Wire sizing is governed by two distinct physical constraints: Ampacity (the current-carrying capacity before the insulation melts) and Voltage Drop (the energy lost to resistance over long distances). While a 14 AWG wire can safely carry 15 Amps, it might only be suitable for a 10-foot run before the voltage drop exceeds the 3% limit recommended for efficient equipment operation.

Aluminum is significantly lighter and cheaper than copper, which is why it is preferred for main service feeders. However, aluminum has higher resistance and requires a larger gauge to carry the same current. Additionally, aluminum requires specialized oxidation-inhibiting paste and specific lugs to prevent high-resistance connections that can lead to fires.

Per the National Electrical Code (NEC), for "continuous loads" (any load running for 3 hours or more, such as an EV charger or a space heater), you should only load a circuit to 80% of its rated capacity. For a 20-amp circuit, your continuous load should not exceed 16 amps to ensure the breakers and wires do not overheat over time.