To read a wire size chart one must examine the column listing various sized gauges of wire, then read across the row to gather the information for that particular gauge of wire. Wire size charts are often designed to show the appropriate amperage for various gauges of wire.
Amperage is a measurement of how much energy is transferred through a wire in one second; the higher the amperage, the more energy is being transferred. Many wire size charts show the "ampacity" of various wires, or the maximum amperage that a wire can safely transfer without overheating. Different gauges of wire have different ampacities, meaning that they can transfer different amounts of energy and are appropriate for different types of wiring.
Wire size charts generally list wires by their American Wire Gauge, or AWG, number. The AWG rating is related through a mathematical formula to the diameter of the wire, with a higher AWG number being associated with a smaller-diameter wire. As the AWG number increases, and the diameter of the wire decreases, the ampacity of the wire generally decreases. Different types of wire may have different ampacity ratings, making it important to find a trusted rating for the wire being used before doing electrical work.