Cable Selection
Free sample · full course section in PlusC&G 2365 Level 3 · Installation
Choosing the right cable: how much current it can carry, how heat and bundling reduce that, and how far it can run. Practised one step at a time.
- Current-carrying capacity
- Correction factors
- Voltage drop
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Original public revision material · A4:2026. For revision only; do not use samples to guide work on site.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Explore key terms
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Current-carrying capacityIz
The current a cable can carry continuously in its installed conditions without its insulation exceeding the temperature it was designed for. Tabulated values assume reference conditions; the real, installed capacity is what remains after correction factors for temperature, grouping and insulation have taken their share.
Read the full Current-carrying capacity glossary entryVoltage drop
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.
Read the full Voltage drop glossary entryCorrection factors
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.
Read the full Correction factors glossary entryDiversity
The design allowance for the fact that connected loads never all run flat-out at once. A kitchen's sockets, oven and kettle sum to a huge figure on paper, but the realistic simultaneous demand is far lower. Applying diversity turns the connected load into a sensible maximum demand, keeping supplies, switchgear and cables from being absurdly oversized.
Read the full Diversity glossary entry
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