You never size a cable exactly at the load current. Electrical loads can fluctuate, and starting currents for motors can be significantly higher than running currents.
You are installing an electric water heater in a garage. how to calculate the cable size
Example: A 2400 Watt electric heater on a 120V supply. $$I = \frac2400120 = 20 \text Amps$$ You never size a cable exactly at the load current
This constraint is fascinating because it ties the cable size to the upstream protective device (circuit breaker or fuse). A slower breaker requires a thicker cable to absorb the fault energy. Installers who ignore this calculation are gambling that a rare fault won't happen—a dangerous bet. how to calculate the cable size
Ideally, voltage drop should be kept below 3% for lighting and 5% for power circuits.