STATION WIRING 629 



acids, or other corrosive elements; thus, water, gas and stray cur- 

 rents cannot reach the cable protected by this material. It is a 

 good non-conductor, doing away entirely with the trouble with 

 stray currents, and it is also an absolute prevention against 

 electrolysis, which destroys many cables, gas and water pipes 

 during each year. 



Control and instrument wiring and field and exciter circuits 

 are invariably run in iron conduit; first, because they are so 

 numerou3 and their directions varied, and second, because of 

 their small size they require protection against mechanical injury. 

 The cheapest and least conspicuous place cf installment is in the 

 concrete floors. 



The practice of choosing a conduit having an inside diameter 

 at least 30 per cent greater than the outside diameter of the cable 

 will give good results, and Table LI II also gives the size of con- 

 duit recommended for different sizes of conductors. All con- 

 ductors of cables for duct service should be stranded to facilitate 

 installation. 



In laying out a conduit job, first ascertain the size and number 

 of wires required, then take the sizes of conduit from Table LIII. 

 One-half inch is usually used for branch conduits and is the small- 

 est size permitted by the National Electric Code. In running 

 several conduits together, a pull-box will be found more economical 

 than elbows for making turns, as one pull-box will take the place 

 of several elbows. Do not pull wires through conduits with a 

 block and tackle, as 'it will not only injure the insulation, but 

 wedge the wires in such shape that they cannot be removed readily 

 if desired. Be careful to ream out the end when conduit is cut, 

 as the bur may otherwise cut through the insulation. Conduits 

 should be securely fastened to walls and ceiling by use of pipe 

 straps or hooks. Plug all exposed ends of conduit in new buildings 

 to prevent plaster and dirt from falling into it. 



Single vs. Multiple Conductors. Low-voltage cables for 

 direct-current service, such as exciter and field leads, are as a rule 

 of the single-conductor type. This, however, does not refer to 

 control and instrument wiring for which multi-conductors with as 

 many as a dozen conductors are used. These are as a rule of 

 different-colored braids so as to facilitate identification during 

 installation. 



Whether single- or multiple-conductor cables should be used for 



