Why do utilities transmit power over long distances at high voltages such as 220 kV or 400 kV instead of at the generation voltage of around 11 kV? What are the limits to going higher?
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Write line current in terms of P, V and power factor, then look at I²R.
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Model answer.
For a given power P delivered at power factor cos φ, the line current is I = P / (√3 V cos φ) for a three-phase line. The current is inversely proportional to the voltage.
1. Lower losses. The copper loss in the line is 3I²R. Since I ∝ 1/V, the loss ∝ 1/V². If the voltage is doubled, the current halves and the line loss falls to one quarter. Going from 11 kV to 220 kV (20 times) cuts the I²R loss for the same conductor by a factor of 400.
2. Less conductor material. For the same percentage loss, the required conductor cross-section also falls roughly with 1/V², so less copper or aluminium is needed. This lowers the cost of the conductors and the towers that carry them.
3. Better voltage regulation. The voltage drop IZ is smaller when the current is smaller, and as a percentage of a higher system voltage it is smaller still.
Why not go infinitely high?
- Insulation costs rise: longer insulator strings, bigger clearances, taller towers and costlier transformers and switchgear.
- Corona loss and radio interference increase at very high voltages, especially in wet weather.
- Line charging current from shunt capacitance increases, and very long lightly loaded lines can show the Ferranti effect (receiving-end voltage higher than sending-end).
So the transmission voltage is an economic choice that balances line losses against insulation and equipment costs. As a rough rule, longer distances and larger power transfers justify higher voltages.
Likely follow-up. "Why not generate at 400 kV directly?" Generator windings cannot be insulated economically for such voltages. Generators work at around 11 to 25 kV, and step-up transformers raise the voltage for transmission.
Original UniCircle question. Last reviewed 28 Oct 2026.