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64
RICHARDSON'S BLUEBOOK OF PROJECTION
rated capacity of a wire is, for all practical purposes, the same — the rise in temperature is too slight to be a factor.
Mil Foot Standard
(13) Calculation of resistance in electrical conductors is based upon what is known as the "mil foot," (14) which is the resistance of a wire of the same characteristics as the one to be calculated, and which is one mil (1/1000 of an inch) in diameter and one foot long.
TABLE NO. 3 STANDARDIZED STRANDING
Strands
■s
1
7/25
7/ 32
7/40
7/ 51
7/ 64
7/ 81
7/ 91
7/102
19/ 64
19/ 72
19/ 81
19/ 91
19/107
19/114
37/ 91
37/ 97
37/102
37/116
61/102
61/107
61/114
61/121
61/128
91/114
91/128
127/114
127/128
Cable
Allowable Carrying Capacities in Amperes
6 OS
o
Outside
Dia. over
Copper
< S
»— i
Table B Varnished
Cloth Insulation
22
4,490
.075
15
18
20
7,150
.096
20
25
18
11,370
.120
25
30
16
18,080
.153
35
40
14
28,740
.192
50
60
12
45,710
.253
70
85
11
58,000
.273
80
95
10
72,680
.306
90
110
14
78,030
.320
100
120
13
98,380
.360
125
150
12
124,900
.405
150
180
11
157.300
.455
175
210
*
217,500
.540
225
270
9
248,700
.570
250
300
11
306,400
.637
275
330
*
347,500
.679
300
360
10
381,200
.714
325
390
*
484,300
.798
400
480
10
633,300
.918
475
565
*
698,000
.963
500
600
9
788,500
1.030
550
660
*
893,100
1.090
600
720
8
1,007,000
1.150
650
780
9
1,191,000
1.250
725
870
8
1,502,000
1.410
850
1020
9
1,660,000
1.480
900
1100
8
2,097,000
1.660
1100
1300
4, JJ'-S
3-5-2
20 25 35 50
70 90 110 130 150 175 210 250 325 350 400 450 500 600 700 750 825 900 1000 1125 1350 1460 1700
*These individual S. Wire Tables.
strands are odd sizes not listed in the American or B.
(15) The resistance of one mil foot wire made of commercial copper is 10.5 ohms at normal temperature of 75 degrees Fahrenheit.
Resistance of Copper Circuits
(16) In applying the mil foot standard we will assume a circuit to be of No. 6 wire and that it is 200 feet in length, having 400 feet of wire. If one foot of