Cable sizing engineering principles
Selecting the proper cable cross-section requires balancing current-carrying capacity (ampacity) against voltage drop and thermal dissipation.
Under IEC 60364 and BS 7671, the tabulated conductor rating must exceed design current, and total voltage drop from origin to load must not exceed specified percentages (typically 3% for lighting and 5% for power circuits).
Worked example: Domestic radial circuit
A single-phase 230V radial circuit carries 16A across a 20-metre run clipped direct.
A 2.5 mm² copper cable rated at 27A produces 5.76V drop (2.50%), well inside the 3.0% threshold (6.90V maximum allowed).
Cable calculator inputs
Load Current
Total full-load current (Amps) of connected appliances.
Circuit Length
One-way distance in metres from distribution board to furthest point.
CALCULATOR QUESTIONS
Electrical cable sizing calculator questions
What is the maximum allowed voltage drop?
In standard low voltage installations, 3% is allowed for lighting circuits and 5% for other power circuits.
Why does cable length affect conductor size?
Longer cable runs increase electrical resistance, causing larger voltage drops and power losses unless conductor cross-section is increased.