Voltage drop calculation ΔU - NIBT 2020
Online calculator to check the voltage drop ΔU of a low-voltage line according to NIBT 2020. Enter current, length, cross-section and operating conditions - the result complies with NIBT articles 5.2 and takes into account the operating resistivity at 70 °C (PVC) or 90 °C (XLPE).
What is voltage drop?
Voltage drop (denoted ΔU) is the potential difference lost along a conductor between the switchboard and the point of use, caused by the ohmic resistance of the cable and, at larger cross-sections, by its inductive reactance. It is expressed in volts or as a percentage of the nominal voltage.
An excessive drop under-supplies the loads: it shortens motor life, makes lighting flicker, derates variable frequency drives and can prevent inductive loads from starting. Checking ΔU is therefore a mandatory step in sizing an LV line.
NIBT 2020 limit values
The NIBT 2020 (chapter 5.2) imposes a maximum of 3 % for lighting and 5 % for socket outlets and motive power. For installations supplied by a private transformer, the overall limit rises to 8 %, the remaining margin between the main distribution board and the point of use then having to be split between the riser, secondary distribution and final circuits.
- 3 % - lighting circuits (all types)
- 5 % - socket-outlet and motive power circuits
- 6 à 8 % - installations on a private transformer (overall limit)
Applied formulas
- Single-phase
- ΔU = 2 × L × I × (ρ × cos φ / S + λ × sin φ)
- Three-phase
- ΔU = √3 × L × I × (ρ × cos φ / S + λ × sin φ)
- DC
- ΔU = 2 × L × I × ρ / S
The resistivity of aluminium is 1.6 times higher than that of copper: for the same cross-section, an Al cable causes a voltage drop about 60 % higher than a Cu cable.
Go further with ElectroCAD
Discover the ElectroCAD suite or check out our other electrical calculation tools - protection sizing, circuit breaker rating, current-carrying capacity, selectivity.
Frequently asked questions
What is the maximum permissible voltage drop according to NIBT 2020?
3 % for lighting circuits and 5 % for socket and power circuits. Up to 8 % overall limit for installations on a private transformer (NIBT 2020 § 5.2).
What is the difference between copper and aluminium cable?
The resistivity of aluminium is about 1.6 times higher than that of copper. For the same cross-section and the same current, the voltage drop of an Al conductor is therefore about 60 % higher than that of an equivalent Cu conductor.
Why distinguish between PVC and XLPE?
The maximum conductor temperature in continuous operation is 70 °C for PVC insulation and 90 °C for XLPE. Since the resistivity ρ depends on temperature (α ≈ 0.004 K⁻¹), a loaded XLPE cable has a slightly higher resistance and therefore ΔU than an equivalent PVC one.
Does the calculation take the reactance X into account?
Yes. The per-unit-length reactance λ is included from 16 mm² upwards according to the CENELEC bands. Below that, its contribution is negligible compared with the ohmic resistance.