STATION WIRING 635 



wires around this. The annular form thus increases the carrying 

 capacity by utilizing more of the- copper and there is a further 

 increase in the capacity due to the larger radiating surface. In 

 view of this fact that the rope core cable has a greater carrying 

 capacity due to its increased radiating surface it could advan- 

 tageously be adopted for all cables, direct-current or alternating- 

 current, for sizes 700,000 cir. mils and above. 



It is apparent from the above that the carrying capacity of a 

 cable depends on so many factors that no table can be given which 

 applies to all conditions, and considerable care should be exer- 

 cised in selecting the size if it is necessary to economize. Tables 

 LIV and LV will, however, serve as a guide for determining the 

 safe current-carrying capacity under three assumed conditions, 

 X, F, and Z. Condition X is such as to require the maximum-size 

 cable while condition Z is the most favorable requiring the mini- 

 mum size. 



The use of these tables is best illustrated by a couple of exam- 

 ples: 



Assume that it is desired to find the safe size of a single-con- 

 ductor, varnished cambric, insulated cable, installed in duct, the 

 operating voltage being 6600 volts and the continuous current to 

 be carried 1000 amperes. 



Referring to the first column in Table LIV we must use the 

 next higher current values or 1075, and it is seen that the cable may 

 have a size from 1,250,000 C.M. to 2,000,000 C.M., depending on 

 the operating condition. Then going to Table LV we find in the 

 eighth line from the top (corresponding to our case) that two con- 

 ditions, Y and X, are given, the former being limited to a 1,000,000 

 C.M. cable and the latter to a 2,000,000 C.M. By comparing the 

 results from the two tables it is apparent at once that the Z con- 

 dition is out of the question entirely and furthermore that the Y 

 condition, corresponding to 1,500,000 C.M., also gives too small a 

 value as this condition was limited to a 1,000,000 C.M. cable. 

 The size must, therefore, correspond to condition X or 2,000,000 

 C.M. 



As another example, assume that a 750-volt varnished cambric, 

 insulated cable in conduit is to carry 175 amperes. What size is 

 required? 



Referring again to Table LIV we have three different sizes to 

 choose from, 4/0, 2/0 and 1/0. From Table LV, sixth line from 



