The Flow of Neutral Current in a Multiwire Circuit

[Joe D.]

In a single phase system 1 load on A phase draws 9 amps and 1 load on B Phase draws 5 amps. Is it correct in thinking that the current starts at the source, travels from one load to another (A phase through load to shared neutral through second load then B phase) then back to the source and the current imbalance goes back on the neutral? Or is it correct in thinking that the current just leaves the source on A & B phase goes through their respective loads and just cancels out on the neutral because of the opposing magnetic fields on the neutral?

 

[zbang]

I'd say neither but the first conceptually is more correct. (Remember that the A and B phase leads are opposite polarities, so when one "pushes" the other "pulls".)

(Assuming a center-tapped transformer feeding the demo)
Let's turn the draws around so A=5 and B = 9. Load A pulls 5 from the A phase and pushes it out to the neutral terminal, load B pulls 9 from that terminal with 5 having passed through load A and 4 coming up the neutral wire from the transformer.

 

[jaggedben]

A is somewhat more correct conceptually, although I don't think the 'either/or' makes complete sense here, nor is current in reality a substance that 'travels' from one place to another. You could say the imbalance returns on the neutral because the rest of it cancels out due to the opposing fields. You could also say that the imbalance 'comes out' on the neutral during one half of the cycle and 'returns' during the other half.

 

[LarryFine]

I agree your first description is more accurate. For all practical purposes, the two halves (attempt to) behave as two independent circuits that happen to share a common conductor. You could do the same thing with two different voltages, or even an AC and a DC source and load.

Just as each line conductor carries current in an attempt to keep the load end at the same voltage as the source end, minimizing voltage drop, so does the neutral carry as much current as it can trying to keep the load neutral at the same voltage as the source neutral point.

Here's a little ditty I wrote a while back that was made a sticky:

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...

forums.mikeholt.com

 

[jaggedben]

. For all practical purposes, the two halves (attempt to) behave as two independent circuits that happen to share a common conductor. You could do the same thing with two different voltages, or even an AC and a DC source and load.

I'm not sure that's helpful. The difference between a center tapped neutral and two truly independent sources with a common conductor is that if you lose the neutral on the former you still have voltage between the two remaining conductors, whereas if you lose the common conductor on the latter you lose all power. That's an important practical difference. So is the fact that current on a true neutral can't be more that on one of the hots, whereas current on the common conductor could be the sum of both currents on the others. In other words, two independent circuits could behave like the OP's A or B, whereas a center-tapped single phase source only behaves like A. (If I understood his gist.)

 

[GoldDigger]

...

Here's a little ditty I wrote a while back that was made a sticky:

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...

forums.mikeholt.com

Although it iseems to be arranged in stanzas,I will not accept it as a dtty unless you provide the tune to go with it.