AC Power Question

@cleeds 

Will you please explain what "back-current" is? If you have a 14-3 cable, then each of the hot wires come from different legs of your service, correct? (After all, that's why they can share a neutral.) Please explain how a changing load on one leg can influence the voltage on the other leg.

If your 14-3 cable is connected to the same phase (leg) of your AC service, that's a code violation because of the shared neutral, no?

On a shared neutral such as 14/3, the back current is reverse flow from the red leg affecting the neutral potential for the black leg and vice-versa. That’s why shared neutral circuits sometimes use a smaller gauge neutral conductor. When both sides are steady loads, such as lighting, you can do shared neutral, but in the case of the circuit powering my clothes washer, the other leg powered a lighting circuit, which caused the lights to flare when the washing machine was changing cycles and electrical demand from it surged. 

Shared neutral circuits must never have both hot conductors on the same leg of breaker panel. That would cause the neutral load to reach double the current rating, and create fire hazard. Shared neutral should be against code in my opinion. The correct use for 14/3, 12/3, 10/3 etc... is 240 volt loads that require a neutral conductor, or sub panels. 

Multiwire branch circuit.

https://www.youtube.com/watch?v=eVamt9IdQd8

As my questions should suggest, I understand how these circuits work. What I don’t understand is how the load on one phase can influence the current on the other phase, unless there is something wrong with the electric service in the first place.

Shared neutrals are absolutely allowed under NEC and I see no reason to avoid them.

Two circuits on the same phase require their own neutral. In other words a 12/3 romex termination at the panel needs different phases, code changed where now the breakers for those circuits need a mechanical tie to shut them off simultaneously, or a dpdt breaker. Older homes will not have this feature, before the code was revised a single breaker was acceptable for each 110V phase as long and the romex run was of different phases. Too lengthy to get into.
Anyways, Jea brought code revisions in regards to the breakers to my attention months back, quite a few code revisions since I left the field.
With that said, the two circuits of same phases sharing the same neutral is one of those ’shall not’ do and has been code for as long as I remember. It's pretty much common sense.