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CURRENT RECTIFICATION 313
waste of power wherever current costs are high. It is much more economical and more efficient to install wires larger than the Underwriters call for. The saving on current consumption will be much greater than the initial cost of the larger wires.
(123) Calculate (see page 66) the difference in cost per working hour between, for example, a No. 6 wire loaded to capacity, and a No. 4 wire conveying the same amperage. Multiply this by the estimated number of hours of operation per year. You will be surprised at the saving particularly if both current cost and amperage used are high.
(124) In installing the main projection room circuit and the circuits feeding the projectors, spots, effect projectors and stereopticons, be sure to make ample allowance for possible future increases in current consumption. You may think this an unnecessary suggestion but the cost is little and the precaution is wise. It is both expensive and troublesome to be forced later to pull out all the wires and install larger ones, or to run an auxiliary circuit. In any event install conduit large enough to accommodate larger wires, should they become necessary.
(125) The voltage drop of any wire circuit may be ascertained by multiplying the one-way length of the circuit by 21 feet; multiply the product by the amperes and dividing the result by the circular mil area of the wire. This will give you the voltage drop.
(126) Before purchasing a motor-generator set for the projector arc supply, make sure that the generator performance curve is flat (constant voltage over the amperage range of the generator) at the level at which the set is to operate.
Installation
(127) It is extremely unwise to locate a motor-generator set in a basement or any place far removed from the projection room. A good set is quiet enough to be installed in a room immediately adjacent to the projection room, provided it is placed on a support or foundation sufficiently solid to avoid vibration. Furthermore the set