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NOTES ON COPPER WIRE. 467
from 5 to 10 horse power may have 400 to 500 circular mils for
each ampere of current that they convey. In larger and more
expensive dynamos there may be over 600 circular mils in the
armature winding for each ampere carried.
In field coils and similar work we often find as much as 1200
circular mils for each ampere conveyed.
In lines about 400 to 500 circular mils are used per ampere
of current when the wire is small as No. 10, but the larger the
wire the more circular mils are required per ampere. (See
Underwriters’ Rules )
Table No. 87 gives the carrying capacity of round copper
wire according to the underwriters’ rule as the standard adopted
for interior wiring.*
TABLE No. 87.—Carrying Capacity of Copper Wire.
Size Amperes Amperes. 1
Size Amperes. Amperes.
B. & S.Gage. B.& S.Gage.
18 3 5 4 65 92
16 6 8 3 76 110
14 12 16 2 90 131
12 17 23 1 107 156
10 24 32 127 185
8 33 46 00 150 220
6 46 65 000 177 262
5 54 77 0000 210 312
The lower limit is specified for rubber-covered wires to
prevent gradual deterioration of the high insulations by the heat
of the wires, but not from fear of igniting the insulation. The
question of drop is not taken into consideration in this table, but
this may be calculated by the formulas given on page 463.
The carrying capacity of the No. 16 and No. 18 wire is
given in the table, but smaller wire than No. 14 is not allowed to
be used in interior wiring.
Example.
What is the smallest size copper wire allowed for a line
supplying current for 36 incandescent lamps of 16-candle power
at 110 volts ?
Solution
:
A 16-candle power lamp at 110 volts will use about one-half
of one ampere of current ; 36 lamps will, therefore, take about
18 amperes, and if we are to follow the lower limit No. 12 is
too small as that is only good for 17 amperes, but No. 10 will be
of ample size.
* For instructions regarding wiring, see " Rules and Requirements of the
National Board ot Five Underwriters for the Installation of Electrical Wir-
ing," which may be obtained from any local electrical inspection bureau.
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