Check 1: current-carrying capacity
Find the design current Ib. For a three-phase load, Ib = P ÷ (√3 × U × cos φ). Then choose a size whose tabulated rating, after correction, is at least the breaker rating In. The correction factors come from IEC 60364-5-52 (or the maker's tables): ambient or ground temperature, how many circuits are grouped together, and the installation method (in air, on tray, in conduit, buried). In Saudi summers the temperature factor alone can cut the usable rating noticeably.
Check 2: voltage drop
Long runs lose voltage. IEC 60364-5-52 suggests limits of about 3 % for lighting and 5 % for other uses (for installations fed from the public low-voltage network); the project specification may set its own. The quick method uses the maker's mV/A/m value for the cable size and installation:
Voltage drop (V) = mV/A/m × Ib (A) × length (m) ÷ 1000
Worked example
A 400 V three-phase feeder carries 50 A over 80 m in a copper multicore XLPE cable. If the cable data sheet gives about 2.5 mV/A/m for 16 mm², the drop is 2.5 × 50 × 80 ÷ 1000 = 10 V, which is 2.5 % of 400 V: within a 5 % limit. If the run were 200 m, the drop would be 25 V (6.25 %) and you would move up a size. Always use the mV/A/m and current ratings from the data sheet of the cable you buy.
Check 3: protection and short circuit
The breaker must protect the cable: Ib ≤ In ≤ Iz, and the cable must withstand the fault current until the breaker clears it. For long final circuits, check that the earth fault loop is low enough for the breaker to trip in the required time; this sometimes forces a larger size than the current rating alone.
Use our calculator, then send the list
Our cable size calculator applies these checks with correction factors for Saudi conditions. When you have the sizes, send the list and we quote the cable with matching glands, lugs and breakers on one offer.
Common questions
What is the maximum allowed voltage drop?
IEC 60364-5-52 gives about 3 % for lighting and 5 % for other circuits in installations supplied from the public LV network. Project specifications or the utility may set different limits.
Why do I need a bigger cable in summer conditions?
Cable ratings are tabulated for a reference temperature. Higher air or ground temperatures reduce how much current the cable can carry, so a correction factor below 1 applies.
Does a bigger breaker need a bigger cable?
Yes. The cable must be able to carry at least the breaker rating after corrections (In ≤ Iz). Raising the breaker without upsizing the cable leaves the cable unprotected.
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This guide is general information. Final selection must follow the project design, the applicable standards and the maker's data.