STATION WIRING 633 



should be allowed than for ordinary cable practice. Since such 

 leads are not usually protected by any automatic circuit breakers, 

 it is good practice to select a cable for this purpose with an insu- 

 lation thickness 50 per cent greater than the normal working 

 voltage of the generator. 



Control and Instrument Wiring. Under this class would be 

 grouped the control circuits for oil switches, rheostat and governor 

 motors, etc., secondaries of current and potential transformers 

 and all other similar conductors. These conductors are always 

 of a very flexible rubber-covered weatherproof multi-conductor 

 type, installed in iron conduit. Occasionally where the location 

 is very damp a lead covering may be desirable. With this cable 

 it is possible to pull it through a conduit some 100 feet in length 

 with four standard conduit bends in the run. 



The best practice is to lay the conduits in the floor and let 

 them terminate as near the switchboard sill as convenient. Fre- 

 quently the ends of the conduits are bent to point upwards and 

 cut to extend just a short distance above the finished floor. This 

 often necessitates a number of visible crossings of the leads where 

 the conduits cannot be run to the desired point. To obtain a 

 neater construction, a pull-box with cover can be provided in the 

 floor along the back of the board, and the conduits arranged so as 

 to terminate in the walls of the box. Provision is then made for 

 bringing the leads from this box to the desired point at the bottom 

 of the board, the necessary splices and crossings being made in the 

 box. 



High-tension Wiring. For circuits above 13,200 volts, bare 

 conductors are generally used because of the increased cost of 

 ordinary insulation for such high voltages, and because such con- 

 ductors are necessarily spaced far apart and generally located at a 

 considerable distance from the floor. They are, therefore, rigidly 

 mounted on insulators and carefully guarded. 



Size of Cables. (Current-carrying Capacity.) For the com- 

 paratively short runs encountered in power stations the size of 

 the conductors is generally governed by the permissible current- 

 carrying capacity and this, in turn, is determined within practical 

 limits by the maximum temperature which the insulation sur- 

 rounding it will withstand. First, the temperature must not be 

 high enough to cause too rapid a rate of deterioration of the insu- 

 lation. This temperature is, roughly, 85 C. for saturated paper, 



