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.
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What is the purpose of a Multi Wire Branch Circuit?
As defined earlier, an MWBC is essentially two circuit that share one neutral. The only purpose of an MWBC is to save time and money. Copper is expensive and if you can save dozens of feet of copper by running less electrical cable, this will result in decreased time and money.
The most common place we see MWBC’s is at the garbage disposal and dishwasher. This is because its easy to run cable to the kitchen cabinet, and then split it off into the two different circuits.