James,
The ratings are based on the maximum temperature at which the cable should be allowed to run, hence the fact that the "clipped direct" rating is higher because it will lose heat to the surrounding air more quickly than an enclosed cable. (The "clipped direct" ratings are
too high in the current Regs., in my not-so-humble opinion, but that's another story.
)
So in the situation you describe, you would have to go with the lower current and limit the cable to its enclosed rating.