A feeder must carry 340 A continuous. You’re handed a table: 4/0 ACSR “Penguin” is rated about 340 A at 75 C and about 460 A at 100 C. Is 4/0 Penguin big enough at the 75 C rating? Decide, then check.
At the 75 C rating, Penguin sits at exactly 340 A, the same number your feeder needs. That leaves zero spare. A conductor loaded to its rating with no margin has no room for load growth, a hot day, or a small error in the load estimate, so in practice you either size up to the next conductor or lean on the 100 C rating, where Penguin reaches about 460 A. Read every ampacity number as a ceiling you stay under, not a target you hit.
Ampacity is a limit on conductor temperature, not a magic property of the metal. Current flowing through a conductor heats it by I-squared-R losses, and the rating sets the hottest the conductor is allowed to get. The table answers one question: how many amperes drive the conductor up to that temperature limit and no further.
That is why the same Penguin shows two different numbers. A higher allowed conductor temperature lets the wire shed more heat to the air, so it carries more current, paid for with more sag and faster annealing that shortens its life. The 75 C rating is conservative and protects the conductor. The 100 C rating buys roughly a third more capacity from the identical wire, which is why utilities sometimes re-rate older lines to 100 C instead of restringing them.
Every ampacity number rests on assumed conditions, and you must state them. The values here assume 25 C ambient air, 2 ft/s wind, 0.5 emissivity, and full sun. Drop the ambient on a cold winter night and the same conductor carries more; raise it on a still summer afternoon and it carries less. An ampacity quoted without its ambient is incomplete.
Now read the table across sizes. Penguin 4/0 ACSR gives about 340 A at 75 C. Step up to Linnet 336.4 kcmil and the 75 C rating jumps to about 530 A. Step up again to Hawk 477 kcmil and you reach about 670 A at 75 C. Bigger conductor, more metal, more surface to shed heat, so more current before the temperature cap. To size a feeder, start from the continuous peak load, add margin for growth, then pick the first conductor whose rating clears that number at the conductor temperature your standards allow.
Every component on this list, conductor, switchgear, cutouts, regulators, carries its own lead time and supplier landscape. DistroForge report editions track capacity and lead times across the distribution equipment categories covered in this track.