Current-carrying capacity

Multipliers applied to a cable's tabulated current rating to reflect how it is actually installed. High ambient temperature, grouping with other loaded circuits, contact with thermal insulation and the type of protective device all de-rate it. The cable size that survives every applicable factor — and still beats the design current — is the one you install.

The voltage lost along a cable as current flows through its resistance. It is Ohm's law working against you, growing with both load current and run length. BS 7671 limits how much of the supply voltage may be dropped in the installation's own wiring, so a long run can force a larger conductor than the current alone would need.

Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a short circuit drives an enormous current that must be broken in milliseconds. Protective devices are chosen to handle both.

Current is the rate at which electric charge flows, measured in amperes (A), often called amps. In a simple closed circuit, the supply drives current through the load and back to the source. Current is not used up by a lamp: energy is transferred there. A cable current rating describes how much current it can carry in stated conditions, not how much it always carries.

An overload occurs when an otherwise sound circuit carries more current than its intended rating because of the loads connected to it. Several high-power appliances on one extension lead can exceed its rating even without a short circuit. Overload is a kind of overcurrent, and can cause overheating. An RCD alone does not provide overload protection; that is a different protective function.