Find the smallest copper cable that safely carries a load and keeps voltage drop within limits, for a given run length and installation method. Results update as you type.
The design current the cable needs to carry, and the phase configuration.
Copper, PVC-insulated conductors. One-way cable run length.
Derating for ambient temperature and cables grouped together.
The cable must be able to carry the design current continuously without overheating. Ambient temperature and grouping with other circuits both reduce a cable's safe carrying capacity, so the design current is scaled up before comparing it against the cable's rated (tabulated) capacity.
The smallest cable whose tabulated rating (Iz) is ≥ It is selected.
The breaker protects the cable, so its rating must sit between the design current and the cable's actual on-site carrying capacity — using a standard breaker size, since you can't buy a breaker rated at an arbitrary in-between value.
Where Ib is the design current, In is the breaker's standard rated current, and Iz is the cable's actual carrying capacity after temperature and grouping derating. If no standard breaker size fits this range for a given cable, the calculator steps up to the next cable size.
Even if a cable can carry the current safely, a long run can drop too much voltage before it reaches the load. If the selected cable's voltage drop exceeds the allowable percentage, the calculator steps up to the next cable size and re-checks.
Current ratings and mV/A/m figures are typical values for PVC-insulated copper conductors, based on commonly published IEC/SANS-style reference tables (30°C ambient, single circuit). Standard breaker sizes reflect a common IEC 60898-style MCB range. They are indicative for preliminary sizing only.
This tool provides an indicative cable size and breaker rating based on typical published current-carrying capacity and voltage drop tables for PVC-insulated copper conductors. It does not replace the current SANS 10142-1 tables, correction factors specific to your exact installation method, or professional judgement. Always verify the final cable and breaker selection against SANS 10142-1 and have the design reviewed by a qualified electrician before installation.