Power lines carry electricity at voltages up to 765,000 volts — enough to arc across several meters of air. This seems dangerous, but it's actually the key to efficient power delivery. High voltage is the reason your electricity can travel hundreds of kilometers from a power plant.
Why High Voltage?
Power = voltage × current. To deliver 100 MW of power, you can use high voltage with low current, or low voltage with high current. The problem is that wire losses = I²R — they depend on current squared.
At 100 kV delivering 100 MW: current = 1,000 A. At 10 kV: current = 10,000 A — and losses are 100× higher. This is why transmission lines use extremely high voltages.
Voltage Levels in the Grid
- Generation: 10–25 kV (power plant output)
- Step-up transformer: Increases to 115–765 kV for transmission
- Sub-transmission: 33–115 kV (regional distribution)
- Distribution: 4–35 kV (neighborhood lines)
- Service drop: 120/240 V (to your home)
Each voltage change uses transformers, which are 98–99% efficient. The total journey from plant to home is about 95% efficient.
AC vs DC Transmission
AC (alternating current) dominates because transformers easily change voltage levels. However, HVDC (high-voltage direct current) is increasingly used for very long distances (> 500 km) and undersea cables because it has lower losses and doesn't suffer from reactive power.
China's ±1,100 kV HVDC line transmits 12 GW over 3,300 km — the world's longest and highest-capacity power line.
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