As a transformer bank the use of open-delta is relatively inefficient where most of the loads are three-phase since the bank has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of the closed delta bank of three units. When the open-delta bank is used because the three-phase loads are small, you find transformers of different sizes being used. A larger transformer for the single-phase load and a smaller transformer for the lesser, three-phase loads.