Figs. 839 and 840.—Wrong and right methods of loop wiring. In general, when a large percentage of loss is allowed with lamps at short distances, the size of wire, calculated simply in accordance with the resistance rules, will be found too small to carry the current safely. This fact is often overlooked, and even though wires may have been correctly calculated for a uniform percentage of loss, they will become painfully hot simply because the table of carrying capacity was not consulted. The cross connection of mains wherever possible, for the purpose of equalizing the pressure, will also often reduce the heating effects of the current. An example of this is shown in the above figures. A circle of lights was wired as in fig. 839, and after the current had been turned on, the wires of the circle became hot, and there was quite a perceptible difference of candle power between the lights near A and those near B. Investigation disclosed the fact that the loop, contrary to instructions, had been left open. A few inches of wire as shown in dotted lines remedied the fault. A better arrangement, however, is shown in fig. 840.
EXAMPLE.—What is the proper size of wire for a 10 H.P. motor, run at 220 volts, allowable drop 2 per cent. on 200 foot circuit.
Substituting the given values in the formula on page 758:
Circular mils = 10 × 746 × 200 × 21.6 220 × 4.4 × .9 = 36,991.
The nearest larger value to this result, in the table of carrying capacities of copper wire (page 731), is 41,740, corresponding to No. 4 wire, B. & S. gauge.
In all cases the size of the wire thus formed should be compared with that allowed by the underwriters for full load current of motor, plus 25 per cent. of that current, and if the size calculated happen to be smaller than the allowable size, it should be increased to the latter, otherwise it will not pass inspection.
| H. P. | Per Cent. Eff. | Watts Input | 50 Volts | 100 Volts | 220 Volts | 500 Volts |
|---|---|---|---|---|---|---|
| ¾ | 70 | 800 | 16 | 7 | 4 | 2 |
| 1½ | 70 | 1600 | 32 | 15 | 7 | 3 |
| 3 | 75 | 2980 | 60 | 27 | 14 | 6 |
| 5 | 80 | 4660 | 93 | 42 | 21 | 9 |
| 7½ | 85 | 6580 | 132 | 60 | 30 | 13 |
| 10 | 85 | 8780 | 176 | 80 | 40 | 18 |
| 15 | 85 | 13200 | 264 | 120 | 60 | 26 |
| 20 | 85 | 17600 | 352 | 160 | 80 | 35 |
| 25 | 85 | 21900 | 438 | 199 | 100 | 44 |
| 30 | 90 | 24900 | 498 | 226 | 113 | 50 |
| 40 | 90 | 33200 | 664 | 301 | 151 | 66 |
| 50 | 90 | 41400 | 828 | 376 | 188 | 83 |
| 60 | 90 | 49700 | 994 | 452 | 226 | 99 |
| 70 | 90 | 58000 | 1160 | 527 | 264 | 116 |
| 80 | 90 | 66300 | 1330 | 608 | 302 | 133 |
| 90 | 90 | 74600 | 1490 | 678 | 339 | 149 |
| 100 | 90 | 82900 | 1660 | 755 | 377 | 166 |
| 120 | 90 | 99500 | 1990 | 905 | 453 | 199 |
| 150 | 90 | 124000 | 2480 | 1130 | 564 | 248 |
| Appx. | Appx. | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| K.W. | 125 Vs. | 250 Vs. | 500 Vs. | H.P. | K.W. | 125 Vs. | 250 Vs. | 500 Vs. | H.P. |
| 1. | 8 | 4 | 2 | 1.3 | 30. | 240 | 120 | 60 | 40. |
| 2. | 16 | 8 | 4 | 2.7 | 37.5 | 300 | 150 | 75 | 50. |
| 3. | 24 | 12 | 6 | 4.0 | 40. | 320 | 160 | 80 | 53. |
| 5. | 40 | 20 | 10 | 6.7 | 50. | 400 | 200 | 100 | 67. |
| 7.5 | 60 | 30 | 15 | 10. | 60. | 480 | 240 | 120 | 80. |
| 10. | 80 | 40 | 20 | 13. | 75. | 600 | 300 | 150 | 100. |
| 12.5 | 100 | 50 | 25 | 17. | 100. | 800 | 400 | 200 | 134. |
| 15. | 120 | 60 | 30 | 20. | 125. | 1000 | 500 | 250 | 167. |
| 20. | 160 | 80 | 40 | 27. | 150. | 1200 | 600 | 300 | 201. |
| 25. | 200 | 100 | 50 | 34. | 200. | 1600 | 800 | 400 | 268. |
To determine the current required for a motor, as for instance, the 10 H. P. motor under consideration, multiply the horsepower by 746, and divide the product by the voltage of the motor multiplied by its efficiency as follows: (10 × 746) ÷ (220 × .90) = 37.6 amperes.
