Testing and proper breakers for a multi wire branch circuit?

Question

How do you determine if an outlet is part of a unidentified multi-wire branch circuit? Also, does code say MWBCs need to be on double throw circuit breakers and is it permissible to link CBs with that hole in the handle?

Answer 1

The easy way to tell if an outlet is powered by an MWBC is to turn off the breaker(s) known to supply power to that MWBC and see if the outlet's power goes away.

The US National Fire Protection Agency's National Electrical Code does specify that all nongrounded conductors in an MWBCs must be controlled by a double-pole breaker or marked pair of linked breakers.

It's absolutely permissible to link breakers together using that hole - that's what the hole is there for. In most cases, though, it's a better idea to use a unitized double-pole breaker; that way you're likely to get the best possible match and service life between the two individual internal breakers.

Note that it is NOT permissible to link two breakers with different CURRENT ratings together in that manner - all nongrounded conductors in an MWBC must be the same size wire, and so if two linked individual breakers are used, then they MUST have the same current ratings.

Answer 2

How do you determine if an outlet is part of a unidentified multi-wire branch circuit?

Here are three possible scenarios:

• There are 2 different hot wires (not a splice) and only 1 neutral wire connected to the outlet. This can be a single 240 V circuit, or it could be two 120 V circuits on a MWBC.
• Tracing the hot wire within the panelboard. Remove the cover of the panelboard and look for the cable or conduit where the circuit enters. If that circuit has more than one hot wire (connected to more than one breaker pole) then it could be a MWBC.
• Tracing the entire circuit. For the sake of updating outlets or other repairs, this is sometimes how MWBCs are discovered when they are not properly identified at the panel. By tracing an outlet to a splice where there are two separate hot wire colors, this reveals the presence of a MWBC.

Note that measuring voltage is NOT a valid method. It is possible to have a MWBC installed wrong, such that the hot wires have zero voltage between them. This is why the other visual search methods are necessary.

Also note that turning off a single pole breaker identified for an outlet is not a reliable method. If the outlet is part of an unidentified MWBC, then there is a shared neutral involved and it will not get disconnected by the circuit breaker.

Does code say MWBCs need to be on double throw circuit breakers.

No, it is not required but sometimes still the easiest/best solution. The technical term for this is "common trip". NEC 240.15 provides an exception for MWBC if they are strictly used for line-to-neutral loads (120 V appliances only).

is it permissible to link CBs with that hole in the handle?

Yes, using a listed handle tie that is designed for that purpose. NEC 210.4(B) requires "a means that will simultaneously disconnect" the MWBC. This requirement has been in place for decades already.

Answer 3

You need to identify each pair of hot wires with there corisponding neutral first try starting at the romex sheath in the panel with the power to the panel OFF and follow each to there termination and label them. And relocate breakers as necessary. If this is not possible it is much more difficult .If you can find the first box that each MWBS is run to you can use a breaker finder tracer and a continuity tester to locate each wire at the panel. Tracer for the hots and continuity for the neutral with the wire taken loose at both ends. Continuity test needs to be done with all power off. Continuity can be used for all wires and is easiest with 2 people and a 2way radio or cell phone. If you aren't sure of how to test continuity tell me and I'll explain.