Why would turning on one breaker trip another?

Question

Before rearranging the main living room, I replaced two 15A outlets that were previously hidden. (The new outlets are the kind that are hard for kids to jam things into.) After doing this I discovered, or rediscovered after forgetting, that the circuit appears to be redundant. I can’t get both 15A circuits to stay up. Turning on one while the other is on makes the other pop.

Now, as I said, I can’t recall if I discovered this before and forgot. The house could stand to be rewired with a new box, but that’s a crazy pipe dream that won’t add value to the home. Absent that, should I be concerned here? Should I replace the two circuits? Am I misreading how these circuits work? Does this pair allow 30A? Surely not. I double checked my work with the outlets.

Answer 1

Somewhere the circuits have been bridged together where they shouldn't be. And if the hots are out of phase with each other, then turning them both on creates a direct path from one hot to the other, and a short, which is tripping your breaker.

If you're seeing this kind of wiring issue, then there are almost certainly others that are also there, and the proper solution is to gut your house, find all the hidden junctions, fire hazards, and other wiring issues, and likely rewire the whole mess.

Since you'll likely ignore that advice, find the mid-point between these two circuits, break it into two by unhooking the load connection from the outlets on each end of the two circuits, label the line, and bind the hot and neutral together on each end of that disconnected line so that no one reconnects the circuit accidentally. To find the midpoint, start unhooking one side of an outlet and see what still works when you turn on one of the two circuits. And make sure the kids aren't trying to put things in the outlets while you're testing.

Answer 2

Was one of the outlets switched? Look at the outlets you removed and see if the brass bar connecting the two sides of the duplex outlet was cut (on the side by the screws you attach the wires to). If so, you will have to do the same thing to separate the circuits. One might be "hot" and the other switched and are fed from different breakers.

You might be able to test this theory by turning off the wall switch that could have previously controlled the outlet. If that stops it, it's probably the answer.