This value increased by 25 per cent. of itself (37.6 × .25 = 9.4 amp.) is equal to 37.6 plus 9.4 = 47 amperes. In the table of carrying capacities of copper wire (page 731), 46 amperes is given as the allowable carrying capacity of No. 6, B. & S. gauge, rubber covered wire; therefore No. 5 wire must be used.
Calculations for Three Wire Circuit.—In all cases of interior conduit work, and in most cases of inside open work, the main feeders from a three wire source of supply are installed on the three wire plan, and the sub-feeders and distributing mains, on the two wire plan, except where the application of the method necessitates the use of unwieldy sizes.
In laying out sub-feeders and mains, the total load, under normal operating conditions, should be divided as nearly as possible into two equal parts, and one part connected on each side of the neutral part of the entrance cut out, or the neutral bus bar of the switch board or panel board in an isolated plant, thus making the load on each side of the neutral wire of the feeder as near equal as possible.
Fig. 841 shows a three wire panel board with connection for 12 mains; those shown in solid lines as A, B, C, etc., being connected between the neutral wire and the negative wire of the feeder, and those shown by dotted lines as G, H, I, etc., being connected between the neutral wire and the positive wire of the feeder. The total load consists of ninety-one 16 candle power lamps, which are so distributed that the positive wire of the feeder carries the current for 46 lamps, and the negative wire, 45 lamps, the neutral wire carrying the difference or current for 1 lamp.
The proper size of wire for the mains may be calculated as already explained, but in calculating the outer wires of the three wire feeder, the neutral wire should be disregarded and the outer wires connected as a two wire circuit carrying the total load of 91 lamps at the over all pressure of 220 volts.
EXAMPLE.—Ninety-one 16 candle power lamps consuming 3.1 watts per candle power at a pressure of 110 volts, will require a current of
16 × 3.1 × 91 110 = 41 amperes.
The distance from the entrance cut out to the main or feeder switch is 200 feet, then for a 2 per cent. drop, or a loss of 110×.02=2.2 volts, the cross sectional area of the wire will be,
41 amperes × 200 feet × 21.6 2.2 volts = 80,509 circular mils.
Fig. 841.—Three wire circuit panel board with connections for 12 mains. The wires shown in solid lines as A, B, C, etc., are connected between the neutral wire and the negative wire of the feeder, and those shown by dotted lines, as G, H, I, etc., are connected between the neutral wire and the positive wire of the feeder.
The joint resistance of the lamps on a three wire system, however, would be four times greater than on a two wire system; consequently the resistance of the outer wires of the feeder in this case will be four times greater for the same percentage of loss, and the cross sectional area of each of the outer wires will be, 80,509÷4=20,127 circular mils. According to the underwriters' rules, this value compels the use of No. 6 B. & S. gauge wire.
If the lamp voltage, 110 volts, be used, the two wire formula (5) given on page 748 must be modified to,
circular mils = amperes × feet × 21.6 drop × 4
but if an over all voltage, (that is, the voltage between the outer wires), of 220 volts be used, the two wire formula does not require any modification.
