AC Power Question

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.

cleeds1,945 posts01-25-2019 7:08am

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

The load on one Hot Line does not affect the load on the other Hot Line. What can be affected is the voltage measured from the hot to neutral due to a severely unbalanced multiwire branch circuit due to voltage drop on the heavily loaded hot to neutral circuit. The shared neutral is part of the circuit.

sleepwalker65 stated in his post a 14-3 w/grd 3 wire multiwire branch circuit feeds the laundry room outlet on the 2nd for the washing machine and a ceiling lighting circuit. (One Hot Line for the wall receptacle for the washing machine, the other Hot Line for a ceiling lighting circuit). The electrical panel is located in the basement on the opposite end of the house. How long, distance, is the multiwire branch circuit?

(First this must really be an old house. For as long as I can remember the circuit for a (laundry area) cloths washing machine per NEC is 20 amp, #12awg minimum copper wire.) A muliwire branch circuit should have never been used in this instance. Does it meet bare minimum NEC code ? Yes..... I wouldn’t have though......

So back to the load on the Hot Line to neutral load created by the washing machine. Say the distance from the electrical panel to the wall outlet for the washing machine is over 100ft, or more. (It could very well be. 3000 sq ft house.) What do you think is happening to the current draw of the motor during the wash cycle? Do you think it is steady or fluctuating? As the current is fluctuating what is the voltage doing on the circuit? I bet it is also fluctuating due to Voltage drop on the line created by the load on the motor in the wash cycle.

If a clamp on amprobe was clamp on the neutral at the end of the 14-3 where it splits off to 2 two wire circuits, (one for the washing machine, the other for the lighting), you will see the varying amperes of the motor load. If you measure the voltage from either Hot Line to the common neutral you will also see the voltage fluctuating. If the ceiling lights are incandescent bulbs you will see the voltage fluctuations caused by the voltage drop on the neutral wire.

As for light bulbs burning out faster than they should normally, I would be looking for a loose or corroded neutral connection starting at the electrical panel neutral bar connection to the junction where the circuits split off the 14-3.

Also worth mentioning Motors are rated in HP. As the voltage drops due to voltage drop current goes up. More current more heat. Too much heat will cause terminations to overheat, corrode, and can eventually cause them to fail.

As for the use of multiwire branch circuits. They have been around ever since 3PH 4 wire polyphase and split phase single phase distribution power transformers. Common place especially in office buildings and industrial facilities.
They save/saved money. Less wire and fewer conduits and smaller sized conduits. Less labor cost man hours.

Then came the age of computers. Starting in the 1980s data centers in office buildings. PCs, (Personal Computers), started showing up on workers desks connected to the main frame. 1990s even more PCs on workers desks. Around the early 2000s problems were starting to show up. All the while, more PCs were being installed in office buildings. Electricians were being called to trouble shoot electrical problems on 4 wire multiwire branch circuits. What they found was the neutral conductor at the electrical panel connection to the neutral bar was corroded, burnt, and the insulation on the wired was discolored and in some cases burned near the termination. HARMONICS! All them PCs.

The solution to the problem? Increase the size of the neutral one size larger than the phase conductors. So if the phase conductors are # 12 the neutral conductor is #10. Did that work? Yeah, for the branch circuit wiring........

Then come the 2008 NEC code edition. Something new for multi wire branch. For electrical safety reasons all ungrounded conductors of multi wire branch circuits shall be required to have all ungrounded conductors of the multi wire branch circuit to be de-energized simultaneously. Either a multi pole breaker or if single pole breakers are used an approved common tie device must be used that opens all breakers with one movement. What do you think this did for the benefit of using multiwire branch circuits in office buildings build after the 2008 NEC was adopted by AHJs across the US? (For many years there were, still may be some, AHJs that did not adopt this section of the 2008 NEC. My State did in Jan of 2009)

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My situation is a 15yr old house governed by Canadian electrical code, which is for the most part consistent with the US NEC. The reason the builder used 14/3 for those branch circuits was purely economics based. In Canadian electrical code, a clothes washer circuit is only required to be 15 amp. Should it be on its own? - yes! Should it be 12/2 instead? - yes! But, the reality is this house was built in a large development project, and they didn’t do things to meet ideals, they based every decision on the bottom line. One fine day, I will get around to running a dedicated 12/2 for that clothes washer. As an intermediate stage, I removed the lighting circuit from the other leg of that 14/3 and put it on a dedicated 14/2, which has improved the lifespan of lightbulbs.