Page:Electricity (1912) Kapp.djvu/237

Rh hour when the peak in the lighting load occurs depends on the kind of premises lighted. In offices it is between five and six; in residential districts between seven and eight, because at that time the kitchen premises are fully lighted, the bedrooms are used by people dressing for dinner, and at the same time the reception-rooms must be lighted up. Whatever the property lighted, there is a great variation in the demand for current at different times of the day, and the cables must be designed to be equal to the maximum demand that may occur.

If the street main is fed at the home end, the total current sent into it at that end is proportional to its length. The resistance is also proportional to its length, and since the drop is proportional to the product of current and resistance, we find that for a given density of supply expressed at so many amperes per yard run, the voltage drop is proportional to the square of the length. If, then, instead of feeding the street main at one end, we feed it in the middle only, we substitute two half-mile lengths for the single mile and we quarter the voltage drop. To do this we require a separate cable, the so-called "feeder," from which no current is taken