Neutral Current Cancelation

[shawn73]

I was reading an article on transformers at www.federalpacific.com/university/kfactor/chapter3.html It states the following..."In a 3-phase, 4-wire system, single-phase line-to-neutral currents flow in each phase conductor and return in the common neutral. Since the three 60 hertz currents are separated by 120?, when balanced they cancel each other. The measured resultant current is equal to zero." I don't understand, if the current is zero, how does anything work?? Lights don't come on with 0 current flow. What are they trying to say? If I remember correctly it is legal to share a neutral in a 120V branch circuit if the loads are fed from 3 different phases. Is this the reasoning? (I have always ran a neutral for every circuit anyway, but I have heard of people sharing)
Also,
In chapter 2 of the same website, they state that the neutral connection of a transformer should be rated at 200% of the phase ratings of the phase connections, to account for harmonics. Is it a good idea to oversize individual branch circuit neutrals at 200% or just the neutral between the panel board and the transformer? It seems that the theory would follow.

Thanks. I like this website, everyone is very helpful.
Shawn

 

[charlie b]

Re: Neutral Current Cancelation

I have to leave the office shortly, and I don?t have time for a complete answer. So here is the short answer:

For one load, current flowing out from the panel along the Phase A conductor will go through the load and return to the source via the Phase B conductor. For a second load, current flowing out from the panel along the Phase A conductor will go through the load and return to the source via the Phase C conductor. For third load, current flowing out from the panel along the Phase B conductor will go through the load and return to the source via the Phase C conductor. Etc, etc, etc.

Hope that helps.

 

[spsnyder]

Re: Neutral Current Cancelation

Ed MacLerin has some great diagrams on this site that may help. Here's an example mathmatically how to verify that the currents in the neutral cancel.

3 phase balanced load

A = 5<0-degrees = 5 + j0
B = 5<120-degrees = -2.5 + j4.33
C = 5<240-degrees = -2.5 -j4.33

Add the currents vecorally for the nuetral current. A+B+C = 0 + j0 = 0.

Try it graphically as well to prove it.

Hope this helps.

 

[spsnyder]

Re: Neutral Current Cancelation

As far as your comment on powering a load with no current... In my example each of the phases, A, B, and C would have 5 amps, the neu. would have 0 amps,

 

[steve66]

Re: Neutral Current Cancelation

Don't bother oversizing any neutrals unless more than one phase shares the neutral. (This is the case between the transformer and the panelboard). Even then, it is open for debate if this is really necessary. Maybe if all (or almost all) the loads are non-linear (like a computer room), but otherwise you probably don't have to make the neutrals larger than the phase conductors.

Steve