520 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 



tus, etc., then each group should be separately fused after the 

 wattmeter, for then if one part of the line is cut out the rest can go 

 on drawing current. For example, if the arc lamp were misman- 

 aged it ought not to be possible to blow out the fuse for the house 

 lights, and the reverse. 



720. Location of fuse blocks. The general rule is that there 

 must be a fuse block wherever there is a change in the size of the 

 wire used. These fuse blocks must be in cabinets in plain sight and 

 readily accessible. Usually also, with every fuse block there is a 

 knife switch. 



721. Capacity of fuses. The rated capacity of fuses should 

 not exceed the allowable carrying capacity of the conducting wire 

 (see tables 694, 695), and circuit breakers should not be set more 

 than 30% above that allowable capacity. 



The allowable capacities for incandescent lamp lines are as 

 follows : 



55 volts or less 12 amperes 



55-125 volts 6 amperes 



1 25-250 volts 3 amperes 



For electric lighting each special circuit or line should not be used 

 for a current greater than will give a power of 660 watts. This 

 would mean for example, that if one wished to use 60 watt lamps 

 there could be only 1 1 of the lamps on a single line. If 40 watt 

 lamps were used then there might be as many as 16 lamps on a line, 

 etc. 



In using flat-irons and other heating devices on an electric lamp 

 circuit, care must be exercised not to turn on any lights on that 

 branch of the circuit. 



Likewise in using the small arc lamp for drawing with the micro- 

 scope, ultra-microscopy, etc., where from four to six amperes of 

 current are needed, one should not use incandescent lights on 

 that line at the same time, for the current would exceed the allow- 

 able amount and probably blow a fuse. 



722. Replacement of fuses. As fuses are liable to blow out 

 it is well to have a supply on hand, then the burnt out ones can be 



