Maximum cable lengths
The 13A extension leads provided on the website are a maximum length of 25m (15m for hot tub leads). This is not an arbitrary length, but one set with due consideration to cable resistance and voltage drop.
Imagine watering your garden with a 20m hose pipe. The hose pipe exerts resistance on the flow of water. The longer the pipe, the more resistance is experienced by the water as it flows through. In reality, this means that the longer the hosepipe, the less pressure there is at the end.
This same principle applies to electricity flowing through a cable. The longer the cable, the more resistance is exerted on the flow of electricity. The table below shows that the resistance of a 1.5mm cable increases from 0.07ohms at 5m, to 0.6ohms at 45m.
It's not just the length of a cable which increases the resistance. Imagine watering your garden again with the 20m hosepipe, but this time the pipe is as thin as a drinking straw. The smaller in diameter a pipe is, the greater resistance it exerts against the flow of water. Again, this principle applies to the flow of electricity. The table above illustrates that at a length of 20m a 2.5mm thick cable has a resistance of 0.16ohmd, but a thinner 1.5mm thick cable has a resistance of almost double at 0.27ohms.
Just like the pressure is reduced at the of a long hose pipe, voltage is reduced at the end of a cable. The longer the cable, the greater the reduction in voltage at the end. The smaller the diameter of the cable, the more the voltage reduces with each metre. Using an extension lead with cable which is too small in diameter, or is too long, results in the voltage at the socket end being reduced below an acceptable limit. Known as voltage drop, this is a big issue.
Why? As the voltage drops, the appliance will draw more current to compensate. If this exceeds a certain threshold, the 13A plug may start to overheat and in extreme cases the plug/fuse holder may start to melt. 13A fuses are unlikely to blow when the current only slightly exceeds a safe level, even over prolonged periods of time. See our guidance Why does my plug get warm for further details.
This helps explain why a garden heater can run perfectly fine on a high quality 1.5mmsq thick extension lead, but may melt the plug if used with a lead which is much longer or has thinner cable. DIY store extension leads are often supplied with extremely long lengths, and generally use thinner 1.25mmsq cable.
Maximum Cable Lengths
The table above indicates the voltage drop experienced by varying thickness cables at different lengths. As the maximum permissible voltage drop for a 230v extension lead is 11.5v, the table below demonstrates the maximum lengths we can safely make. Due to the increasing resistance of the cable, RCDs are mandatory at longer lengths.