The proper size of wire may also be calculated by means of the formula
drop 2 × distance × amperes = resistance per foot . . . . (1)
Example.—If in calculating a three wire feeder, the over all voltage be 220 volts, the drop = 4.4 volts, twice the distance = 400 feet, and the current = 20.5 amperes, then,
4.4 volts 400 feet × 20.5 amperes = .0005365 ohms per foot.
In the table of the properties of copper wire which gives the resistance of various sizes of wire, it will be noted that at all of the given temperatures No. 8 wire has a resistance greater than the value just calculated, therefore, No. 6 B. & S. gauge wire should be used for the outer wires of the feeder. In the table the resistance is given per 1,000 feet, hence it is only necessary to move the decimal point to obtain the resistance per foot.
If the calculation be based on the lamp voltage, 110 volts, the formula (1) must be modified to
drop × 4 2 × distance × amperes = resistance . . . . (2)
In this case, drop = 2.2 volts, 2 × distance = 400 feet, and current = 41 amperes, then,
2.2 volts × 4 400 feet × 41 amp. = 8.8 16,400 = .005364 ohms.
Size of the Neutral Wire.—In three wire circuits, the size of the neutral wire will depend to a great extent upon operating conditions. In the case of installations which occasionally have to be worked as two wire systems, the cross section of the neutral wire should be equal to the combined cross section of the two outer wires.
For interior wiring which must pass inspection, the neutral wire must always be twice the size of one of the outside wires. However, for general distribution, if it be reasonably sure that the system will always be worked three wire and that the drop in the two outer wires does not exceed 1½ per cent., the cross section of the neutral wire may be made smaller than that of one of the outer wires. In such a case the size of the neutral wire may be calculated for a maximum unbalancing of 25 per cent., when the current in one of the outer wires is 75 per cent. of the current in the other outer wire.
For instance, suppose that in a balanced system, the total load on each of the outer wires of a feeder be 211 amperes, and that on account of certain operating conditions, this load has to be divided unequally so as to put 242 amperes on one of the outer wires, and 181 amperes on the other outer wire. In this case the neutral wire will carry 60 amperes, or 25 per cent. of the current carried by the heavier outer wire.
If the drop in the outer wires exceed 1½ per cent., the cross section of the neutral wire will have to be equal to or larger than that of each of the outer wires, otherwise the drop in the neutral wire will exceed ½ volt with an unbalancing of 25 per cent.
The term wiring is commonly understood to mean the methods employed in laying the conductors used for the transmission and distribution of electrical energy for lighting, power, and other purposes. Interior wiring, comprises the various methods of installing the conductors from the entrance devices in the walls or other parts of the buildings to the lamps, motors, and other electrical apparatus within the buildings.
The different methods of interior wiring may be conveniently grouped into the following general classes:
1. Open or exposed wiring;
2. Wires run in mouldings;
3. Concealed knob and tube wiring;
4. Rigid conduit wiring;
5. Flexible conduit wiring;
6. Armoured cable wiring.
Open or Exposed Wiring.—This method of wiring possesses the advantages of being cheap, durable and accessible. It is used a great deal in factories, mills and buildings where the unsightly appearance of the wires exposed on the walls or ceilings is of no consequence.
Ques. What kinds of wires are suitable for this method of wiring?
Ans. Either rubber covered or slow burning weather proof wire.
Rubber insulation should always be used where the wire is in a damp place, such as a cellar, and either weather proof or rubber insulation may be used to protect it against corrosive vapors.
Figs. 842 to 844.—Open or exposed wiring. Fig. 842, wires passing through beams. The holes are bored at an angle and wire run through in zig-zag course. Porcelain tubes are used where the wire passes through beams; fig. 843, cleat work across beams, the cleats are carried by boards attached to the beams; fig. 844, method of carrying wires on cleats around beams.
Ques. How are the wires installed?
