Which cable cross-section do I choose?

You do not choose a cross-section by feel, nor by copying the previous cable. Four steps, in this order: design current, current rating, voltage drop and short-circuit withstand. The largest cross-section that comes out of those steps is the cable.

The design current is step 1. For a three-phase load of 40 kW at 400 V with cos φ 0.9 that is 40,000 / (√3 × 400 × 0.9), over 64 A. Calculate with the actual load, not just the nominal rating of the protective device.

Then the current rating. The datasheet gives the permissible current per installation method: in the ground, in air or in conduit, for a single circuit. On top of that come correction factors for ambient temperature and grouping from NEN 1010 and VDE 0298-4. Six grouped circuits in air nearly halve the rating.

Step 3, voltage drop, is the one that most often decides the size. A second example: a copper XLPE cable such as YMvK carries 63 A three-phase over 80 metres, at cos φ 0.9. In 10 mm² on a wall or in a tray (installation method C), that cable still passes on current rating, but the voltage drop at operating temperature rises to over 4.5 percent. With a 3 percent limit it becomes 16 mm². On long routes this is almost always the step that sets the cross-section.

That leaves the short-circuit withstand: can the cable carry the fault current until the protection trips? On short routes with high-rated protection, this step occasionally still calls for the next size up. For medium voltage, the screen cross-section counts as well.

The Cross-section tab of our cable selector estimates steps 1 to 3 and shows the assumptions. For the specification we verify the result with the quote, including the choice between copper and aluminium and the drum lengths. Our advice is an aid; the responsible designer or installer checks it and makes the decision.