Reference
The rules that sit underneath several calculators at once.
Each of the 26 calculators explains the rule it applies, on its own page, next to the answer. These articles are for the rules that do not belong to any single tool — the ones you meet in three different calculations and have to get right in all of them.
All static, all readable without JavaScript, and all naming the code section they describe so you can go and read the original.
What the calculators do, which code editions they follow, and how to check a result by hand.
The same arithmetic sends you to a bigger device in one article and a smaller one in another. How to tell a floor from a ceiling.
Why you derate in the 90 °C column but size in the 75 °C one, and what NEC 110.14(C) actually restricts.
What makes a load continuous, why the factor is applied twice, and why a 40 A charger needs a 50 A breaker but a 40 A cooktop does not.
Publication and adoption are different events, often years apart. How to find the edition your inspector actually enforces.
How the tables behind these articles are sourced, and which edition each follows, is set out in the methodology.