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Why do power lines use high voltage?

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Quick answer

Power lines run at high voltage to cut energy wasted as heat. Power is voltage times current, so a higher voltage carries the same power with a smaller current. Heat loss grows with the square of current, so ten times the voltage means roughly one hundredth of the loss. Transformers later lower the voltage for homes.

Power lines use high voltage because the same power can then be carried with a much smaller current, and resistive heat loss in the wire rises with the square of the current. Transformers step voltage up at the power station for the long journey and back down near homes for safe use.

Step by step

  1. A power station generator produces alternating current at a moderate voltage, typically in the range of 10 to 25 kilovolts.
  2. A step-up transformer beside the station raises that voltage to hundreds of kilovolts while lowering the current by the same ratio.
  3. The small current travels along thick aluminium conductors on tall towers, losing only a small share of energy as heat.
  4. Grid substations near towns use step-down transformers to bring the voltage down to distribution levels such as 11 kilovolts.
  5. A final transformer on a pole or in a street cabinet lowers it to household voltage for sockets and lights.

Key things to understand

  1. Electrical power equals voltage multiplied by current, so raising the voltage lets the same power flow with less current.
  2. Every conductor has resistance, and the heat it wastes equals the current squared multiplied by that resistance.
  3. Raising voltage tenfold cuts current tenfold and so reduces heating losses in the line roughly a hundredfold.
  4. Transformers make this possible for alternating current, stepping generator output up to hundreds of kilovolts for long-distance lines.
  5. Substations then step the voltage down in stages until homes receive a safe supply such as 230 volts.

Labelled diagram

  1. Generator

    Produces alternating current at roughly 10 to 25 kilovolts inside the power station.

  2. Step-up transformer

    Raises voltage to hundreds of kilovolts and lowers current by the same factor.

  3. Transmission line

    Carries a small current over long distances so little energy is lost as heat.

  4. Substation

    Step-down transformers reduce voltage to distribution levels such as 11 kilovolts.

  5. Local transformer

    Lowers voltage once more to the household supply that reaches wall sockets.

From power station to plug at falling voltage

Real-world examples

  • India's national grid uses 400 kV and 765 kV AC lines plus ±800 kV HVDC links to move power between regions.
  • Britain's National Grid carries bulk power on 275 kV and 400 kV lines strung between large steel pylons.
  • Undersea HVDC cables link the electricity grids of several European countries, including Britain and Norway.
  • A green pole-mounted transformer on an Indian street lowers 11 kV distribution voltage to 230 V for nearby houses.

Common mistake

✗ Myth: High-voltage lines carry more electricity than ordinary household wiring because the voltage is higher.

✓ Fact: Voltage is only one part of power; a high-voltage line carries a deliberately small current. The high voltage exists to reduce losses, not because the line holds more electricity per se.

People also ask

Why is electricity transmitted at high voltage?
Electricity is transmitted at high voltage to reduce the current needed for a given amount of power. Line losses depend on the square of the current, so a smaller current wastes far less energy as heat in the wires. This lets power stations send electricity hundreds of kilometres with modest losses.
What voltage are overhead power lines?
It depends on the line's role. Long-distance transmission lines commonly run at 220 kV to 765 kV, regional lines at 66 kV to 132 kV, and local distribution lines at around 11 kV in India and the UK. Household supply is then 230 V, or 120 V in North America.
Why do we use transformers in power transmission?
Transformers change AC voltage efficiently using two coils wound on an iron core. A step-up transformer raises voltage for low-loss transmission, and step-down transformers later reduce it to safe levels. Without them, raising and lowering voltage on such a large scale would be impractical and wasteful.
Why not transmit at even higher voltages?
Higher voltages need taller towers, longer insulators and wider spacing between wires to stop sparks jumping through the air. Above a certain level, corona discharge also leaks energy and creates noise, so engineers balance line losses against equipment cost.
Why is high voltage so dangerous to touch?
Voltage is the push that drives current through your body, and hundreds of kilovolts can force a lethal current through skin and tissue. It can also arc across a gap of air, so getting close to a line is dangerous even without touching it.
What is HVDC and why is it used?
High-voltage direct current lines carry power as steady DC rather than alternating current. They lose less energy over very long distances and undersea cables, but need costly converter stations at each end to change AC to DC and back.
Do thicker wires also reduce losses?
Yes, a thicker conductor has lower resistance and therefore wastes less heat. However, metal is heavy and expensive, so raising the voltage is a far cheaper way to cut losses than making transmission cables ever thicker.

Part of the guide: Energy & Electricity at Home →