Changing Tandem Breakers with Potential Unmarked MWBC

Question

I have two adjacent 20A tandem breakers in my panel. I need to change one of the breakers on each tandem to 15A (in order to protect 14 gauge wires on the respective circuits).

The possibility exists that an MWBC is present amongst the four circuits in question, but it can’t be confirmed (or rejected) - the panel box is crowded and the relevant cable jacket(s) are recessed in the wall.

I want to swap the 20A tandems with BD1520 tandems, so that each of the four phase conductors originates from the original phase (preserving the voltage between any potential MWBC hot wires). Outside of the potential code violation (which I inherited) for not having a handle-tie (if indeed an MWBC exists), would this possible solution create any electrical hazards that I may be overlooking?

The label/diagram for the panel is missing.

Answer 1

If it's not in conduit, same color = not MWBC.

If you can establish positively that your house is wired with cables and not conduits, the colors of the wire will tell the tale. Cables always have different colors for each wire (otherwise how would you tell them apart).

Given that both blacks are the same color, that can't happen with MWBC in a cable - only in a conduit. So if you can confirm that circuit does not go into a conduit, then the same colors prove it is not a MWBC.

Eat your cake and have it too - use a quadplex

You don't need to say "except for the lack of handle ties". You can have handle-ties also.

Who's to say the 20's aren't also MWBCs?

Simply use a 15/20 quadplex which has handle ties on both.

Any of the Eaton BQ, BQC or BRD quadplexes should do it for a Westinghouse, Challenger, Bryant etc. panel that uses the pictured breakers and has the CTL notches you showed in another question.

Answer 2

Handle ties not required until 2008 so there may not have been a code violation.

you may not be able to see what cables connect to what neutrals but there are a few ways to figure it out.

First how many hot and how many neutrals same number not a MWBC.

Ok you have 1 neutral and 2 hots that is a MWBC the trick to finding the pair is to turn both hots off lift the neutral put a wire nut on it.

Turn 1 hot on With a load like a lamp) if it did not work that is one hot for that neutral turn the hot off and try the other hot with a load again with the neutral lifted if the second hot did not work you have identified the 3 wires of a mwbc do verify the lamp is good LOL.

You can use this basic test with multiple neutrals just make sure to have 1 of the 2 suspects turned off because a MWBC can work without a neutral if both loads are similar you won’t know because it works but turn 1 off and the other dies also.

This is how I have figured out multiple MWBC’s in pipe when they were not identified 6 hots 3 neutrals same method, I hope it helps you.