This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The number of lamps in any building usually depends upon the building owner's wishes, but when he does not decide this point the architect or electrician should decide for him. Calculations for the number of lights are usually based upon the number of 8 candle-power required to light 100 superficial feet of floor space. The following table will give an approximate idea of the number of lights required for ordinary purposes at a height of 7 feet 6 inches above the floor level :-
Description of Room.
Number of 8 c.p.
Lamps per 100
Square Feet of
Lecture-rooms, music-rooms, entertainment halls, schoolrooms, etc. ...
Dining-rooms, drawing-rooms, principal halls, staircases, and landings ...
3 to 4
Showrooms for the display of goods ...
3 to 4
Churches, chapels, shops, warehouses ...
Bedrooms, minor rooms of residences, small passages, etc.
The higher the lights are placed above the floor level the less, of course, will be their illuminating effect, and it is usually reckoned that an 8 c.p. lamp placed 7 feet 6 inches above the floor level will have the same illuminating effect as a 16 c.p. lamp 12 feet above the floor level.
A rough and ready rule for the number of lights in a dwelling-house is to allow 3 c.p. of light per 10 superficial feet of floor space.
In the school illustrated in Plate VI. the right-hand schoolroom has a floor space of 590 square feet. The number of 8 c.p. lamps required to light it, according to the above table, will be -
Area of floor x 4 = 27'. 3" x 21'. 9" x 4 = 24 100 100
The central classroom requires fourteen 8 c.p. lights, and the infants' classroom twenty 8 c.p. lights.
These powers can be approximated most nearly by the use of 8 c.p. lamps arranged in clusters of three lamps, thus :-
2 rows of
4 clusters of
The lamps are arranged as pendants at a height of 7 feet 6 inches from the floor, this height being well out of the reach of the children and convenient for even diffusion without unnecessary loss of light.
The reason for using 8 c.p. is that lamps of this candle-power lend themselves better to a symmetrical arrangement and more equal diffusion than lamps of any other power for the particular example treated.
It is very necessary that schoolrooms should be amply lit with well-diffused light, and the walls should be of a light colour to assist in the general diffusion of light, so that the shadows may be as light as possible.
The corridor need not be so well lit as the classrooms, for which reason two pendants of three lights each have been used, allowing approximately three 8 c.p. lamps per 100 square feet of floor space.
The girls' cloakroom is lit by two 8 c.p. lamps - one in the centre of the room, and one over the lavatory basins. The boys' cloakroom has only one light, placed approximately centrally. Two 8 c.p. lights are placed in the front entrance, and one in the side entrance. One 8 c.p. light is placed outside each entrance, to guide the children to the doors in dark or foggy weather, and to prevent them from stumbling on the steps. One 8 c.p. light is also placed inside the coal-store, and is controlled by a switch just within the coal-store door.
All the lights, save the coal-store light, are controlled from switches fixed to the subdistributing boards in the classrooms. These distributing boards are encased, so that they are out of the way of the children, and can only be touched by the mistresses or masters who possess the keys to the cases.
The lights in the entrances are fixed near the ceiling, well out of reach of the children.
The supply is brought in at the most convenient point to the supply company's mains, and carried through the main fuse and meter to the main distributing board, where the mains are connected to the terminals of the main switch before the bus bars are reached. Separate leads are taken from the terminals of the main distributing board to the bus bars of the subsidiary distributing boards. Separate circuits are then taken from these to each cluster and to each separate lamp. It would, of course, be more economical at the outset to so arrange the lights that they might be switched on in groups, but this often necessitates great waste of current. In the right-hand classroom, for instance, if each row of four lights were controlled from one switch only, it would mean that on occasions when one cluster would afford ample light, four clusters at least would have to be switched on.
All wires are taken by the shortest routes from point to point, unless for appearance sake they must be taken otherwise. Thus when cables are concealed within wooden casing fixed to the walls it is usual to fix the casing either vertically or horizontally, although very frequently economy might be effected by running the casing and cables diagonally.
In the case under consideration it is assumed that wood casing is to be used, and the shortest routes from point to point in every case will be obtained by taking the cables vertically up to the roof, diagonally in the roof, and vertically down again.
The Sizes of Cable required is governed by the amount of current they are to carry. Lamps of 8 c.p. usually require about 30 watts to run them, so that, assuming the pressure of the supply to be 100 volts, it follows from the formula C x V = W that
C = 30/100 = o.3 amperes.
Each cluster of three lamps will take 0.9 amperes, and in referring to Column 4 of the table given on page 180 it will be seen that the smallest stranded cable will carry a current of 4.4 amperes, so that it will be perfectly safe to use 3/22 cable in this case for all the circuits from the subsidiary distributing boards through the lamps.
The size of the sub-mains must now be determined. The current flowing in the sub-main on the right-hand side, when all the lamps on this particular circuit are switched on, will be 27 x 0.3 amperes = 8.1 amperes, and the smallest size of cable which will carry this current is 7/22 (see Table). The central and left-hand sub-mains carry 5.4 and 6.9 amperes respectively, so that smaller cables might be used for these; but in practice they are usually made of the same size as the main carrying the largest current. The house main, of course, carries the sum of the currents in the sub-mains, namely, (8.1 + 5.4 + 6.9) = 20.4 amperes, and the smallest cable to carry this current is given by the table as 7/18.