Cable Sizing
Installation · C&G 2365 Units 203/305 · Full course section in Plus
Picking a cable is a chain of checks: start from the design , choose protection, then de-rate for how the cable actually runs — grouped with others, buried in insulation, passing through hot spaces. The tabulated capacity that survives those has to beat the current the protective device lets through.
Then comes : a cable that carries the happily can still deliver mains too weak at the end of a long run, so length becomes part of the calculation. BS 7671 sets the limits; the arithmetic is yours.
Our lessons build the method step by step with calculators inline, then drill it with practice questions that change the numbers — because the exam certainly will.
Free interactive sample
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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.
Common questions
- Can I try Cable Sizing free on this page?
- Yes. The public sample below accepts answers and gives feedback and retries without an account, payment or energy. It is a short revision sample, not the complete course or a full mock paper. The full course section requires Plus.
- Where does Cable Sizing sit in the City & Guilds scheme?
- Cable Sizing maps to Units 203/305. On TradeTraining it lives in the Installation section as bite-size lessons with practice questions.
- What factors affect cable sizing?
- The design , the protective device rating, and the installation conditions: ambient temperature, grouping with other circuits, thermal insulation and the installation method all de-rate a cable's tabulated capacity. over the run length then sets a second, independent limit.
Explore key terms
Open a term to read its meaning here, then close it or explore another. Free to use without an account.
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