The electricity that powers homes and businesses travels across vast distances through a complex network of wires known as power lines. The voltage, which is the electrical potential difference that drives the current, changes dramatically along this path. The specific voltage a line carries is determined by its function within the electrical grid, ranging from hundreds of thousands of volts on massive transmission towers to a couple hundred volts at the point of consumption. This variation ensures that the bulk movement of electricity is efficient while the final delivery to the consumer is safe.
The Physics Behind High Voltage
Utility companies rely on extremely high voltages for long-distance transport to minimize the amount of energy wasted as heat. This energy loss, often called resistive loss, is an unavoidable consequence of sending current through a conductor, which possesses some inherent electrical resistance. The physics governing this process shows that the power lost in the wires is proportional to the resistance of the cable multiplied by the square of the current flowing through it.
This relationship, often expressed as P(loss) = I²R, demonstrates that current (I) is the most significant factor in determining energy waste. Even a small reduction in current results in a much larger reduction in heat loss. To move a set amount of electrical power (P), the current and voltage (V) are inversely related, as P = V x I.
If a utility company doubles the voltage, it can halve the current while still moving the same amount of power. Following the I²R principle, halving the current reduces the power loss by a factor of four. By utilizing high voltages, which keeps the current low, the electrical grid can move tremendous amounts of power over hundreds of miles with minimal energy wasted as heat.
Voltage Levels in Long-Distance Transmission
The highest voltages in the electrical grid are found on the largest, tallest structures, often referred to as extra-high voltage (EHV) lines, which form the backbone of the system. These lines are designed for bulk power transfer, moving electricity from large generation facilities, such as power plants, to major regional substations. The voltage levels used in this phase are between 100 kilovolts (kV) and 765 kV.
Common transmission voltages include 230 kV, 345 kV, and 500 kV, depending on the distance and power capacity. For the longest distances and highest capacity requirements, ultra-high voltage (UHV) lines operate at 800 kV or higher. These voltages make long-distance transmission economically feasible, as efficiency gains offset the high cost of specialized infrastructure.
The exact voltage is based on factors such as total power demand and the distance the electricity must travel. For instance, a 500 kV line handles a greater power load and spans longer distances than a 138 kV line. Massive wires are separated by large insulators to prevent arcing to the tower or the ground. The transmission segment ends when the power reaches a regional substation, where the first major step-down in voltage occurs.
Voltage Levels in Local Distribution
Once electricity reaches a major substation, the system shifts from high-efficiency bulk transfer to safe, localized distribution. At the substation, large transformers step the voltage down from the transmission level (e.g., 500 kV or 230 kV) to a sub-transmission or primary distribution level. These intermediate voltages range from 4 kV to 35 kV.
These medium-voltage lines, known as primary distribution lines or feeders, run along utility poles in neighborhood streets. They carry power from the substation to a localized area, serving as the main arteries of local power delivery. Common primary distribution voltages include 12.47 kV and 34.5 kV.
The final stage involves a local transformer, mounted on a pole or concrete pad, which performs the last voltage reduction before the power enters a building. This step converts the medium voltage of the primary line (e.g., 13.8 kV) to the low, usable voltage required by consumers. In North America, the standard residential service voltage is 120/240 volts, which is safer for household appliances and wiring. Commercial and industrial customers may receive slightly higher voltages, such as 480 volts, but these remain low-voltage within the overall grid context.