0.9  which  equals  31.545  kVA.  The  transformer capacity to meet this demand will be 37.5 kVA. The next  largest  standard  size  transformer  is  50  kVA,  much too large for this demand load, and the next smaller size is too small. However, if the computed total maximum  load  was  25.85  kVA  times  your  demand factor, this would give you 23.26 kVA; therefore, you would need a 25 kVA transformer instead of the 37.5 kVA  transformer. Your next problem is to determine the most suitable  location  for  the  transformer.  That  does  not mean finding the strongest pole but finding the pole that is nearest to the electrical center of the area. The electrical center is the point where a balance is obtained between the total kVA spans to the north and south of the location of the transformer. The kVA span is the product of the number of spans times the kVA load of the pole. To begin with, assume that you are going to place the transformer on pole K (fig. 4-15). Then figure the total kVA spans to the north and south of this location. A chart will simplify your calculation. kVA Spans North of kVA Spans South of Pole K Pole K 1 × 4.25 =   4.25 l × 9.1 =    9.1 2 × 1.3 =   2.6 2 × 14.2 = 28.4 6.85 37.5 Total kVA spans north Total kVA spans south of pole K = 6.85 of pole K = 37.5 You can see that if you placed the transformer on pole K, it would be at an imbalanced electrical center; that is, it would be too far away from the heaviest loads. So pick another pole. This time choose pole L and make another chart. kVA Spans North of kVA Spans South of Pole L Pole  L 1 × 6.2 =   6.2 1 × 14.2 =   14.2 2 × 4.25    =   8.5 3 × 1.3 =   3.9 14.2 18.6 Total kVA spans south Total  kVA  spans  north of pole L = 14.2 of pole L = 18.6 Pole L is nearest to the electrical center of the area. That is the pole on which you will mount the trans- former. Table  4-1.—Demand  Factor Structure Demand  Factor Housing 0.9 Aircraft maintenance facilities .7 Operation facilities .8 Administrative facilities .8 Shops .7 Warehouses .5 Medical facilities Theaters NAV aids Laundry, ice plants, and bakeries All others .8 3 .5 1.0 .9 Single-phase   distribution   transformers   are manufactured with one or two primary bushings. The single-primary-bushing transformers can be used only on   grounded   wye   systems   if   they   are   properly connected.  Figure  4-16  schematically  shows  the connections of a single-phase transformer to a three- phase 2,400-volt three-wire ungrounded delta primary voltage  system  to  obtain  l20-volt  single-phase  two- wire  secondary  service.  The  connections  for  similar systems   operating   at   other   primary   distribution voltages such as 4,800, 7,200. 13,200, and 34,400 would be identical. Figure  4-17  shows  the  proper  connections  for  a single-phase transformer to a three-phase three-wire Figure   4-16.—Single-phase   transformer   connection   for   120- volt  two-wire  secondary  service.  Transformer  secondary coils  are  connected  in  parallel. 4-12