With respect, mweaver, I very STRONGLY disagree with this statement. I submit that the intended meaning is ?year-round average.? I concede that the NEC does not define or explain the intended meaning of ?ambient temperature,? in the sense used by Table 310.16. I would like to see an NEC revision that makes this point clear. However, I take some comfort in the fact that the laws of Physics are on my side.
I have made this point before, and as websparky had noted, you can look up the earlier discussions. Here is a brief summary of the basis for my interpretation:
The hazard that higher temperatures impose on a conductor is an increase in the rate of deterioration in the insulation system.
For every 10 degree F increase in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will decrease by about 50%.
For every 10 degree F drop in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will double.
The relationship between temperature and the rate of degradation is not linear. If a cable spends one hour at a temperature of 90F (i.e., 4 degrees above the de-rating threshold), it may have to spend 3 hours, or even 5, at a temperature of 82F (i.e., 4 degrees below the de-rating threshold), in order to break even (i.e., on the overall average rate of degradation)
The concept is similar to buying a car that is intended to last you 10 years, but that has a gas tank that cannot be refilled. You start with 10 years worth of gas in the tank, but that assumes you burn gas at the rate of a car moving at 30 miles per hour. Once you run out of gas, you must buy a new car (similar to replacing a cable, once its insulation system has degraded too far). If you drive faster than 30 mph for a short while (say for 2 hours), you burn gas faster than the design rate, and you might run out of gas before the car?s 10 year expected life. If you drive slower than 30 mph, you save gas. But you might have to drive at 20 mph for 6 hours, before you can save enough gas to restore the car?s 10 year expected life. Finally, if you constantly drive below 30 mph, you could, in fact, extend the useful life of the car beyond its expected 10 years.
HOWEVER, the fact that you drove over 30 mph for a single hour would not, by itself, drain the gas tank, and render the car useless. Similarly, if you allow a cable to experience more than an 86F ambient for a limited time, without reducing the current to the de-rated value shown in 301.16, that will not instantly destroy the cable. What it will do is to reduce the cable?s useful life by some small amount. You can make this up by keeping it below 86F for an extended period.
ANOTHER HOWEVER: Websparky is right about needing an EE (and a PE, at that), if you want to take advantage of the lower ambient temperatures. The EE would have to be familiar with the Arrhenius equation and the methodology for applying it. The Arrhenius equation is k=A*exp(-Ea/R*T), where k is the rate coefficient, A is a constant, Ea is the activation energy, R is the universal gas constant, and T is the temperature (in degrees Kelvin). It would be no easy task to apply this to a cable?s insulation system. You would also need an AHJ willing to go along with the EE?s calculated results. You are not likely to get both.
