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ELECTRICAL CONDUCTORS
63
(7) Varnished cloth-insulated wires, smaller than No. 6, may be used only by special permission.
(8) The Board of Fire Underwriters does not recognize anything less than number 18 wire. Nothing less than No. 14 may be used for interior circuit wires.
(9) Table No. 3 gives details of standard wire, not including what is known as asbestos-covered, used for connecting projector table switch with the lamp. (10) Table No. 4 may be used for calculating the resistance of any copper circuit the exact length of which is known.
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TABLE NO.
2
Varnished
Diameter of
Area in
Rubber
Cambric
Other"
No.
Solid Wires
Circular
Insulation,
Insulation,
Insulation
Gage
in Mils
Mils
Amperes
Amperes
Amperes
18
40.3
1,624
3
6
16
50.8
2,583
6
10
14
64.1
4,107
15 ■
18
2».
12
80.8
6,530
20
25
30
10
101.9
10,380
25
30
35
8
128.5
16,510
35
40
50
6
162.0
26,250
50
60
70
5
181.9
33,100
55
65
80
4
204.3
41,740
70
85
90
3
229.4
52,630
80
95
100
2
257.6
66,370
90
110
125
1
289.3
83,690
100
120
150
0
325.0
105,500
125
150
200
00
364.8
133,100
ISO
180
225
000
409.6
167,800
175
210
275
200,000
200
240
300
0000
460.0
211,600
225
270
325
Fur insulated aluminum allow 84 percent of Table No. 1 capacity rating.
(11) In using Table No. 4, suppose we find the projection room feed circuit to be fifty feet long. That would be a total of 100 feet of wire, since there are two wires in a circuit. The wires, measured with a B & S gauge, prove to be No. 5. Examining Table No. 4 we find No. 5 copper to have .3174 of an ohm resistance per each 1,000 feet of length, or .003173 of an ohm per foot. The circuit has 100 feet of wire, hence the total resistance of the whole circuit will be 100 X .0003174 = .03174 of an ohm.
Tables Nos. 2 and 3 give correct data up to capacity of the wires — in other words until the load becomes sufficiently heavy so that to add to it would raise the wire temperature above the approved maximum.
(12) Resistance for any volume of current up to the