The electrical wire size, or gauge, required to complete a particular installation is not calculated, but rather selected from a table. With sizes standardized by the American wire gauge system, this table relates wire gauge to rated ampacity.
Many versions of the table are available online, and individual ones may include or omit example uses for each type of wire. When reading the table, users should note that smaller gauge numbers correspond to thicker wires. It is the larger diameter of these wires that give them higher ampacity ratings, or the amount of current that can be safely conducted without becoming hot or starting a fire.