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NOTES ON COPPER WIRE 463
Example.
A coil of 6 inches middle diameter is wound with No. 18
wire, how many ampere turns will it give when there are 1 10 volts
between its terminals?
Solution:
Six inches middle diameter will give 18.85 inches middle
circumference, = 1.57 feet, as the length of one turn.
The length of one pound of wire No. 18, is given in Table
No. 85, as 203.4 feet.
The resistance in one pound of wire of No. 18 is given in
Table No. 85 as 1.293 Ohms, but for this calculation it is
near enough to take it as 1.3 Ohms.
Inserting these values in the formula, we have:
no X 203.4
N = T . sy T – = 10962 ampere turns.
I -3 * I -57
The number of ampere turns in a given coil will, at constant
voltage, be about 1 % times larger for each number of larger wire.
Following formulas give the electrical resistance in lines
of copper wire :
1 0.8 XL X C _ IQ.8 XL X C
d =
I E= d
X E d X E
IS -r^ Q \S T L
10.8 XL " 10.8 X C
C =Current strength in ampere. E —Volts lost in the line.
L = Total length of copper wire in feet in the line (meas-
ured back and forth)
.
d = Diameter of copper wire in circular mils/
Example.
What size of copper wire is required to transmit 20 ampere
600 feet when the loss in voltage shall not exceed 5 volts. Note
the distance is 600 ft. ; the length of the wire is therefore 1200 ft.
Solution
:
10.8 X 1200 X 20 ....
a = = 51840 circular mils.
Looking in the table we find that No. 4 wire is 41743 circu-
lar mils, and No. 3 wire is 52634 circular mils ; therefore, if
the loss in the line shall not exceed 5 volts, we must use No. 3
wire.
Important : The size of copper wire used for electrical
wiring must never be smaller in diameter than what is required
by the underwriters’ regulations.
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