Cable Sizing Calculator

Most cable sizing calculators online assume a 30°C ambient. On a UAE site in August that assumption will undersize your cable by a full size or more. This one derates properly for temperature, grouping and installation method.

How this calculation works

Sizing a cable is four steps, and skipping any one of them is how cables end up running hot.

Design current. Start with the load. For a three phase load: I = (kW × 1000) ÷ (V × √3 × power factor). For single phase: I = (kW × 1000) ÷ (V × power factor).

Protective device. The breaker or fuse rating must be at or above the design current, and at or below the cable’s derated capacity. The cable protects itself only if the device trips before the cable overheats.

Base current rating. Every conductor size has a tabulated current-carrying capacity, and it depends heavily on how the cable is installed. A cable clipped to a wall in free air carries substantially more than the same cable buried in insulation or bunched in a conduit with five others.

Derating — this is where UAE sites differ. Three factors multiply together:

Ambient temperature. Tabulated ratings assume 30°C for cables in air. At 45°C ambient the correction factor for a 70°C thermoplastic cable is around 0.79 — meaning the cable carries roughly 21% less than the table says. For cables in direct sun on a site, surface temperatures go considerably higher again.

Grouping. Cables bunched together can’t shed heat. Six circuits in one tray might carry only 57% of their individual rating.

Installation method. Buried, in conduit, in trunking, in free air — each has its own correction.

Multiply all three, apply to the base rating, and compare against your design current. If it falls short, go up a size and check again.

Then check voltage drop separately. On long site runs, voltage drop usually governs the cable size, not current capacity. A cable can be perfectly adequate thermally and still deliver unusable voltage at the far end. Use the voltage drop calculator to confirm.

Worked example

A 75 kW pump 120 metres from the distribution board, three phase 400 V, 0.85 power factor, cables in a tray with three other circuits, ambient 45°C.

Design current: (75 × 1000) ÷ (400 × 1.732 × 0.85) ≈ 127 A.

Select a 160 A protective device. Now the derating:

• Ambient 45°C: ×0.79
• Grouped, four circuits on a tray: ×0.77
• Combined: ×0.61

The cable needs a base rating of at least 160 ÷ 0.61 = 262 A to carry a 160 A device safely under these conditions. That’s a substantially larger conductor than the 127 A design current alone would suggest — roughly double.

Then check voltage drop over 120 m. At that length it may well push you up another size again.

The point of the example: the honest answer here is more than twice the cable a naive calculation gives. Cables sized on design current alone, without derating, are the most common electrical fault we’re called to on UAE sites.

Common mistakes

Using a calculator that assumes 30°C. The single biggest source of undersized cable in this region.

Forgetting grouping. One cable on a tray and six cables on a tray are very different installations.

Ignoring voltage drop on long runs. Anything over about 50 metres deserves a check.

Sizing to the load instead of the protective device. The cable has to survive whatever the breaker will let through, not just the normal load.

Reusing a cable size from a previous job. Different route, different grouping, different answer.

FAQ

Why do UAE sites need larger cables than the tables suggest? Because published current ratings assume 30°C ambient. At 45°C a cable carries roughly 20% less. Bunch it with other circuits and you can lose 40% or more of the tabulated capacity.

What derating factor should I use for UAE ambient? For 70°C thermoplastic insulated cable in air, roughly 0.79 at 45°C and 0.71 at 50°C. XLPE cables tolerate heat better. Always check the manufacturer’s tables for the specific cable — these are indicative.

Does voltage drop or current capacity determine cable size? Whichever gives the larger cable. On short runs, current capacity usually governs. Beyond roughly 50 metres, voltage drop often takes over.

What size cable do I need for a 100 kVA generator? A 100 kVA set at 400 V delivers around 144 A. After derating for a 45°C ambient and typical grouping you’re likely looking at 70–95 mm² per phase, depending on run length and installation method. Run your actual figures through the calculator.

Do I need to size the neutral the same as the phases? For balanced three phase loads, usually not. But with significant non-linear load — VFDs, LED lighting, IT equipment — triplen harmonics add in the neutral and it can carry more current than any phase. Size it full where harmonic content is high.