This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
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.
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?
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.
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.
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?