How much voltage a run loses between the source and the load — volts dropped, the percentage, and the voltage that actually arrives, for copper and aluminum, single and three phase.
For #10 copper, 20 A, 150 ft one way, 240 V single-phase, 3.0 % recommendation, the engine returns:
3.11% drop (7.5 V, 232.5 V at the load) — EXCEEDS the limit
Rendered from the same calculation library the form uses — computed at startup, never typed by hand, and golden-tested against the engine on every run. Note this example deliberately shows a run that exceeds the recommendation — the tool's job is to tell you, not to flatter the run.
Every foot of conductor has resistance, and the longer the run, the more voltage is consumed by the wire before it reaches the load. Motors run hot, lights dim, and electronics misbehave at the end of undersized long runs. The commonly used limits are recommendations for performance rather than hard requirements in most circumstances — which is exactly why the calculator reports the number rather than a bare verdict, and why your AHJ or the equipment manufacturer may hold you to something stricter.
Method scope, stated honestly: Chapter 9 circular-mil (K-constant) method. Ignores reactance, power factor and raceway material — fine for short, high-power-factor runs; less accurate for large conductors, steel raceway, or motor loads.