CHAPTER XL 

 REACTANCE OF CABLE, SINGLE-PHASE. 



As stranded cables are very commonly used for trans- 

 mission lines, it is desirable to have a special formula for 

 the reactance of cables. An outline will be given of the 

 method of obtaining the formula for a seven-wire cable. 

 This formula was used in preparing the reactance tables 

 in Part III. 



A seven-wire strand consists of a central straight wire, 

 with six wires of the same size laid around it in a spiral. 

 The spiralling of the wires increa*ses the resistance of the 

 cable by an amount which is taken as i%. The spiralling 

 also increases the outside diameter of the cable by about 

 T 6 o of i%. (See Chapter VII.) 



In calculating the reactance of the cable, the first step is 

 to plot the flux density at various distances from the center 

 of the cable. (See Fig. 15.) For points entirely outside 

 the cable, the flux density obeys the law 



27 



where x is the distance from the center. The total voltage 

 due to these lines which cut the entire cable is 



2log e - X io~ 9 

 P 



volts per ampere per centimeter. When x is less than the 

 radius of the cable, the flux at the distance x is 



10 x 

 76 



