Distribution Grid Structure

The transmission grid moves bulk power at 115-765 kV across hundreds of miles. But no home or business uses electricity at 345,000 volts. The distribution grid steps voltage down through several stages to deliver power at 120/240 V to your outlet. It is the last mile of the electricity system, and it is where most outages occur.

Electricity flows from transmission to distribution through substations that step voltage down to 4-35 kV for primary distribution feeders. These feeders carry power along streets on overhead lines or underground cables. Distribution transformers (the cylindrical cans on utility poles or green boxes at ground level) step voltage down again to 120/240 V for residential service drops, or 480 V for commercial buildings.

Trace the voltage levels. Transmission: 115-765 kV. Substation steps down to primary distribution: 4-35 kV. Distribution transformer steps down to service: 120/240 V residential, 480 V commercial. Each step trades lower voltage for higher current over shorter distances.

Why do most power outages originate in the distribution system rather than transmission?

Exposure and redundancy. The transmission grid is built with redundant paths (N-1 contingency: any single element can fail without losing load). Distribution feeders are typically radial, not networked: one path from the substation to the customer. A fallen tree on a distribution line has no backup route. Roughly 90% of all customer outage minutes originate in the distribution system.

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