Re: Ambient correction
The physics of the situation would tell you that it is OK. But the code official might make it difficult for you to take advantage of that fact. You might be called upon to produce an engineering report that includes meteorological data and a calculation method called the "Arrhenius Equation." I think you need to pull new cable or find another solution.
Here is a summary of the physics: High temperatures degrade a cable?s insulation system. But it?s not linear. A rule of thumb is that adding ten degrees to the year-round average ambient temperature will cut the expected life of the cable?s insulation system in half. But that also means that any time that the cable spends under the 86F temperature (i.e., on which 310.16 is based) will put some ?cable life? into a ?life bank,? in that the insulation system degrades more slowly, and will last longer than expected. So if you could produce a reliable history of temperature versus time on a day by day basis for several years, it could be possible to calculate the overall rate of degradation. If this calculation proves that the cable would degrade more slowly than it would at a flat temperature of 77F, then perhaps you could use the 1.08 factor. But I think such a calculation would cost more than pulling new cable.