416 



TABLE 424.— AUXILIARY TABLE FOR COMPUTING WIRE RESISTANCES 



For computing resistances in ohms per meter from resistivity, p, in microhm-cm (see Table 

 386, etc.). e.g., to compute for No. 23 copper wire when p = 1.724: 1 m = 0.0387 + .0271 -f 

 .0008 -(- .0002 = 0.0668 ohms; for No. 11 lead wire when p = 20.4 : 1 m = 0.0479 + .0010 = 

 0.0489 ohms. The following relation allows computation for wires of other gage numbers: 

 resistance in ohms per m of No. n wire = 2 X resistance of wire No. (n — 3) within 1 percent: 

 e.g., resistance of m of No. 18 = 2 X No. 15. 



TABLE 425.— SAFE CURRENT-CARRYING CAPACITY OF COPPER WIRE, FOR 

 DIFFERENT CONDITIONS, IN AMPERES PER CONDUCTOR* 



Varnish cambric insulators 



Rubber insulators 

 in enclosed and exposed conduit 



Impregnated paper insulation 



Three con- 

 ductor cable 



in under- 

 ground duct 



Single 



conductor 



cable in air 



98 



173 

 234 

 352 



78 



134 

 177 

 264 



* These values are for voltages in the range up to 5,000 or 7,000 and for 75 to 100 percent time load, 

 ambient temperature 30°C and copper temperature 75-80°C. Adapted from Publication No. P-29-226 of 

 the Insulated Power and Cable Engineers' Association. For other values see these tables. 



SMITHSONIAN PHYSICAL TABLES 



