For a large building, separate circuits may be run to the top floor up two or more vertical corners. One of these circuits is shown in Pig. 52. The terminal board of the dynamo is shown at s, corresponding to the junction box in a public supply system, and a b c represents one set of feeders, which is connected to central points on the subfeeders e f, these again being connected to the mains m n at such points as will insure the best distribution of current. Tap circuits are provided for the lamps on the several floors, cut-outs being placed at all branches.

In calculating the size of the feeders, the proportionate number of lamps for each one is considered as determining the current it carries; for instance, if there are two feeders at opposite corners, each may be regarded as supplying-cur-rent to one-half the lamps on each floor, although the lamps may be connected to a common set of mains.

For the three-wire system, three wires for each set of feeders or mains are carried throughout, except in the small tap circuits.

74 Wiring In Large Buildings 502

Fig. 52.

Examples For Practice

1. A group of thirty-three 110-volt 16-candlepower lamps is located 325 feet from the closet. What size feeder will be required, a drop of 2 1/4- volts being allowed ? Ans. No. 8 B. & S.

2. A 6-light electrolier is set up at a distance of 35 feet from the mains. There are three lamps of 16 candlepower, two of 32 candle-power, and one of 50 candlepower, the circuit being 55 volts. With a drop of 1 1/2 volts, what size wire should be used ? Ans. No. 13 B. & S.

3. What would be the drop in the circuit mentioned above, if No. 12 wire were substituted for No. 13 ? Ans. 1.16 volts.

4. A hall 30 feet wide by 95 feet long is provided with seventy-five 16-candlepower 110-volt lamps, the wiring being arranged in the form of a loop, and the lamps mounted on the walls. Allowing a drop of 2 volts, what size wire should be used? Ans. No. 3 B. & S.

5. What size conductor will be required to carry the current for four hundred and fifty-two 110-volt 16-candlepower lamps at a distance of 374 feet, the allowable drop being 2 per cent?

Ans. 815,052 circ. mils area.

6. On a three-wire system there are to be installed sixty-two 110-volt 16-candlepower lamps at a distance of 235 feet from the closet, a drop of 1 1/2 volts being allowed. What size conductor will be required ?

Ans. No. 3 B. & S.

Testing

75. When wiring is being installed, it should be frequently tested, to insure the discovery and correction of faults in the insulation, while they may be easily repaired.

The method employed is to free one end of the conductor from contact with other objects, and connect the other end to one terminal of a testing instrument, the second terminal being "earthed" or "grounded"through a gas or water pipe. The resistance of such a circuit will necessarily be very high, preventing the passage of any appreciable current; but, if there be a fault at any point, a current of larger volume will flow through.

Testing 503

Fig. 53.

A convenient form of testing instrument is the magneto shown in Fig. 53. The terminals, or binding posts, to which connections are made as already described, are on the upper side at t , t'. A crank handle on the outside of the case turns the pinion p through the gear-wheel g, thereby revolving the armature of a small dynamo. If there is a fault on the line and connections are made as directed, the bell will ring on turning the crank. Care should be taken to ascertain that the gas or water pipes used for the connections are actually in communication with the street mains. The wiring will be passed by the insurance inspectors if a magneto that will ring through a resistance of 20,000 ohms shows no grounding of the circuit.