§525 — Voltage drop: the 3%/5% limit and when it becomes decisive

The guide on cable cross-section & current-carrying capacity covers how Iz = Itabel × f1 × f2 × f3 determines whether a cable can thermally handle the current. That is not the only requirement a cable must meet: §525 of NEN 1010 also sets a recommended limit on voltage drop between the supply point and the load — a cable whose cross-section comfortably satisfies the Iz requirement can still produce too much voltage drop over a long cable run.

The recommended limit values

Application	Maximum voltage drop
Lighting	3% of nominal voltage
Other applications (socket outlets, motors, heat pumps)	5% of nominal voltage

On a 230 V installation this works out to roughly 6.9 V for lighting circuits and 11.5 V for other circuits, measured between the installation's supply point and the furthest load on that circuit.

Why this is a separate check

Voltage drop is not a thermal phenomenon like the Iz calculation, but a consequence of the resistive (and, at larger cross-sections, also inductive) impedance of the conductor itself: the longer the cable, the greater the voltage drop at equal current and cross-section. A cable that comfortably satisfies Iz ≥ Ib based on the correction factors from §523 can still, over a cable length of say 40-50 meters (an outbuilding, a shed, a long outdoor run to an EV charger), produce a voltage drop of more than 5% — the two checks are independent of each other and must both be performed separately.

A practical rule of thumb

A commonly used practical formula for copper conductors on single-phase circuits:

Uval (%) = (2 × L × Ib × cos φ) / (56 × A × Un) × 100

where L is the single (one-way) cable length in meters, Ib the load current in A, A the cross-section in mm², Un the nominal voltage, and 56 the conductivity of copper in m/(Ω·mm²). When in doubt about the exact constant for the material and frequency used: the tables in the NEN 1010 annex are authoritative, this formula is a practical rule of thumb for a first estimate.

Practical relevance

For long cable runs — a garden shed, a detached garage, an EV charger supply to the driveway — voltage drop is often the determining factor for the cross-section to choose, not the current-carrying capacity. A cable that would suffice at 2.5 mm² based on Iz sometimes needs to be upsized to 4 mm² or 6 mm², purely to stay within the voltage-drop limit.

Common mistakes

Performing only the Iz calculation and skipping voltage drop — especially on long runs to outbuildings this results in a cable that is thermally adequate but delivers a noticeably lower voltage at the load (dimming lights, slower motor start-up).
Applying the 3% limit to a mixed circuit with both lighting and socket outlets — the stricter 3% limit applies as soon as lighting is present on the circuit, not the more lenient 5% limit.
Calculating voltage drop over the straight-line distance instead of the actual (sometimes detouring) cable route — a cable that has to go around a structural obstacle is longer than the straight-line distance to the load.
Calculating voltage drop for the whole circuit from the transformer/meter instead of from the installation's supply point — §525 applies to the installation itself, not to the supplying network.