The physics behind the entire issue is not as simple as that, Smart $. The whole reason for limiting the ?ampacity? is to limit the degradation of the wire?s insulation system. Jon (Winnie) had the right concept: temperature is the bad guy in this equation, and higher temperatures cause faster degradation.
The temperature that the insulation sees is a combination of the surrounding air temperature and the increase in temperature caused by current flow. We all know and understand this portion.
The math to which Jon referred is called the ?Arrhenius Equation.? Start with this: If you surround a wire with 30C air, and keep that surrounding temperature (notice how I cleverly avoided using the word ?ambient??) constant for decades and decades upon end, and if you push through the wire a constant current that exactly matches the tabulated (or calculated) ampacity, then we will have reasonable assurance that the wire?s insulation system will remain in acceptable condition for its design lifetime. I heard once that the design lifetime is 40 years. I have no proof of that value, but let?s use it for discussion purposes.
What the Arrhenius Equation would tell us is that if we were to take a second identical wire with the same current flow and surround the second wire with air at 40C, and if we maintained that temperature constant for decades upon decades on end, then the second wire?s insulation system would degrade as much in 20 years as the first wire?s insulation system would degrade in 40 years. The rule of thumb is that if you add 10C to the temperature, you cut the lifetime in half.
On the other hand, if the surrounding temperature is kept at or below 20C, and if we keep the same current constant, then the lifetime of wire?s insulation system could go as long as 80 years. From a practical standpoint, we know that wires do not carry their rated ampacity value for decades on end. Loads are turned on and off, and load values are seldom as high as the calculations would predict. We also know that the surrounding temperatures vary from day to day and from month to month. So yes there will be hot summer days, and the rate of degradation gets worse during those days. But whatever time is spent at the peak surrounding temperature will not result in the same amount of degradation as would have been experienced if that temperature had been kept constant forever. Also, during the cooler months, the rate of degradation is much slower.
This is the reason that I am not in favor of basing an ampacity calculation solely on the highest temperature that will ever surround the wire during its entire lifetime.
