NM wire gauges range from 14 AWG to 6 AWG. The following show the most common wire gauges:
The chart to the right shows the proper circuit breaker to be used for the wire gauge in a circuit.
Current rating of a circuit is defined by the lowest current rating of wires and circuit breakers on that circuit.
Commercial Electric cable ripper
This ripper from Commercial Electric has wire guaging holes to help determine wire gauge. The wire guaging holes include 14, 12, 10, 8, or 6 gauge solid wire.
Click on image for larger view
When a circuit is rated for 15 amps, the wires and circuit breakers must be rated for at least 15 amps. The circuit is rated for the lowest current rating of wire or circuit breaker. For example, if the wire is rated for 15 amps, the circuit should not have a 20 amp breaker even if the breaker trips frequently.
When a circuit is rated for 20 amps, the wires and circuit breakers must be rated for at least 20 amps. It is important that a 20 amp (or higher) circuit breaker NOT be used on a circuit with a wire rated for 15 amps or less.