Electrical Wiring Color Coding System - The Spruce Aug 30, 2022 · Electrical Cable and Wire Color Markings . Non-metallic (or NM) 120-volt and 240-volt electrical cables come in two main parts: the outer plastic sheathing (or jacket) and the inner, color-coded wires. The sheathing binds the inner wires together, and its outer markings indicate the number of wires and size of wire (gauge) within the sheathing.