If a Feeder ckt supplies only 3-ph and 208 1-ph loads, will adding the neutral conductor minimize voltage imbalance ? Or does neutral accomplish nothing ?
If you have only balanced three phase loads and your single phase load is connected phase to phase, there is no need for the neutral. Think about the current flow. What do you expect to see on the neutral under these conditions?
Is this a theoretical question or are you experiencing voltage imbalance and seeking a solution?
There are cases where connecting the neutral will solve voltage inbalance, but this is because a ground fault exists that will be revealed (in a bad way) when you connect the neutral to ground.
In my opinion, if the system is supplied by a low-voltage star connected transformer and the consumers are phase-to-neutral [one phase] supplied, the three phase voltage will stay equilibrated if the loads are different only if it is a continuous return of neutral conductor to the source.
For instance let's take a building of 3 dwelling units. Let's say apartment A load is 1000 w at 208 V. Phase A to neutral resistance is then ResA=208^2/1000=43.26 ohm.
Let's say apartment B load is 100 w and Phase B to neutral resistance will be ResB=208^2/100=432.64 ohm.
Now, let's say no load for apartment C. If no neutral is provided-that means the neutral is disconnected at the common building supply cable- then from live A to live B will be 208*sqrt(3)=360 V.[See the sketch]
The common current flowing from A to B=360/ (432.64+43.264) =0.75646 A.
Then the A load will get 0.75646*43.26=32.72 V and the load B 0.75646*432.64=327.27 V.
If a neutral continuous conductor is provided the voltage will be equilibrated and the each individual current will flow back to the source [transformer].
I believe that you misunderstood the OP. The system is 208V Ø-Ø (208/120Y - common in USA) and there are no loads connected Ø-N.
I agree with others that there is no need for a neutral conductor. There is a need for an equipment grounding conductor to ensure tripping for ground faults.
