Multi-wire circuits can be confusing to people.
How multi-wire circuits function, and problems related to them, are important to understand for electrical safety. In a multi-wire circuit, the neutral (grounded conductor) is shared between two circuits. Most circuits have their own neutral. Generally speaking, multi-wire circuits perform just fine as long as the rules of installation are not violated.
Each leg of the multi-wire circuit must terminate on a different bus bar.
Because bus bars are 180 degrees out of sync with each other, the neutral current can travel on the neutral wire safely. If they were to terminate on the same bus bar, the current from the two circuits gets added together. This is because the circuits are no longer 180 degrees out of sync with each other. Wiring the circuits this way can result in overloading the neutral. The amount of current leaving the breaker however, would not be more than normal and the circuit breaker would not trip even with the overheating wire.
The following read-along, video-blog attempts to discuss some of these issues. Please pause or rewind the video where necessary to suit your own level of understanding and learning curve. The demonstration board is set up to do many more experiments than what this post is about, so pay more attention to the overlays in the presentation and ignore all the rest. Perhaps some of the other components will show up in a future post.
Charles Buell, Real Estate Inspections in Seattle