Ans. They are laid on some cornice, wainscoting, beam, or other architectural feature suitable for the purpose, by means of porcelain knobs and cleats, as shown in figs. 842 to 844.
Ques. What are the disadvantages of open wiring?
Ans. The wiring is not sufficiently protected from moisture, and the effects of fire which will destroy the insulation of the wires; it is also liable to mechanical injury.
Figs. 845 to 847.—Splicing. Figs. 845 and 846, making a wire splice, and the twist completed; fig. 847, a wrapped joint on large wire. Splicing of wires or joining a branch to a main wire should always be made by twisting the wires together or twisting one wire around the other, so that the joint will be mechanically strong enough to carry all strain which may come upon it, without any soldering. The joint should then be carefully tinned and soldered in order to give good electrical contact and to avoid corrosion along the contact surface. Where wires are too large to be twisted together, the ends are given a short bend and the two wires wrapped firmly together with a smaller bare copper wire, after which the joint is thoroughly tinned and soldered, preferably by pouring hot solder over the joint. The joint is then insulated by wrapping it with two layers of pure rubber, and three layers of tape, sufficient to make the insulation thickness equal to that of the wire, after which the whole joint should be painted with water proof paint.
Ques. How far apart should the wires be placed?
Ans. When installed in dry places and for pressures below 300 volts, the insulators should separate the wires 2½ inches from each other and ½ inch from the surface along which they pass. For voltages from 300 to 500, the wires should be separated four inches from each other and one inch from the surface along which they pass.
If the wiring be in a damp place, the wires should be at least one inch from the surface.
Figs. 848 to 850.—Crossing of wires. Where wires cross each other, tubes should be used except in case of large stiff wires as in fig. 848; here one wire may be bent down and carried under the other; fig. 849, short bushing strung on the wire—this method is usually unsatisfactory, especially where a large number of wires cross each other; fig. 850, wires crossing each other through tubes. Flexible tubing, such as circular loom may be used in crossing wires in dry locations. Insulators should always be provided where wires cross to support the wires, thus preventing the upper wires sagging and touching those below.
Figs. 851 to 853.—Methods of wiring across pipes. The wires should preferably run over rather than under the pipes. Fig. 851 shows crossing with circular loom, and fig. 852, one in which a tube is used. Both of these methods are satisfactory in the case of gas pipes, but for steam pipes or water pipes which are liable to leak or sweat and drip moisture, the crossing should be above as shown in fig. 853. On side walls where vertical wires run across horizontal water pipes, the latter should be enclosed and the moisture deflected to one side.
Ques. How should wires be protected when run vertically on walls?
Ans. They should be boxed in or run in a pipe as shown in fig. 854, the covering extending 6 feet above the floor.
When placed inside a box there should be a clearance of at least one inch around the wires; the box should be closed at the end as shown, and the wires protected where they enter the top with bushings. When the wires are placed in a pipe they should be first encased in a piece of flexible tubing that will extend from the insulator below the end of the pipe to the first one above it.
Fig. 854.—Methods employed in open wiring when run vertically on walls. Either a box casing or iron pipe should be used to protect the wires. The covering need only extend six feet above the floor.
Ques. What kind of incandescent lamp receptacle or wall socket is best adapted to exposed wiring?
Ans. One which does not have exposed contact ears, an approved form being shown in fig. 859.
Practical Points Relating to Exposed Wiring.—Some of the principal points which should be remembered in this connection, together with the methods which may be applied to special cases, may be briefly stated as follows:
1. In interior wiring no wires smaller than No. 14 B. & S. gauge should be used, except as allowed by the underwriters, and no more than 660 watts should be allowed to a circuit.
2. Tie wires should have an insulation equal to that of the conductors which they secure.
3. In all cases, whether the wires be run on knobs, split insulators, or cleats, the wires should be supported at intervals of at least 4½ feet, and if exposed to mechanical injury, the supporters should be placed at closer intervals.
