An ampere foot may be defined as the product of one ampere multiplied by one foot.
The unit ampere foot is used in figuring motor circuits or currents designed to carry a mixed load.
Fig. 820.—The center of distribution of a circuit coincides with the geometrical center of the group of lamps when the lamps are of uniform size and spaced equal distances apart. The center of distribution is here indicated by the dotted line A B.
The ampere feet of a main are found by multiplying the maximum load in amperes by the distance from the fuse block to the electrical center of the load.
Thus if the center of distribution be 50 feet from the fuse block and the maximum load is 9 amperes, the number of ampere feet is equal to 9 × 50 = 450.
Electrical Center of Distribution.—The electrical center of a circuit depends upon the distances between the lamps and the fuse block; also the relative sizes of the lamps.
It may be defined as the sum of the lamp feet for each section divided by the number of 16 candle power lamps in the circuit.
If the lamps be of uniform capacity, and placed at equal distances apart, the center of distribution will coincide with the geometrical center of the group of lamps. However, if the lamps vary in size, and be irregularly spaced, the electrical center will not coincide with the geometrical center unless the lamps be symmetrically arranged so as to compensate for the difference in sizes and spacing.