Measuring voltage drop in practice — multimeter versus calculation

The guide on §525 covers the permitted voltage-drop limits (3% for lighting, 5% for other circuits) and the accompanying calculation formula based on cable length, cross-section and load current. This article covers an additional, practical question: how voltage drop is actually measured on an existing installation, rather than only calculated — and which common measurement mistakes produce a misleading result.

Why a calculation does not always suffice

The calculation formula from the §525 guide assumes the nominal conductor resistance at a reference temperature (typically 20 °C) and the specified load current Ib. In practice, the actual situation deviates from this in two ways: the conductor temperature under operating load is often considerably higher than 20 °C (for a PVC-insulated cable, up to roughly 70 °C at full load), which raises copper resistance by roughly 20% or more relative to the 20 °C value, and the actual load current at a given moment can deviate from the design value Ib. A measurement under real operating conditions therefore gives a more direct picture of the actual voltage drop than the theoretical calculation alone.

The measurement method: voltage at the source and at the load, under load

To actually measure voltage drop, voltage is measured simultaneously (or shortly after one another, during a stable load) at two points: at the connection point of the circuit (for example the circuit breaker in the distribution board) and at the furthest load on that same circuit — while the circuit is actually carrying a representative load, as close as possible to the design value Ib or the actual expected operating load. The difference between the two voltage readings, expressed as a percentage of the nominal voltage, is the measured voltage drop.

Why a measurement without load is meaningless

Voltage drop is a direct consequence of Ohm's law (U = I × R): without current through the conductor there is — aside from measurement noise — no voltage drop to measure, regardless of cable length or cross-section. A common mistake is measuring the voltage at the load without an actual, representative load connected or switched on during the measurement: this almost always produces a voltage value that barely differs from the source voltage, wrongly suggesting there is no voltage-drop problem. For a meaningful measurement, the circuit must actually be delivering current to a load representative of normal use during the measurement.

Why R1+R2 is not a substitute for a voltage-drop measurement

The R1+R2 continuity measurement (see the related guide on this test) determines the resistance of the phase and protective conductor in series, using a small, fixed test current that is far below the circuit's actual operating current. This is a valuable measurement for verifying earth-fault loop impedance, but it gives no direct statement about the voltage drop the installation actually experiences during normal operation, at the real load current. An installer who only measures R1+R2 and concludes from that alone that the voltage drop is acceptable is conflating two different measurements with a different purpose.

Instrumentation: two multimeters, a data logger, or a dedicated tester

In practice, voltage drop is measured with two simultaneously read multimeters (one at the source, one at the load), with a voltage data logger that records at both points simultaneously (useful for a load that varies over time, such as a starting motor), or with a dedicated installation tester that has an integrated voltage-drop function. When using two separate multimeters, it is important that both instruments are read simultaneously — or at least within the same stable load period — because a varying load (for example a cooling compressor switching on elsewhere in the installation) can otherwise distort the difference between the two readings.

Practical relevance

For a complaint about dimming lighting, slower motor start-up, or a noticeable voltage dip when switching on a heavy load on a long run, an actual voltage-drop measurement under representative load is a more direct diagnostic tool than a purely theoretical recalculation based on the specified cable length and cross-section alone — especially when the actual conductor temperature or load current is uncertain or deviates from the original design assumptions.

Common mistakes

Measuring voltage at the load without an actual, representative load switched on — without current there is by definition hardly any voltage drop to measure, regardless of any underlying problem.
Confusing an R1+R2 continuity measurement with a voltage-drop measurement — the small, fixed test current of an R1+R2 measurement is not representative of the circuit's actual operating load.
Not taking both voltage readings simultaneously or during the same stable load period — a varying load elsewhere in the installation can distort the measured difference.
Not accounting for the higher conductor temperature under operating load when comparing a measured value with a calculation based on the 20 °C reference resistance — this can produce an apparent discrepancy between measurement and calculation that is in reality explained by the temperature effect.