4. Wires run on bare ceilings of low basements, especially where they are liable to injury, should be protected by two wooden guard strips as shown in fig. 858. The protective strips should be at least ⅞ inch in thickness and slightly higher than the knobs, insulators, or cleats. The two circuit wires should not be run closer than 6 inches apart, and wires run near water tanks must be rubber covered so as to render them moisture proof.
5. Cleats should be used for the wiring of stores, offices, or buildings having flat ceilings, provided the wiring is installed in dry locations.
6. When the installation is exposed to dampness or acid fumes such as those developed in stables, bakeries, etc., the wires should run on knobs or split insulators, and should be rubber covered.
Figs. 855 to 857.—Methods of carrying wires through floors. In passing through floors (or walls) the wires often come in contact with concealed pipes or other grounded material, hence the only way they can be properly protected is by making the bushing continuous. This may consist of continuous porcelain tubes as shown in fig. 855, or short bushings may be arranged in iron pipes as in fig. 856. The method followed in case of an offset in the wall is shown in fig. 857. Sometimes the floor can be taken up and an iron conduit, properly bent, put in place, the wires being reinforced with flexible tubing. Another method is to attach the wires to insulators; in this case the floor must not be put down until the wiring has been examined by the inspector.
7. When wires are run at right angles to beams which are more than 4½ feet apart, a running board should be used and the wires cleated to it as shown in fig. 843. It is desirable, however, to avoid the use of running boards, whenever possible by running the wires parallel with the beams, thus reducing the cost of insulation.
8. In factories or other buildings of open mill construction, mains of No. 8 B. & S. gauge or larger wire, where they are not exposed to injury, may be placed about 6 inches apart and run from timber to timber, not breaking around, and may be supported at each timber only.
Fig. 858.—Method of protecting exposed wiring on low ceilings by two guard strips.
Fig. 859.—Receptacle suitable for use with open wiring, the requirement being that the contact ears should not be exposed.
9. The best location for feeders is on the walls. In dry buildings the fire and weather proof wire can be used with safety; but covered wire must be used on buildings subject to any form of dampness. In all cases where feeders are run on the walls, they should be protected from mechanical injury by boxings at least 6 feet high on each floor. If floor switches be used, they may be mounted on the front of the boxing. In such cases, the holes in the boxing through which the wires pass to the switches should be provided with porcelain bushings.
10. The rosettes, receptacles, sockets, snap switches, etc., used in connection with exposed wiring should conform in all respects to the standards specified by the underwriters.
Figs. 860 to 866.—Various porcelain knobs and cleats. In open work various forms of these devices are used.
Fig. 867.—Porcelain tube for entrance of wire into a building. There must be a drip loop outside to drain off water, and the hole through which the conductor passes must be bushed with a non-combustible, non-absorptive insulating tube slanting downward toward the outside. The object of the inclination is to allow any water that might enter the tube to gravitate to the drip loop.
Fig. 868.—Interior bushing. Wires must be separated from contact with walls, floors, timbers or partitions through which they may pass by non-combustible, non-absorptive, insulating tubes, such as glass or porcelain, except at outlets where approved flexible tubing is required. Bushings must be long enough to bush the entire length of the hole in one continuous piece, or else the hole must first be bushed by a continuous water proof tube. This tube may be a conductor, such as iron pipe, but in that case an insulating bushing must be pushed into each end of it, extending far enough to keep the wire out of contact with the pipe.
Wires Run in Mouldings.—Wooden mouldings are extensively used in connection with the wiring of stores, factories and buildings. The advantages of this type of construction are: simplicity, cheapness, and accessibility, and when the moulding is run straight and accurately mitred it makes a neat job. Any class of wooden moulding wiring, however, is not sufficiently impervious to moisture to render it suitable for use in damp places, and it is liable to be crushed or punctured. Furthermore, it is naturally very combustible. These difficulties are overcome to a certain extent by impregnating the moulding with some kind of moisture repellant, or by coating it both inside and out with water proof paint. Hardwood moulding should be used wherever possible, but soft wood moulding usually conforms much better to the wall line.
