Line voltage drop

Voltage drop

Larger lighting installations can feature significant cable lengths. Based on technical connection requirements  issued by energy providers, the voltage drop in the main power supply system (connecting lines between the supply terminal and the electric meter) at a power requirement of up to 100 kVA must not exceed 0.5%.
According to HD 60364-52, the voltage drop up to the current-using equipment must not exceed a maximum of 4%, i.e. no more than 9.2 V for 230 V and no more than 16.0 V for 400 V.

The voltage drop in electrical lines is calculated according to HD 60364-52: Annex G using the following formula:

or

With:

ΔU

being the voltage drop in volts

b

being the coefficient b = 1 in three-phase circuits; b = 2 in single-phase circuits
Note: Three-phase circuits with an entirely uneven load (only on one phase) are considered single-phase circuits.

ρ1

being the specific electrical resistance of the conductors in uninterrupted operation.
The value at the temperature present in uninterrupted operation is used as the specific electrical resistance,
i.e. the specific electrical resistance at 20°C multiplied by 1.25, or 0.0225 Ω·mm²/m for copper and 0.036 Ω·mm²/m
for aluminium.

l

being the straight length of cable and line installation in metres

A

being the conductor cross-section

cos φ

being the power factor where known; otherwise a value of 0.8 (sin φ = 0.6) is assumed

X

being the reactance per length unit of the conductor where known; otherwise a value of 0.08 mΩ/m is assumed

IB

being the operating current

The relative voltage drop ΔUrel in percent is derived as follows:

With U0 being the respective system voltage in volts.

For purpose-designed luminaires, the following applies to the power factor according to EN 60598:

At a mains voltage of 230 V, this consideration leads to (φ = 0) in the worst case, meaning the following for the maximum permissible line length at a specified voltage drop of 9.2 V (4%):

In case of continuous line through-wiring, this means that

  • IB = 10 A current carrying capacity and A = 1.5 mm2 cable cross-section result in a cable length of 30 m if the overall current flows along the entire length, meaning that the full load is installed at the end of the conductor.

  • IB = 16 A current carrying capacity and A = 2.5 mm2 cable cross-section result in a cable length of 32 m if the overall current flows along the entire length, meaning that the full load is installed at the end of the conductor.

Where the load is evenly distributed along the line length, meaning the luminaires are spaced evenly, the result is that in a single-phase continuous line, the voltage drop based on the permissible maximum current in the conductor is not exceeded as long as the continuous line is not longer than 60 m/64 m. A three-phase continuous line may be twice as long if all three phases bear an even load.

If the maximum currents for the cable cross-section are not reached, the voltage drop is correspondingly smaller and greater line lengths can be realised, as displayed in figure 2.32. In practice, the voltage drop is hardly ever the limiting factor for line length, since elevated inrush currents in electronic control gear units, magnetic triggering of circuit breakers as well as sensitivity of positively-driven breakers and fault-current circuit breakers towards short-term current peaks usually limit the number of luminaires per outer conductor more significantly (see also chapter 2.1.9.5 and chapter 2.1.10.5 "Inrush currents").

Figure 2.32: Dependency of the permissible line length due to voltage drop (x-axis) on the utilised capacity of the conductor in the through-wiring of a continuous line (y-axis) in a single-phase system (left) and a three-phase system (right)