Example

A group of 41 lamps, 16 candlepower, is to be set 1,500 feet from the dynamo on a 110-volt circuit. What diameter of conductor should be used when the drop is to be 5 volts ?

Solution

The number of lamps N = 41, the distance in feet F = 1,500, and the drop in volts E = 5. The lamps being of 16 candlepower, and the voltage 110, we have from formula 5,

Rf = 5/41 x 1,500 = .0000813 ohm per foot. = .0813 ohm per 1,000 feet.

From the wire table we find this corresponds to No. 00 B. & S. Ans.

62. When there are other lamps on the circuit than 16 candlepower, they should be allowed for in estimating the total number as follows:

For one 32 c. p. lamp count 2 of 16 c. p. For one 50 c. p. lamp count 3 of 16 c. p. For one 100 c. p. lamp count 6 of 16 c. p.

Example

An electrolier in a large hall is provided with one lamp of 100 candlepower, four of 50 candlepower, twelve of 32 candlepower, and twenty-four of 16 candlepower. What size of wire will be required for the main conductors from the closet, the distance being 300 feet, the line voltage 110, and the drop 4 volts?

Solution

The number of lamps is to be expressed in terms of 16-candlepower lamps.

1 lamp of 100 c. p.

=

6 of 16 c. p.

4 lamps of 50 c. p.

=

12 of 16 c. p.

12 lamps of 32 c. p.

=

24 of 16 c. p.

24 lamps of 16 c. p.

=

24 of 16 c. p.

Total,...

=

66 lamps = N.

The drop in volts E = 4, and the distance in feet F = 300. Then, by formula 5, the resistance per foot of the wire,

Rf = 4/66 x 300 = .000202 ohm.

= .202 ohm per 1,000 feet.

Reference to Table 1 shows this to be almost exactly No. 3 wire, which should therefore be chosen. Ans.

63. It is always necessary to compare the results obtained by these calculations with a table of carrying capacity, in order that undue heating of the conductors may be avoided. In the example just given, the current is 66 X.5 = 33 amperes, since each 16-candlepower lamp would take .5 ampere, and this is little more than one-half the allowable current for No. 3 wire, as shown in Table 1, so that this wire has a large margin of safety.

64. In calculating the wire for lamps on a 55-volt circuit, formula 5 is modified to read

Rf = E/2 N F'

(6.) since the current used per lamp is doubled. The 16-candlepower lamp is taken as the standard, as before, its current now being 1 ampere, instead of .5 ampere.

Example

A room is provided with a cluster of 24 lamps of 16 candle-power, and 3 of 32 candlepower. The E. M. F. of the circuit is 55 volts, and the line loss from the closet is to be 3 volts, the distance being 75 feet. What size of wire should be used?