Is there a definitive way to know whether MWBCs are present (or not) in an electrical panel?
That is, are there multi-wire branch circuits?
Can one look at the connections at the panel to see a clear answer, and/or measure voltages?
If you can provide links to images, that'd be great.
Please remember that some of us may not know technical terms / acronyms! :-)
This came up in a previous question discussion and it was suggested that I post it as a stand-alone question.
Thank you again.
A multi-wire branch circuit is one in which a single neutral wire carries return current corresponding to more than one hot wire. Under NEC rules, the hot wire(s) and corresponding neutral, if any, must be enclosed in the same cable or conduit.
Look around the perimeter of the panel. There are many places where branch circuit wires come in, either as cables or as loose wires in a conduit. Look closely at each set. Any cable or conduit that contains an odd number of insulated wires may contain a MWBC. Also, any conduit that contains fewer neutral wires (white or gray) than hot wires (any color except white, gray, or green), may contain a MWBC.
For any of those sets of wires identified above as possible MWBCs, follow each wire in the set. If two or more go to circuit breaker hot terminals (ie not the neutral terminal of a GFCI or AFCI breaker) then there's a MWBC in there. In the case of wires in conduit, it's possible that some of them are members of an MWBC while others aren't. That complex scenario is beyond the scope of this answer.
Some might argue with this method because it counts the 120/240 circuits commonly used for clothes dryers and ranges as MWBC. I'm not entirely certain whether NEC considers them to be MWBC or not, but from a practical standpoint it's somewhat irrelevant: like MWBC, these circuits also need to land on a handle-tied circuit breaker. (They also require the breaker be common-trip, whereas for basic MWBC common-trip is optional, but that's whole other ball of wax.)
Depends how your house is wired.
If NM-type cables, you'll see a /3 cable with black/white/red/ground where black and red are both connected to a breaker.
In the current era those should be either a two-pole breaker or adjacent and handle-tied, but earlier codes permitted less obvious and less safe breaker arrangements, so it's not guaranteed.
If you use a voltmeter the hots on a MWBC will be 240V between them, not 0V, but there's no guarantee that a duplex receptacle on an MWBC will be split & connected to both sides. That is a common arrangement, but not guaranteed.
If the wiring is in conduit they can be harder to spot, as there's no requirement for the hots to be different colors - but you will have two hots and one neutral. You'll also have two hots and one neutral for devices like dryers and ovens or ranges that use both 120v and 240V, so you have to sort out what circuit is doing what to be sure you've got the MWBCs located (and you might as well update your panel labeling if that isn't clear for the panel labeling you start with.)
If you have a panel wired with only NM cable, the first clue would be looking for a red wire. Straight 240 volt appliances would use the white and black as hot. Appliances needing 120/240 volts would use black, red and white wires but MWBCs would also use those same colors. Both should have the breakers handle tied if using two single breakers. If you have more handle tied breaker, or double pole breakers, than you do 240 volt appliances, then those extra ones are probably MWBCs. You should be able to get some information from the panel labling,for example, a double pole breaker has two lines written on the panel, one says bedroom A, the other says bedroom B, you can be pretty sure it's a MWBC.
If your panel has conduit, same holds true but could have many different color wires in there.
Presence of red wires is a good clue. However, I would argue there is not a reliable first-glance way to determine where are the MWBC breakers or if there are any. This is particularly true with decades-old panels.
If there's a red wire sandwiched between two black wires, no common trip, and no handle tie, then it's pure guesswork trying to ID the circuit without tracing the wires to see where they are bundled. Measuring voltage will NOT help with this. It's even worse with the cover in place as it might be impossible to identify a MWBC. This is why there are new rules to disallow circuits lacking a single disconnect.
