You're on the right track! Here's the breakdown of why NM-B cable ampacity is restricted despite the 90ºC insulation rating of the individual conductors: Bundle Effect and Heat Dissipation

NM-B cable has multiple insulated conductors bundled together and then sheathed in a plastic jacket. This configuration significantly reduces the ability of the conductors to dissipate heat. The heat generated by the current flowing through each conductor gets trapped within the cable assembly.

NEC Table 310.16 and Termination Temperature Limitations

The National Electrical Code (NEC) specifies ampacity ratings for various wire types in Table 310.16. While the individual conductors are rated for 90ºC, the overall ampacity of NM-B cable is limited to the 60ºC column for several reasons:

Common Terminals

Many common circuit breakers, receptacles, switches, and other termination points are only rated for 60ºC or 75ºC. You must use the lowest temperature rating of any termination point.

To prevent melting insulation, degradation and potential fire hazards. Safety Margin

The NEC errs on the side of safety. Limiting the ampacity ensures that the cable won't overheat, even under continuous load, protecting the insulation from degradation and preventing potential fire hazards.

Exception 310.14(C)

The 90°C rating can be used ONLY for ampacity derating purposes!*

NM-B Sheath

The outer sheath of NM-B cable isn't designed to withstand the same high temperatures as the individual conductor insulation. Sustained high temperatures could cause the sheath to degrade, crack, or become brittle.

In summary: While the individual conductors inside NM-B cable have a 90ºC insulation rating, the bundled configuration, termination limitations, and overall safety considerations mandate using the 60ºC ampacity column in NEC Table 310.16 for typical applications. This is to ensure that the entire cable assembly doesn't overheat and compromise safety. Flag for review