Cable Selection and Sizing Explained

Selecting the right cable is a core part of installation design. BS 7671 sets out how to choose a conductor with the correct current-carrying capacity for its circuit and environment.

What affects cable selection

• Design current — the load the circuit must carry.
• Installation method — clipped direct, in conduit, in insulation, buried, etc.
• Grouping — cables bunched together derate one another.
• Ambient temperature — hotter surroundings reduce capacity.
• Voltage drop — the conductor must be large enough to keep voltage drop within limits.
• Protection coordination — the cable must be protected by its device against overload and fault.

Why it matters

An undersized or poorly installed cable can overheat, degrade its insulation, and become a fire risk. An oversized one wastes money. Correct selection keeps the circuit safe and efficient across its life.

On an EICR

Evidence of overheating, unsuitable cable for the environment, or conductors too small for their protective device is recorded and coded by risk. Correct selection also underpins safe additions such as EV charging.