Ampacity keeps it legal, voltage drop keeps it working — the difference shows up 100 feet from the panel. These are the NEC table calculations you already know, running at truck speed, plus the business math for when the job is yours.
Ampacity is about heat; voltage drop is about performance. A 12 AWG copper run is legal at 20 A but drops 7.9 volts over 100 feet — motors run hot, LEDs flicker, EV chargers fault out. NEC's 3% guideline upsizes wire on distance long before ampacity does.
NEC 210.19/215.2: loads running 3+ hours (EV chargers, heaters, lighting) must be multiplied by 1.25 before sizing wire and breaker. A 40 A EVSE needs wire and a breaker sized for 50 A — that is the rule everyone's cousin skips.
Use them for estimates, bids, and sanity checks — the tables are NEC Chapter 9 and 310.16 at 75°C. Final design belongs to the licensed electrician of record and the AHJ; terminals, bundling, and ambient temperature can all change legal ampacity.