ELECTRIC DISTRIBUTION AND WIRING. 275 



motive force of the large generator into two equal parts, P and Q 

 must have carefully adjusted compound field windings, or the 

 field rheostat of P or Q must be repeatedly adjusted as the cur- 

 rent in the middle main changes in value. 



In the use of a motor-generator balancer it is desirable to 

 keep the system approximately balanced as explained above under 

 (c) and thus greatly reduce the duty of the motor-generator. By 

 careful grouping of the consumers' lamps and motors, the unbal- 

 ancing of a three-wire system may, in practice, be kept within 

 eight or ten per cent., so that the rated output capacity of each 

 of the dynamos of a motor-generator balancer need be only eight 

 or ten per cent, of that of the main generator. 



114. Factors which determine the size of wires in practice. 



There are five conditions which should be considered in selecting 

 the size of wires for distributing electric current, namely : (a) 

 The wire must have sufficient strength to withstand the mechan- 

 ical stresses to which it may be subjected. This condition applies 

 especially to wires strung on poles. (<) The wire must be large 

 enough to carry the prescribed current without becoming so hot 

 as to damage its insulation or to ignite adjacent inflammable ma- 

 terials. This condition applies especially to wires in a building. 

 (c) The wire must be large enough to keep the variations of vol- 

 tage at the lamps, or other receiving units, within certain limits. 

 This variation of voltage is briefly discussed on page 195, from 

 the point of view of feeder control, and it is more fully discussed 

 in Art. 117. This condition, r, applies only to the distributing 

 wires of a constant-voltage system, (d) The size of a wire should 

 be chosen so as to give an economic balance between the cost of 

 the copper and the cost of the power lost in the wire. This con- 

 dition is discussed in Art. 120. (e) In extreme cases the size 

 of a wire may be determined by a consideration of the electric 

 strength of the air or other insulating substance surrounding the 

 wire, inasmuch as the strength of an insulating medium to with- 

 stand the electric stress between two wires due to a given voltage 



