Three Phase Four Wire System

What are some factors in choosing a Four Wire Delta System over a Four Wire Wye System or Visa Vera?

In the 4 wire delta or center tap delta there is usually limits of 5-7.5% of total kVA on the single phase loading. It was meant for control power for the 3 phase load only. I have found most do not get this and overload the center tap capacity. You can tell easily when you take off the cover and Center coil is cooked black and outside coils look good.
3-phase 4-wire wye: In the US, is typically 120/208Y or 277/480Y - the single-phase load must be balanced with about 1/3 served by each "hot" leg. This is typically used to supply mostly three-phase load.

3-phase 4-wire delta: In the US, is typically 120/240V supplied by either an open-delta or closed delta secondary. This is typically used to supply a large single-phase load and a small three-phase load. The bank typically has one transformer that is larger (for example 15-100 or 15-100-15). That larger transformer supplies all of the single-phase load through its center taped secondary winding. It is called the "lighter transformer". It supplies the two "hot" legs and the neutral. The three-phase load is connected to the two hot legs of the lighter transformer and to the "stinger" leg that is supplied by the smaller transformer(s), called "power transformer", that complete the delta. The "stinger" is so named as its voltage to neutral is higher than that to the hot legs.


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