« НазадПродовжити »
From an analysis of this general character a complete specification may be made up, the purposes, conditions and limitations of the installation being known.
In any specification calling for an insulation for a particular purpose, a careful distinction should be made between so-called "weather proof," "moisture proof," and "rubber covered" insulations, bearing in mind the seemingly paradoxical fact that a "weather proof" wire is not necessarily a "moisture proof" wire, nor a "moisture proof" wire always the best wire for a wet place, and that conditions will be met demanding a high grade of insulation where a bare wire will give better satisfaction than the best "rubber covered and braided" wire on the market.
In the determination of the number and sizes of dynamos, reference should be had to the maximum number of lamps that will be lighted at one time, the length of time of maximum load, the length of time of light load, the day load, possibilities of extension, etc.; the sizes of the units being such as to have as near the maximum load as possible on the dynamos when operating, and the least possible amount of idle machinery, allowance being made for possible breakdowns and similar contingencies. The question of variation in load must be determined largely from experience with the given class of installation and one's judgment.
A question that too seldom receives the attention its importance demands is the question of the allowable variation in pressure or maximum difference of potential, especially in large isolated plants, between the centre of distribution and the farthest lamp in the installation. No fixed or arbitrary rule can be laid down, but each case must be considered by itself. The loss should be based not so much on the total number of lamps wired for as upon the maximum number that will be in use at any one time, upon the distances between groups of lamps rather than the total distance from the centre of distribution to the farthest lamp on any circuit, and upon the usual variation in load rather than extreme variations; the object should be primarily to obtain a minimum variation in the candle power of the lamps, and though this is at tained by allowing but a small percentage of loss between the centre of distribution and the farthest lamp, yet for all practical purposes the same result may often be gained with a larger percentage of loss provided it be given the proper distribution. For example, the distribution of losses is evidently entirely different in a theatre from the distribution in an office building for the same limits of variation in the candle-power when operating under the normal conditions of variation in load.
There is 50 per cent. more copper in a circuit figured with 2 per cent. loss than in the same circuit figured with 3 per cent, loss, and the question of which is ultimately the greater economy is by no means an easy one to answer. This question comes with peculiar force when considering the method of wiring, whether it shall be two-wire fifty-volt, twowire one-hundred-volt, three-wire one-hundred-volt, or a system so designed that any given combination of the above systems can be used with a limiting maximum loss for the system using the largest current and the lowest voltage. A very careful analysis should therefore be made both of the conditions at the time of the installation and of conditions that are likely to arise in the more or less remote future which will influence the character of the system to be adopted.
In this connection it may not be out of place to mention two methods of wiring which may be used where economy as well as adaptation to surroundings is a necessity, but which for various reasons have not been very extensively used. If it is desirable to wire for a 100volt system and yet necessary to connect with an alternating system, reducing to 50 volts, the system may be adapted to the wiring by installing converters in pairs and connecting their secondaries in series. If it is desirable to operate a direct current system where the cost of copper becomes excessive on account of distances or other limiting conditions, this item of expense, as well as the accompanying expenses in pole line and labor, can be very materially reduced by using the double three-wire system with a 250-volt dynamo and two lamps in series on either side of the system.
Of paramount importance to bidders estimating on wiring contracts, and to the contractor who secures the contract, are accurate plans showing in detail the locations of the various elements of the installation. A strictly accurate anil reliable estimate cannot be made without plans drawn to scale and having the exact location of each fixture, switch, and meter outlet, together with that of the dynamo if there be one, the switchboard and the risers, or feeders. In a large installation with a small percentage of loss, a difference of 50 feet in the position of the switchboard may entirely alter the location of the risers, or of the distribution boxes, or increase or diminish the cost of the installation by several hundreds of dollars. A single example will illustrate this: figure the cost, including wire, conduit, and labor, of a riser 100 feet in length, feeding 1,000 50-volt 16-c. p. lamps uniformly distributed along its length, on the basis of using a conduit system, the best grade of rubber covered and braided stranded wire, with a maximum loss of 2 per cent. on the 50-volt system; remove the switchboard 50 feet, making the length of the riser 150 feet with the 1,000 lights along the farther 100 feet, and figure the cost by the same method; the difference between the two figures graphically represents the necessity of carefully selecting the centre of distribution and location of the risers, and of knowing their exact position before making an estimate.
Of no less importance is the position of all outlets and the number of lights per outlet, since this determines the number of circuits, the lengths and sizes of tap lines, the method of running the circuits, the number of cutouts, and the amount of labor involved. The location of the switch and meter outlets with reference to the number and position of the lights they control is necessary for the same reason. Outlets, especially side outlets, should be located as far as possible by actual dimensions in feet and inches from floors, ceilings or walls; it is only necessary in some cases to specify that all outlets shall be wired to, leaving the dimensional location of the outlet to the gasfitter, but with the rapid increase of buildings fitted for the electric light only, with its more frequent adaptation to architectural and aesthetic effects, and the thousand and one purposes and places from which other forms of illuminant are excluded, the contractor is compelled in an increasing number of cases to rely upon detailed plans showing their exact location and to work to scale. Several methods will suggest them