Voltage Levels in the Grid

Electricity leaves a power plant at about 20,000 volts, travels across the country at 345,000 to 765,000 volts, arrives at a neighborhood substation at 13,000 volts, and reaches your outlet at 120 volts. That is a 6,000-fold range. Every step exists to solve the same problem: minimizing I²R losses while keeping end-use voltage safe.

The grid uses different voltages at different stages because the physics of power delivery creates a tradeoff. P = VI means that for a given power, higher voltage reduces current. P_loss = I²R means lower current dramatically reduces heat losses. Transmission at 765 kV instead of 120 V for the same power reduces losses by a factor of roughly 40 million ((765,000/120)²). But 765 kV at a wall outlet would be lethal. So the grid steps voltage up for transport and down for use.

The standard stages: generation (11-25 kV), transmission (115-765 kV), sub-transmission (26-69 kV), distribution (4-35 kV), and residential service (120/240 V). Each transition requires a transformer, and a unit of electricity typically passes through 4-5 transformers between generator and consumer.

Worked Example

A 500 MW power plant delivers electricity 300 km to a city. Line resistance: 10 ohms total.

• At 20 kV (no step-up). I = 500,000 kW / 20 kV = 25,000 A. Losses: I²R = (25,000)² x 10 = 6,250,000 kW = 6,250 MW. More than the plant produces.
• At 500 kV (step-up). I = 500,000 / 500 = 1,000 A. Losses: (1,000)² x 10 = 10,000 kW = 10 MW.

What fraction of the plant's output is lost at 500 kV?

10 MW / 500 MW = 2%. Without the step-up transformer, transmission would be physically impossible. With it, losses are manageable.

The voltage architecture of the grid is not arbitrary. Every level exists because the physics of I²R losses demands it.

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