MWBC Neutral Current

[delectric123]

If I have a split circuit 120v receptacle, fed from a MWBC on a 208Y\120v system, and I would have a 10 amp load on each receptacle, the neutral current will also be about 10 amps. what's the technical reason it is that way?

 

[GoldDigger]

The technical reason is that both return current paths go through the same neutral wire but are out of phase by 120 degrees.

If they had been in phase the current would have been 20A.

If they had been 180 degrees out of phase, the sum would have been 0A.
The actual vector addition instead produces a sum of 10A, which is also what the return current would have been if the only load had been on the missing third phase (only with opposite direction.)

Tapatalk!

 

[roger]

For simplicity use SQRT I?A + I?B + I?C - (IA x IB) - (IB x IC) - (IC x IA).

When you are only using two legs of a wye just enter those two numbers and shorten the formula.

Roger

 

[delectric123]

so if the voltage would be 120/240 1 ph. instead of 208y/120, the phases would be 180 deg apart thereby making neutral current 0?

 

[Dennis Alwon]

so if the voltage would be 120/240 1 ph. instead of 208y/120, the phases would be 180 deg apart thereby making neutral current 0?

Correct with single phase power the current on the neutral will be the difference between the phases. If you have 15 amps on one and 10 on the other then the neutral will see only 5 amps. In your case of 10 amps and 10 amps then the neutral sees 0 amps

 

[delectric123]

Correct with single phase power the current on the neutral will be the difference between the phases. If you have 15 amps on one and 10 on the other then the neutral will see only 5 amps. In your case of 10 amps and 10 amps then the neutral sees 0 amps

would that also be the same on a 240/120 delta system where the neutral is the center-tap of Phases A & C?
I fully understand the idea of why MWBCs work, but I can't get an image in my head how it works technically, how to explain it to somebody who thinks the neutral will be overloaded, maybe someday...sigh..

 

[GoldDigger]

One way to satisfy yourself of the result for the 120 legs of the high leg delta is this:
A 240 load from A to C will not cause any current in the neutral, nor will current from B to either A or C.
And equal loads from A to neutral and from C to neutral will cause opposite currents in the neutral.
So it is no different from a straight 120/240 single phase three wire.

Tapatalk!