54 



TRANSMISSION LINE FORMULAS 



spiralling, but an average value of i% is assumed in mak- 

 ing up the tables in Part III. The cross section of the 

 cable is assumed to be equal to the sum of the cross sections 

 of the individual wires. The weight per unit length of the 

 cable calculated from this cross section must be increased 

 by the same percentage as the above increase in resistance, 

 due to the extra length of the outside wires. Since the 

 cross section in Fig. 9 does not cut the outside wires ex- 

 actly at right angles, their sections as shown in the figure 



are really ellipses, and the di- 

 ameter of the cable is slightly 

 greater than 6 pi. However, 

 this difference is small and has 

 been neglected in the figures 

 for diameter of cable tabulated 

 in Part III. 



The number of wires in a 

 strand varies in practice ac- 

 cording to the degree of flexi- 

 bility and mechanical strength 

 desired by the user. The num- 

 ber of wires per strand in the tables represents average 

 practice for overhead lines. The larger cables often have 

 19 or even 37 wires. 



The section of a i9-wire strand is shown in Fig. 10, and 

 it is seen that the maximum diameter is 5 times the diam- 

 eter of one of the individual wires. The same increase of 

 i% in resistance is allowed as with a 7 -wire strand. 



There is only a very slight difference in the reactance 

 and capacity of a 7-wire and a i9-wire strand of the same 

 sectional area, so that values listed for 7 wires may be 

 used for 19 wires, and vice versa, without very much error. 



I 

 U-- 2?- 



Fig. 10. ig-Wire Strand. 



