What is the maximum overcurrent protection allowable for #12 THHN or THWN wire when used for general wiring?

The maximum overcurrent protection allowable for #12 THHN or THWN wire when used for general wiring is indeed 20A fuse or circuit breaker. This is governed by the National Electrical Code (NEC), which specifies the appropriate overcurrent protection for different conductor sizes.

For #12 AWG wire, the NEC allows a maximum of 20 amps for branch circuits, which is why a 20A circuit breaker or fuse is the correct answer. This rating ensures that the wire does not exceed its safe carrying capacity and reduces the chance of overheating, which could lead to insulation failure or fire hazards.

Using a breaker rated above 20 amps with #12 wire could lead to overloading of the conductor and is not consistent with safe wiring practices. This is critical to ensure the safety and reliability of electrical installations.