Fig. 869.—Standard wooden moulding for encasing wires. Wooden moulding must not be used in concealed or damp places, nor be placed directly against a brick wall where sweating may introduce moisture that may ultimately cause a short circuit. Wooden moulding for concealing electrical conductors is prohibited by ordinances in some cities.
Ques. For what conditions is wiring in mouldings suitable?
Ans. It is adapted to installations in which the wires have to be laid after the completion of the buildings.
Ques. Describe the moulding usually employed.
Ans. It is made of hardwood in two pieces, a backing and cap, so constructed as to thoroughly encase the wire.
It should provide a one-half inch tongue between the conductors and a solid backing which should not be less than three-eighths of an inch in thickness under the grooves; it should be able to give suitable protection from abrasion.
The inside of the moulding and the cap must have at least two coats of waterproof material, or else the whole moulding must be impregnated with moisture repellant.
Only one conductor is placed in a groove.
The backing is secured to the walls or ceilings by means of wire nails. The wires are then laid in the grooves and the capping put in place and fastened by small brads.
The wires should be continuous, and only rubber covered wire should be employed.
Wooden moulding is made in a great variety of size and design. The general appearance of this type of moulding being shown in fig. 869.
Ques. What other kind of moulding is used?
Figs. 870 to 872.—Metal moulding. An approved form consists, as shown, of two pieces: base (fig. 870), and cap (fig. 871), so formed as to snap together, the cap snapping over the base as in fig. 872. The entire moulding should be galvanized or coated with a rust preventive. When the base is held in place by screws or bolts from the inside surface, depressions must be provided so that the heads of the screws will be flush with the surface of the moulding.
Ans. Metal moulding, as shown in figs. 870 to 872.
Metal moulding is permitted on circuits requiring not more than 660 watts and where the pressure is not over 300 volts. Special fittings must be used with this class of moulding so that it is continuous both mechanically and electrically. The moulding should be grounded. The installation rules are practically the same as those governing conduit work.
Ques. What is a kick box?
Fig. 873.—"Kick box;" a device used to protect wires encased in porcelain tubes where they pass through floors.
Ans. A fitting, as shown in fig. 873, for protecting wires at the points where they enter or emerge from the floor.
Ques. How is moulding work installed on brick or plaster walls which are liable to dampness?
Ans. A backing board must be put on before the moulding is used.
Ques. How should moulding be placed on a ceiling with respect to appearance?
Ans. The appearance is improved if the moulding be carried through to the side of the room, even if part of it be not used. This will give a neat and finished appearance to the ceiling as shown in fig. 874.
Moulding should always be run in as inconspicuous position as possible, and if it be necessary to run it on the open ceiling, it should be arranged so as to form regular panels. Often it can be run so as to take the place of a picture moulding or as a part of the baseboard so that it becomes merely a part of the wooden trim of the building; and in certain cases it should be made of material to match the rest of the trim.
Fig. 874.—Treatment of moulding work on ceilings. All installations should be planned out so as to conform to symmetrical designs, as far as practicable with the proper distribution of the lights, etc., and all runs finished off, whenever necessary, by "dead" mouldings continued to the walls to improve the appearance. In the figure the sectioned portions show the location of the dead moulding. Sometimes, especially in the wiring of private houses the use of special moulding is necessary. In such cases the shape and kind of wood should match that of the finish or trim of the room, and the receptacles should be stained to match the moulding. When the moulding is run along the walls, the capping may be made to match the trim or the picture moulding already in place, thus giving an apparently concealed job. In this kind of work the feeders can be run through the spaces between the walls, and if flexible tubes such as circular loom or flexiduct be used, no splice box will be necessary where the system of wiring changes and single braided rubber wire may be used throughout.
Ques. What is the usual character of moulding work?
