What tool is used to determine the size of an unmarked wire?

The tool used to determine the size of an unmarked wire is a wire gauge. A wire gauge is specifically designed to measure the diameter of electric wires, allowing technicians to identify the appropriate size based on standardized wire gauge systems (such as AWG - American Wire Gauge). This measurement is critical when selecting the right wire for electrical applications to ensure it can handle the required current without overheating, thereby maintaining safety and performance standards.

A wire stripper is designed to remove insulation from wires but does not provide a measurement of wire size. A caliper, while capable of measuring wire diameter, is not as precise or standardized for wire sizes as a wire gauge. A multimeter is primarily used for measuring electrical properties, such as voltage, current, and resistance, rather than for sizing wires. Therefore, the wire gauge is the most appropriate and commonly used tool for determining wire sizes in electrical work.