Table 310.16 provides the allowable ampacity of insulated conductors based on their size and type, assuming they are installed in a raceway or cable. However, Section 240.4(D) requires that overcurrent protection devices be selected to protect conductors from damage due to overcurrents. For 14 AWG THHN wire, the maximum overcurrent protection device that can be used is 15 amps, as specified in Table 240.4(D).Using a 20 amp overcurrent protection device on a 14 AWG THHN wire would violate Section 240.4(D) because the overcurrent protection device would not be able to protect the conductor from damage in the event of an overcurrent. This could result in the conductor overheating and potentially causing a fire.It is important to note that the allowable ampacity of a conductor is not the same as the maximum overcurrent protection device that can be used on that conductor. The allowable ampacity is the maximum current that the conductor can safely carry without overheating, while the maximum overcurrent protection device is the maximum current that the overcurrent protection device will allow to flow through the conductor before it trips.In general, it is always best to use the smallest overcurrent protection device that is allowed by the Code for the conductor size and type being used. This will help to ensure that the conductor is adequately protected from damage due to overcurrents. Flag for review