because the current can come from D through the

wiring of the building to B.

There would, of course,

A

B

STREET MAINS

C

D

vice-entrances of the

building, but it is bad practice to depend

upon building fuses to prevent street damage; for it allows excessive currents to

flow unnecessarily through the interior wiring. With such

FIG. 47.-Current shunted through a Build- an arrangement as ing round a Catch-box.

that shown in Figure

47, either all the lamps should be run from one set of mains, or else the wiring in the building should be divided into two distinct parts and one fed from each set of mains.

17. At the entrance of every building there shall be an approved switch placed in the service conductors by which the current may be entirely cut off.

[The switch required by this rule to be approved must be double-pole, must plainly indicate whether the current is "on" or "off," and must comply with Sections a, c, d, and e of Rule 26 relating to switches.]

This "service-switch" is placed at the point where the wires enter the building, for the same reasons that such a switch is used with high-potential circuits. (Page 90.) It is usually, however, placed just inside the building instead of outside. It is to be a double-pole switch, that is, one that will break both sides of the circuit as does that used in connection with the motor. (Fig. 24.) With such a switch, current may at any time be cut out of the building entirely, and if in mercantile buildings a practice can be made of turning it "off" every night, there is no question of electrical troubles at a time when no one is about. It is convenient too, when trouble occurs in any part of the installation, for it is not necessary then to wait for the arrival of some one having a knowledge of electrical matters, but the current may be immediately cut off from the interior wires. The base of the switch must be of waterproof and non-combustible material, such as slate or porcelain, to prevent leakage, and damage from arcing. The form may be like that shown in Figure 24 or it may be a double-pole switch similar to that shown in Figure 36.

18. Conductors:

a. Must have an approved insulating covering, and must not be of sizes smaller than No. 14 B. & S., No. 16 B. W. G., or No. 4 E. S. G., except that in conduit installed under

Rule 22, No. 16 B. & S., No. 18 B. W. G., or No. 4 E. S. G. may be used.

[Section a. In so-called "concealed" wiring, moulding and conduit work, and in places liable to be exposed to dampness, the insulating covering of the wire, to be ap proved, must be solid, at least three-sixty-fourths of an inch in thickness, and covered with a substantial braid. It must not readily carry fire, must show an insulating resistance of one megohm per mile after two weeks' submersion in water at 70 degrees Fahr., and three days' submersion in limewater, with a current of 550 volts, and after three minutes' electrification.

(See List of Wires, page 80.)

For work which is entirely exposed to view throughout the whole interior circuits, and not liable to be exposed to dampness, a wire with an insulating covering that will not support combustion, will resist abrasion, is at least one-sixteenth of an inch in thickness, and thoroughly impregnated with a moisture repellent, will be approved.]

It will be noticed that the insulating covering of wires used for low-potential wiring is to be the same as that required for high-potential wiring, except where the work "is entirely exposed to view throughout the whole interior circuits, and not liable to be exposed to dampness." For this excepted work the better grades of the wire termed "Weatherproof" meet the requirements. So-called "Underwriters'" wire is not allowed under any conditions.

No. 14 B. & S. (Brown & Sharpe Gauge), No. 16 B. W. G. (Birmingham Wire Gauge), and No. 4 E. S. G. (Edison Standard Gauge) are all of nearly the same size, about one-sixteenth of an inch in diameter. The B. & S. gauge is the gauge commonly used in this country. No wire smaller than No. 14 B. & S. gauge is allowed, except in conduit, because it is otherwise not strong enough mechanically. Small wires strung on ceilings may easily become stretched and thus not only be, by the displacement, more liable to further damage, but the sectional area may by this stretching be so reduced that the wire will heat when the normal current passes. There is also less probability that very small wires will be properly protected by the fuses.

b. Must be protected when passing through FLOORS; or through walls, partitions, timbers, etc., in places liable to be exposed to dampness by waterproof, non-combustible insulating-tubes, such as glass or porcelain.

Must be protected when passing through walls, partitions, timbers, etc., in places not liable to be exposed to dampness by approved insulating bushings specially made for the purpose.

[Section b. Second paragraph. Except for FLOORS, and for places liable to be exposed to dampness, Glass, Porcelain, metal-sheathed Interior Conduit and Vulca Tube, when made especially for bushings, will be approved. The two last named will not be approved if cut from the usual lengths of tube made for conduit work, nor when made without a head or flange on one end.]

c. Must be kept free from contact with gas, water, or other metallic piping, or any other conductors or conducting material which they may cross (except high-potential conductors) by some continuous and firmly fixed non-conductor creating a separation of at least one inch. Deviations from this rule may sometimes be allowed by special permission.

d. Must be so placed in crossing high-potential conductors that there shall be a space of at least one foot at all points between the high and low tension conductors.

e. Must be so placed in wet places that an air-space will be left between conductors and pipes in crossing, and the former must be run in such a way that they cannot come in contact with the pipe accidentally. Wires should be run over all pipes upon which condensed moisture is likely to gather, or which by leaking might cause trouble on a circuit.

f. Must be so spliced or joined as to be both mechanically and electrically secure without solder. They must then be soldered, to insure preservation, and covered with an insulation equal to that on the conductors.

[Section f. All joints must be soldered, even if made with the McIntyre or any other patent splicing device. This ruling applies to joint and splices in all classes of wiring covered by these rules.]

Rule 18 (b) is not always properly read. Non-combustible tubes are always required in floors. In walls, partitions, etc., they are required only in places liable to be exposed to moisture. (Pages 61, 89.) The only