Stop Guessing Cable Sizes: The Quick Guide That Saves Your Install
Cable sizing is where many DIY and rushed installs go wrong. A cable that “looks thick enough” can still overheat, drop voltage, or fail inspection. Getting this right protects equipment — and people.
What actually decides cable size?
- Current (amps) the circuit will carry continuously
- Length of the run (voltage drop grows with distance)
- Installation method (clipped direct, conduit, buried, bundled)
- Ambient temperature and grouping with other cables
- Protective device rating (MCB/MCCB must protect the conductor)
Rules of thumb (start here, then verify)
These are starting points for typical copper PVC cables in normal conditions — always confirm with local standards and manufacturer tables:
- Lighting circuits (up to ~6–10 A): commonly 1.5 mm²
- Socket circuits (up to ~16–20 A): commonly 2.5 mm²
- Cooker / water heater (20–32 A+): commonly 4–6 mm²
- Small motors / workshops: size for full-load amps and starting current
Long runs need a thicker cable even if the amps look low — voltage drop makes LEDs flicker and motors struggle.
Three mistakes we see often
- Upsizing the breaker instead of the cable — That hides the symptom and raises fire risk.
- Ignoring aluminium vs copper — Different materials need different sizes and terminations.
- Mixing colours / poor labelling — Future you (or the next electrician) will thank you for clear phase, neutral, and earth identification.
Quick checklist before you energise
- Continuity and insulation resistance tests
- Earth continuity to metalwork and boards
- Correct polarity at sockets and switches
- Labels on DB ways matching the schedule
- Cable supports and bend radii that will not crush insulation
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