Measuring voltage drop under load

Annex G (table G.52.1) recommends max. 3 % voltage drop for lighting and max. 5 % for other installations when the installation is supplied directly from the public low-voltage network (type A). With a private supply, e.g. its own transformer (type B), the figures are 6 % and 8 %. The annex is informative, not a legal requirement. Lighting is more sensitive and therefore has the stricter limit.

Load the circuit with the load it is designed for. The voltage drop increases with the current, so a measurement without load says nothing about the operating situation.

Measure the voltage at the start of the circuit (the board) and at its end (the load) simultaneously, or use the instrument's built-in voltage drop function (Z measurement), which calculates the drop from the loop impedance.

Convert the voltage drop into a percentage of the nominal voltage (230 V phase / 400 V line). Compare with the formula ΔU = (k · L · I · cos φ) / (κ · A) from the voltage drop calculator to reveal unexpected deviations.

If the 3 %/5 % limit is exceeded, the cable is too long or too thin for the load. The solution is a larger cross-section, a shorter run or relieving the circuit — not ignoring the drop.

Even if the voltage drop is acceptable, current-carrying capacity (installation method) and short circuit must always be checked separately according to the DS/HD 60364-5-52 tables. The largest of the three requirements determines the cross-section.

Record the measured load together with the voltage drop — a drop without a stated load cannot be verified.