The size tells you how many cables fit in the box. Each conductor counts as one allowance. Selection of the right cable depends on power load, environmental conditions, and voltage drop considerations. The following step-by-step guide will show you how to calculate the correct size of cable and wire, or any other conductor, for electrical wiring installations with solved examples in both British or English and SI Systems, i., Imperial and Metric Systems, respectively. Practice good wiring: secure. This article examines the sizing of electrical cables (i.