Electrical
A long run to an outbuilding is where voltage drop bites. Size the cable for the distance, not just the current.
A 20 A circuit fifteen metres away is a different cable from the same circuit at two metres. Current decides whether the cable survives; length decides whether anything at the far end works properly.
Drop is proportional to length, so doubling the run doubles the volts lost. That is usually the moment a cable size that looked generous stops being enough.
Ohm's law is the whole story: volts, amps, resistance and power, any two giving the rest. The circuit tool lets you push the numbers around until the relationships are obvious rather than memorised.
Inductive loads draw more current than their real power suggests. Poor power factor means cable and breakers sized for current you are not usefully using — and on a commercial tariff, a penalty on top.
Ampacity keeps the cable safe; voltage drop keeps the tools working. Put your real distance in — it is almost always the input that decides the answer.