Hawkins Electrical Guide v. 08 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

The capacity of circuits is readily calculated by applying the following formulae:

	38.83 sc 10-3
C =	——————	per mile, insulated cable with lead sheath;
	log (D ÷ d)
	38.83 × 10-3
C =	——————	per mile, single conductor with earth return;
	log (4h ÷ d)
	19.42 × 10-3
C =	——————	per mile of parallel conductors forming metallic circuit;
	log (2A ÷ d

in which

C = Capacity in micro-farads;
for a metallic circuit, C = capacity between wires;

sc = Specific inductive capacity of insulating material;
= 1 for air, and 2.25 to 3.7 for rubber;

D = Inside diameter of lead sheath;

d = Diameter of conductor;

h = Distance of conductors above ground;

A = Distance between wires.

Frequency.—The number of cycles per second, or the frequency, has a direct effect upon the inductance reactance in an alternating current circuit, as is plainly seen from the formula.