Does 310.15(A)(2) Exception apply?
Cheers, Wayne
No. The "stub rule" applies only to conductor ampacities. Not to termination ampacities.
Termination ampacity should always be your starting point. Then you apply conditions of use as needed. Even though terminations are obviously less than 10 ft, and less than 10% of the overall length. Terminations are simply too critical of a location, and too dissimilar from the rest of the circuit, to take credit for neglecting their reduced ampacity.
One place you might see this exception, is when you have an HVAC unit on the roof, and there is 50 ft of wiring inside the building, and only a small 2 ft stub on the rooftop to connect to the device. The stub on the rooftop is in direct sunlight, but the remaining circuit still has enough mass to dominate the conditions of heating inside the conductor, that it draws the heat away from the small portion in direct sunlight.
Another place you might see this, is in photovoltaic applications of a group of PV inverters and a common wireway for the AC output (suppose DC is separate). In wireways, you are allowed up to 30 wires without derating. The instant you add the 31st wire, you drop down to the derated ampacity of 31 wires in a raceway. So in a group of 11 three-phase string inverters, the first 10 string inverters don't require ampacity adjustment until their wires are added with the 11th inverter. So for the last (perhaps 3) inverters, you might need to upsize because the higher ampacity is more than 10% of the overall length. But for the majority of them, you can ignore ampacity adjustments.
), the conductors must be derated 80% just for the number of CCC and looking at 672A ampacity. However, temperature correction for 21-25?C (1.05; entirely feasible for an underground run) would take the ampacity back up to 706A and compliantly protected with an 800A breaker as long as the factored load is not more than 706A.
