Example: A 2400 Watt electric heater on a 120V supply. $$I = \frac2400120 = 20 \text Amps$$
Ideally, voltage drop should be kept below 3% for lighting and 5% for power circuits. how to calculate the cable size
Most examples use copper, but aluminum is lighter and cheaper per amp, though less conductive and more prone to creep and corrosion. A direct 1:1 substitution fails; an aluminum cable must be roughly two sizes larger than copper for the same ampacity. The interesting question becomes systemic: for a long transmission line, the weight savings of aluminum (fewer pylons) may outweigh its larger size. For a building riser, the larger conduit needed for aluminum may erase cost savings. Example: A 2400 Watt electric heater on a 120V supply
The first step is determining the actual current (Amperes) your load will draw. This depends on whether the system is single-phase or three-phase. A direct 1:1 substitution fails; an aluminum cable
Note: Amperage ratings vary based on whether the cable is in conduit, buried in insulation, or clipped to a wall.