Bill;
I'll keep this message's content low
and the answers more consise than indepth, but even the most brief examples will still have a lot of reading involved, so please do not be discouraged or offended.
A quick and simple cover on the Grounded Conductors:
For a single phase 3 wire system, such as the ones used in Residential areas [one transformer], the grounded conductor will work as a "Neutral" because it will carry the unbalanced amount of current in a 120 volt 3 wire multiwire circuit.
An example circuit:
Phase A has a load of 10 amps at 120 volts, Phase B has a load of 15 amps at 120 volts. The "Neutral" [center tapped conductor] will carry 5 amps, which is the left over [unbalanced] load. 10 Amps is common to Phase A and Phase B, so they will carry this "balanced" load current between them. The remainder of current flows on the center tap [Neutral]. If the load on Phase A is 10 amps and the load on Phase B also is 10 amps, there will be no current flowing on the Neutral and the system [circuit] is said to be balanced. This is why it's called a Neutral. It carries the unbalanced current from which ever Phase has the highest load.
As you can see from this example, if the loads are split up evenly between the two Phases, the neutral will have a very light load on it between the service and the power transformer.
The above example also works for multiwire circuits from a 3 phase 4 wire Delta system, however if the "High Leg" is used for a 208 volt 2 wire circuit, this current will not balance out the neutral.
Harmonic distortion on L-N circuits / loads causes the neutral to carry the THD current as unbalanced current. This can overload the neutral conductor[s] if excessive.
Now for the 4 wire Wye system:
The grounded conductor for this system will typically carry the highest load. Each Phase coil is connected at a common point, which is where the 4th wire is derived from. This makes it possible to have a dual voltage system [multiwire circuits]. Loads can be connected L-L for 208 volt 1 phase loads, or L-G [Line to Grounded conductor] for 120 volt loads, along with 3 phase L-L-L 208 volt loads. Same goes for 277/480, 347/600, 2400/4160 and the like.
The common conductor works with either 3 or 4 wire L-G multiwire circuits as follows:
Each L-G circuit is a somewhat direct connection to that transformer coil. There is no center tapped winding like the 1 phase system. This makes the grounded conductor common to all three phases [phase coils].
The common conductor will carry the highest load from a multiwire circuit.
An example circuit:
Phase A = 3 amps, Phase B = 5 amps, Phase C = 1 amp. The load on the common [grounded conductor] will be 5 amps, which is the higher load on the ungrounded conductors.
Another circuit example:
Phase A = 10 amps, Phase B = 10 amps, Phase C = 10 amps. Common [grounded conductor] = 10 amps. 10 amps is the level of current flowing.
These loads are, of course, connected L-G. As you can see, the common does not offer a Neutral state, as did the 1 phase system.
There is an exception to this. When the loads are completely Resistive [Incandescent lamps, electric ovens, heaters with no motors and water heaters], the common will become a Neutral. This would be if the multiwire circuits were connected to only the resistance loads, with no Reactive loads included. One other way to look at that is to say loads with unity power factor [100% PF] could apply here. In this case, the common will carry the unbalanced portion of current. If the currents are equal, there will be no current flowing on the common. This could be determined to be a "Perfectly Balanced Circuit". On the Technical side, even pure resistance loads still contain some Inductive Reactance [Xl] and some Capacitive Reactance [Xc]. Heaters with large - long coils are definitely Xl, along with Quartz Halogen lamps.
All other types of loads which are not pure resistance [Incandescent lamps, or electric resisance heating elements] are Reactive loads. They have a power factor and create a lead or lag current in the AC sine wave. These Reactive loads do not balance across the common for L-G loads.
The way these levels are figured is by using Vectors. Draw Vector lines of a certain scale size [length], separate at 120 degree angles, then connect the intensity of the L-G currents together via a trapazoid. To explain anymore on the Vectors, or the theories behind the current flow, would make the last message seem like a 2 paragraph leaflette!
The above examples and explainations might look long, but they are the shortest, most brief, simplest to understand, lowest tech terms and still consise way I can possibly explain this to anyone. It's much easier to understand face to face where example circuits can be drawn.
I understand that this is completely opposite from what you were told before, so I understand fully why it might be difficult to grasp. Besides, this is Electrophysics / Electrical Engineering! They just don't print this stuff on the back of a box of Coco Puffs
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My suggestion [as always] is to find reference materials, such as Electrical Engineering books or Electrical Circuits / components analysis and theory for AC type books. They can explain a lot better than me
.
If you have the chance[s] to, make an on site analysis of some existing multiwire circuits to determine the loads on each conductor. Using a clamp on type ammeter, measure the current on different multiwire circuits at the panel / subpanel at as many different projects as you can and to 1 phase + 3 phase systems. Gather and collate your data to base a conclusion for your self of how different system types, load types, etc. work out.
If you wish to contact me directly by E-mail, feel free to do so. Click the link below to send me a message:
E-mail me at
adst@SoCA.com Good luck!!
Scott.