Final answer:
The primary disadvantage of a 480-volt system for long-distance power transmission is the increased power loss due to the lower voltage, which is less efficient compared to systems using higher transmission voltages to minimize power losses.
Step-by-step explanation:
A disadvantage of using a three-phase 480-volt system to supply power to a building is that it is unreasonably low for a transmission voltage, which can lead to more power being dissipated in the transmission lines. Long-distance transmission lines typically operate at much higher voltages, in the range of hundreds of kilovolts, with the intent to minimize power losses across great distances.
The reason behind this is the relationship between voltage, current, and power loss, which is described by the equation P = I^2 × R, where P is the power loss, I is the current, and R is the resistance. Hence, transmitting power at high voltages allows for less current and subsequently lower power losses since power plants generate electricity at high voltages that are stepped up using transformers for long-distance transmission and then reduced for safe residential and commercial use.