Ans. Usually, a certain part of the work will be run as concealed, that is, inside the partitions, the wires being "fished" from the moulding to the outlet.
Practical Points Relating to Wiring in Mouldings.—The following practical points will be found useful in the satisfactory execution of any class of wiring with wooden moulding:
1. Wooden moulding should never be concealed, and should not be used in damp places or in buildings subject to acid fumes, such as ice houses, breweries, or stables, etc.
2. Wooden moulding selected for use should be formed of good straight stock and free from knots, knot holes and other imperfections. The saving effected in the lower cost of second hand moulding does not compensate for the additional cost increase in its working.
3. When wooden moulding is used in connection with solid pipe or flexible tube conduit, an iron box or conduitlet must be installed where the system of wiring changes, as shown in fig. 875. The pipe conduit is secured to the box by means of lock nuts, with porcelain bushings or flexible tubes protecting the wires. In all cases the loom should run up to the moulding.
Fig. 875.—Method of tapping outlets for feeder circuits when wiring with wooden moulding.
Arc Light Wiring.—All wiring for high voltage arc lighting circuits should be done with rubber covered wire. The wires should be arranged to enter and leave the building through an approved double contact service switch which should close the main circuit and disconnect the wires in the building when turned "off" and be so constructed that they will be automatic in their action, not stopping between points when started and to prevent arcing between points under any circumstances, and should indicate plainly whether the current is "on" or "off." Never use snap switches for arc lighting circuits. All arc light wiring of this class should be in plain sight and never enclosed, except when required, and should be supported on porcelain or glass insulators which separate the wires at least one inch from the surface wired over. The wires should be kept rigidly at least eight inches apart, except, of course, within the lamp, hanger board or cut out box or switch. On side walls, the wiring should be protected from mechanical injury by a substantial boxing, retaining an air space of one inch around the conductors, closed at the top (the wires passing through bushed holes) and extending not less than seven feet from the floor. When crossing floor timbers in cellars or in rooms, where they are liable to be injured, wires should be attached by their insulating supports to the under side of a wooden strip not less than one-half an inch in thickness.
Arc Lamps on Low Pressure Service.—For this service there should be a cut out for each lamp or series of lamps. The branch conductors for such lamps should have a carrying capacity about 50 per cent. in excess of the normal current required by the lamp or lamps to provide for the extra current required when the lamps are started or should a carbon become stuck without over fusing the wires. If any resistance coils be necessary for adjustment or regulation, they should be enclosed in non-combustible material and be treated as sources of heat; it is preferable that such resistance coils be placed within the metal framework of the lamp itself. Incandescent lamps should never be used for resistance devices. These lamps should be provided with globes and spark arresters, as in the case of arc lamps on high voltage series circuits, except when the closed arc lamps are used.
4. Wooden moulding should never be run in elevator shafts, or shafts of any kind, and should never be run on the inner side of the outside walls of the buildings, as these locations are usually subject to dampness.
5. In laying out feeders it is usually cheaper to use iron conduit in a shaft, than to run moulding through the floor timbers.
6. When tapping outlets for feeder circuits, an iron outlet box with cover should be used, as shown in fig. 886. The one splice box is held up to the outlet box already installed by means of two long screws, and the loom is run right up to the moulding so as to leave no exposed wire.
Fig. 876.—Circular fixture block for outlet from moulding work on ceiling.
7. Wherever fixture outlets are installed, a circular fixture block as shown in fig. 876 should be used, to give a good support for the fixture and to make a neat backing for the fixture canopy. The wires should be brought through the fixture without cutting and disfiguring the canopy.
Concealed Knob and Tube Wiring.—This method of wiring should be discouraged as far as possible, as it is subject to mechanical injury, is liable to interference from rats, mice, etc. As the wires run according to this method are liable to sag against beams, laths, etc., or are likely to be covered by shavings or other inflammable building material, a fire could easily result if the wires become overheated or short circuited.