What is an MWBC?
The Richmond Home Inspector
MWBC is an acronym for multi-wire branch circuit. It is a method of wiring when an electrician uses one cable, for two circuits. The two circuits share one neutral. A more technical definition would be when two ungrounded conductors share one grounded conductor. Although I have read strong opinions against the use of MWBC’s, there is nothing wrong with them when wired properly.
It can be confusing to understand, but here are the basics.
First, both breakers for the MWBC should be tied together. This is reflected in Virginia Residential code E3701.5.1 which states “Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors a the point where the branch circuit originates.
An ungrounded conductor is the hot conductor. So in simpler terms most homeowners should understand, the breakers need to be tied together so the power will be shut off to both hot wires in the MWBC.
This is done to prevent an electrician from being shocked. If an electrician doesn’t disconnect both hot wires in an MWBC, or does not kow he is working on an MWBC, the circuit he is working on will be energized from the other hot wire in the MWBC.
Second, the MWBC breakers must originate from opposite bus bars. Each residential panel has two bus bars powered from L1 and L2.
When wired improperly an MWBC can cause the neutral to become overloaded. When both circuits originate from the same line (same bus bar) the current running through the neutral will be the sum of both circuits. When both circuits originate from opposite phases (different bus bar) the current running through the neutral will be difference of both circuits.
Let’s consider 15 amp circuits in a scenario where both circuits draw 10 amps at the same time. If both circuits are on different phases, the current through the neutral will be 0 (the difference between both circuits.) If they are on the same phase, the current through the neutral will be 20 amps (the sum of both circuits).
20 amps is too high for 15 amp circuits on a 14-3 cable. The neutral will be overloaded. This can cause the insulation to deteriorate and can cause arc fault fires.