JD
Well-Known Member
For 30 years, my standard protocol on setups was to turn everything on (as well as throw a few questionable scenes up) and throw an amprobe on the feeds to make sure there was reasonable balance. Never did I find a higher current on the neutral then was available from any of the hots. A little understanding of 3 phase geometry makes it clear that it would be impossible. First some terms:
WYE = three windings with their low end tied to neutral and their high end providing the hots. Usually 120 volts to N and 208 volts between hots.
DELTA = three windings with their ends tied to each other in the shape of a triangle. Usually 240 volts between hots.
A transformer (source) will be one of the two. A load may be either or a combination of both. Watch what happens in the following three load cases based on equal loads on a WYE source:
One leg loaded: 100% on the affected hot, 100% on the Neutral
Two legs loaded: 100% on both affected hots, 33% on the Neutral
Three legs loaded: 100% on each affected hot, 0% on the Neutral
What happens here is that our load changes from Wye to Delta as it balances out. The current finds its return feed through the other operating hots. A similar thing happens on single phase circuits using a 120-0-120 source. If the two legs are balances, the neutral does very little. In a three phase system, each phase leg lags the prior leg by 120 degrees as compared to 180 degrees in the single phase application.
Back to the real world. No such thing as a perfectly balanced load. During the course of scene and intensity changes the neutral will do a lot of work, but never, I repeat never, over 100% of the work that any given leg will handle.
One last note: What holds true on (our) load end does not hold true in the transformer box itself! Even though our load may be pusdo-delta, the transformer is not! Current between windings ALL flows through the neutral buss, and it may see currents of 300% of the rated load as each winding seeks its backflow through the buss. The transformer box however, is none of our business!
WYE = three windings with their low end tied to neutral and their high end providing the hots. Usually 120 volts to N and 208 volts between hots.
DELTA = three windings with their ends tied to each other in the shape of a triangle. Usually 240 volts between hots.
A transformer (source) will be one of the two. A load may be either or a combination of both. Watch what happens in the following three load cases based on equal loads on a WYE source:
One leg loaded: 100% on the affected hot, 100% on the Neutral
Two legs loaded: 100% on both affected hots, 33% on the Neutral
Three legs loaded: 100% on each affected hot, 0% on the Neutral
What happens here is that our load changes from Wye to Delta as it balances out. The current finds its return feed through the other operating hots. A similar thing happens on single phase circuits using a 120-0-120 source. If the two legs are balances, the neutral does very little. In a three phase system, each phase leg lags the prior leg by 120 degrees as compared to 180 degrees in the single phase application.
Back to the real world. No such thing as a perfectly balanced load. During the course of scene and intensity changes the neutral will do a lot of work, but never, I repeat never, over 100% of the work that any given leg will handle.
One last note: What holds true on (our) load end does not hold true in the transformer box itself! Even though our load may be pusdo-delta, the transformer is not! Current between windings ALL flows through the neutral buss, and it may see currents of 300% of the rated load as each winding seeks its backflow through the buss. The transformer box however, is none of our business!