The National Grid carries electricity from power stations to homes and businesses across Britain. It transmits at very high voltage — up to 400,000 V — to reduce heat losses in the cables. Transformers step the voltage up for transmission and down again to the 230 V used in homes.
What is the National Grid?
The National Grid is the high-voltage electricity transmission network that connects power stations (where electricity is generated) to the local distribution networks that supply homes and businesses.
Key components:
- Power stations — generate electricity at typically 25,000 V (25 kV)
- Step-up transformers — increase voltage to very high levels (typically 132 kV to 400 kV) for long-distance transmission
- High-voltage transmission cables — thick aluminium cables (often on tall pylons) carry electricity over long distances
- Step-down transformers — reduce voltage in stages to levels safe for use (33 kV → 11 kV → 230 V for homes)
The National Grid in England, Scotland, and Wales is managed by National Grid plc. Scotland and Wales have their own distribution networks that connect to the same system.
Why is electricity transmitted at high voltage?
The answer lies in the relationship between power, voltage, and current:
Power = voltage × current
P = V × I
If power is fixed (the power station generates a set amount), then voltage and current are inversely related — increasing voltage means decreasing current, and vice versa.
Now consider how cables lose energy. The energy dissipated as heat in a cable is given by:
Power lost = current² × resistance
P_lost = I² × R
The cable's resistance (R) is fixed. So:
- High current → large I² → large energy lost as heat
- Low current → small I² → small energy lost as heat
Conclusion: transmitting electricity at high voltage reduces the current for the same power output, which dramatically reduces heat losses in the cables. Typically, the National Grid uses voltages of 132,000 V to 400,000 V for this reason.
Worked example:
A power station transmits 100,000 W (100 kW) along cables with a resistance of 10 Ω.
| Transmission voltage | Current (I = P/V) | Energy loss (I²R) |
|---|---|---|
| 1,000 V | 100 A | 100² × 10 = 100,000 W (all lost!) |
| 10,000 V | 10 A | 10² × 10 = 1,000 W (1% lost) |
| 100,000 V | 1 A | 1² × 10 = 10 W (0.01% lost) |
At 100,000 V, the transmission losses are a tiny fraction of what they would be at 1,000 V. This is why the National Grid uses very high voltages.
What does a transformer do?
A transformer is a device that changes the voltage of an alternating current (a.c.) supply. Transformers only work with a.c., not d.c.
Step-up transformer: increases voltage (and decreases current)
- Located between the power station and the high-voltage transmission cables
- Example: 25,000 V from the power station → 400,000 V for transmission
Step-down transformer: decreases voltage (and increases current)
- Located at sub-stations along the distribution network
- Example: 400,000 V → 33,000 V → 11,000 V → 230 V for homes
How a transformer works (basic): A transformer has two coils of wire wound around an iron core. An alternating current in the primary coil creates a changing magnetic field in the iron core. This changing magnetic field induces a voltage in the secondary coil. The ratio of the voltages equals the ratio of the number of turns:
V₁/V₂ = N₁/N₂
Where V₁ and N₁ are the voltage and number of turns in the primary coil, and V₂ and N₂ are those in the secondary coil.
Why is 230 V used in UK homes?
The UK mains supply is 230 V alternating current at 50 Hz. This is the result of step-down transformers reducing the transmission voltage to a level that:
- Is safe enough to use in homes (though still dangerous — mains electricity kills)
- Provides enough voltage to power household appliances without requiring very thick (and expensive) wires in the home
- Is standardised across Europe (the EU standard is also 230 V)
At 230 V, the current drawn by most appliances is manageable — a 1,150 W hair dryer draws about 5 A. If the same hair dryer ran at 23 V, it would need to draw 50 A, requiring much thicker wiring.
What are pylons and why are the cables so high?
The high-voltage cables of the National Grid are suspended on tall steel pylons for two reasons:
- Safety: cables at 400,000 V are extremely dangerous. Keeping them high in the air prevents people or vehicles from accidentally coming near them. The high voltage can arc (jump) through air over considerable distances.
- Efficiency: underground cables at very high voltages require expensive insulation and are difficult to repair. Overhead cables on pylons are cheaper to build and maintain for long-distance transmission, though underground cables are often used near towns and in areas of natural beauty.
Frequently asked questions
Why does high voltage reduce energy loss in cables?
For a fixed power output, increasing voltage means decreasing current (because P = V × I). Heat loss in cables depends on current squared (P_lost = I²R). Halving the current quarters the heat loss — a very effective saving. This is why the National Grid uses voltages of hundreds of thousands of volts for long-distance transmission: even a small reduction in percentage losses represents enormous energy savings across the whole network.
What is the difference between a step-up and a step-down transformer?
A step-up transformer has more turns in the secondary coil than the primary coil. This increases the output voltage while decreasing the current. Step-up transformers are used at power stations to boost the voltage for transmission. A step-down transformer has fewer turns in the secondary coil — it reduces the voltage and increases the current. Step-down transformers are used at sub-stations to reduce transmission voltages to safe levels for homes and businesses.
Why can't transformers work with direct current (d.c.)?
Transformers rely on a changing magnetic field in the iron core to induce a voltage in the secondary coil. A steady d.c. current produces a constant magnetic field, which does not change — so no voltage is induced in the secondary coil and the transformer does not work. Alternating current (a.c.) continuously reverses direction, creating a constantly changing magnetic field that induces a continuously changing (alternating) voltage in the secondary coil. The UK mains supply is a.c. at 50 Hz for this reason.
Is the electricity in my home the same as in the National Grid cables?
No — the electricity reaching your home has been transformed multiple times. National Grid cables carry voltages of 132,000 V to 400,000 V. Local sub-stations step this down in stages: typically to 33 kV, then 11 kV, then finally to 230 V for homes. The frequency (50 Hz) remains the same throughout. The electricity is always alternating current (a.c.) — the same type produced by generators in power stations and required for transformers to work.
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