit 15.
• When circuit 15 is attached to the outlet and on and circuit 14 is connected but off, all lights on circuit 15 work
• When circuit 15 is attached to the outlet and on and circuit 14 is disconnected, no lights work on circuit 15 and the outlet does not work.

It is as if for all lights to work on both circuits, both circuits need to be connected to this outlet. Any ideas?

Answer 1

This sounds like a multi wire branch circuit. The tabs would only need to be broken on a receptacle hot side if both hots went to the same receptacle.

In years past handle ties were not required but are now and are a good idea.

To be sure at the panel you could remove the dead face and will probably find the 2 hots go to the same cable. If in conduit they should be bundled prior to entry with there neutral.

A handle tie is a good idea because you can still get a shock on the neutral of 14 is in use and you are working on 15 or the other way around and a non contact voltage detector won’t always signal a neutral.