400 



ELECTRICAL EQUIPMENT 



any line and the neutral, while for three-phase work the line wires 

 are tapped directly, the voltage between these beingVs times the 

 single-phase. This system results in a copper saving of 56 per 

 cent, assuming that the four wires are of the same cross-section. 



If the main three-phase line potential is fixed, this method 

 offers no saving; on the contrary, it requires 33 per cent more 

 copper. In any case, however, the use of the four-wire system 

 gives increased flexibility, and the neutral wire carries all un- 

 balanced currents. 



This system is mostly used for a combination of motor and 



AAAAAA 

 c, d, 



< 57.7 > 



FIG. 244. 



lighting loads. The lighting service is operated from a 2300-volt 

 phase voltage and the power service from the 4000-volt line voltage. 



Transformers are sometimes designed so as to be suitable 

 for either delta-delta or delta-Y connection, in order to permit the 

 user to increase the capacity of a transmission line by raising 

 the line voltage, which can be accomplished by changing the con- 

 nection from delta to Y on the high voltage side. Such trans- 

 formers are necessarily more expensive than they would be if 

 designed for straight delta-delta, and used at the lower voltage 

 only, because they must be insulated to withstand the higher line 

 voltage. 



The rating of a group of delta- Y-connected transformers is 

 the same as for the straight delta-delta connection. 



Where power is transmitted with delta-Y step-up and Y-delta 

 step-down transformers, service may be maintained with one 

 step-down transformer cut-out, the connections being made as 

 shown in Fig. 245. 



