STATION WIRING 639 



steel work. No ground connection for this service should be of 

 less than No. 6 B. & S. flexible cable. 



For open high-tension wiring utilizing bare conductors, the 

 size depends on the current to be carried as well as the heat- 

 radiating conditions. For very large alternating currents, such as 

 in low-tension bus-bars of large size, the skin-effect may be appre- 

 ciable, requiring a low current density. As a rule, this may vary 

 anywhere from as low as 300 to 400 amperes per square inch to 

 1500 amperes per square inch, depending on the conditions. This 

 is dealt with more fully under the section on "Bus-bars," page 565. 



For very high-voltage work using copper tubing the sizes given 

 in Table LVI are quite common. 



Corona Limit of Voltage. Attention must also be given to 

 the possibility of the formation of corona when the size of high- 

 tension conductors is determined. Table LVI I gives the highest 

 safe three-phase voltage for any given size of wire. The values 

 are based on sea level but may be corrected for other altitudes 

 by the correction factors given in table LVIII. 



Economical Considerations. In determining the size of a 

 conductor the economical side of the problem should not be lost 

 sight of, although it may not be of such great importance for the 

 station wiring as for the distribution or transmission system. 

 The most economical area is that for which the annual outlay 

 equals the annual cost of the energy loss, and according to this rule, 

 the cheaper the power, the less should be the capital outlay for 

 the conductors, thus allowing a smaller size to be used and a corre- 

 spondingly increased loss. In general the cost of ducts, insulators 

 and supports may be considered as not affected by the variation 

 in size, but that the outlay is only affected by the comparative 

 cost of the cable itself. 



Voltage Drop. In a continuous-current circuit, the drop at 

 the terminals of a circuit with resistance R and traversed by 

 a current I ampere, is IXR volts. Likewise in an alternating- 

 current circuit the drop in voltage of a circuit with an impe- 

 dance Z, traversed by a current of / effective amperes, is / X Z 

 volts. 



The voltage drop in alternating current circuits, therefore, 

 depends on both the resistance and reactance, but with wires close 

 together, as in conduit work, the reactance will generally be small. 

 The drop should be calculated for the given power-factor, load, 



