40 



PRINCIPLES OF ELECTRICAL DESIGN 



is wound. If the number of turns per layer is very small, there 

 will be an appreciable loss of space due to the turning back of the 

 wire at the end of each layer. 



Insulation on Spooh or Metal Forms. The materials used for 

 insulating between the winding as a whole and any grounded 

 metal by which it is supported include mica, micanite paper and 

 cloth, pressboard, "presspahn," varnished cambric, oiled linen 



24 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 

 Siae of Wire, B & S Gauge 



FIG. 15. Space factors for wires of circular cross-section. 



or cotton (empire cloth), cotton tape, etc. The voltage that 

 some of these materials will withstand before breakdown is 

 approximately as follows: empire cloth (usually 7 to 8 mils thick) 

 will rarely puncture with less than 600 volts per mil; mica will 

 withstand about 800 volts per mil; and micanite paper or cloth 

 which affords also an excellent mechanical protection can 

 generally be relied on to withstand 400 volts per mil. A large 

 factor of safety is usually allowed, especially on the lower 

 voltages. With a good quality of insulation, the total thickness 

 between the cotton-covered wires and the supporting metal work 

 should have the following values: 



