How is the reserve capacity of a power line calculated?

Modified on Wed, 15 Nov, 2023 at 11:23 AM

Balcony PV systems feed into the existing consumer circuit. The maximum current carrying capacity of the line may be exceeded on individual line sections.

Each line is protected by a circuit breaker. This switches off automatically as soon as a line overload occurs. Typically, several sockets and loads are protected by a common circuit breaker.

Due to the additional power of the balcony PV system, the currents from the public power supply network and the balcony PV system can add up. The current from the balcony PV system is not detected by the circuit breaker, which can lead to a line overload.

The following formula can be used to easily determine whether the existing line with the existing circuit breaker is sufficiently dimensioned:

Iz = In + Ig

Iz indicates the permissible current carrying capacity of the line, which should be greater than the sum of the rated current of the protective device In (circuit breaker) and the balcony PV system Ig.

The following table shows an example use case:

Load capacity of copper cables and lines for fixed installations in buildings with a nominal cross-section of 1.5 mm2 at an ambient temperature of 25°C and with 2 loaded cores. With a larger cross-section or a different type of cable, the permissible current carrying capacity is different, so that this must be considered separately in accordance with DIN VDE 0298-4.
Installation type	On thermally-insulated walls	In electrical installations	On walls	In the air
Current carrying capacity Iz	16,5 A	17,5 A	21 A	23 A
Line reserve (Iz-In) with a 16 A line circuit breaker	0,5 A	1,5 A	5 A	7 A
Line reserve (Iz-In) with a 13 A line circuit breaker	3,5 A	4,5 A	8 A	10 A
Solarnative PowerStick Balcony (PSB-350-xx)	1x (350 W)	2-4x (600 W limited)	2-4x (800 W limited)
Max. rated current Ig	1,5 A	2,6 A	3,5 A

In the example, the cable is designed for a continuous load of 16.5 A (in thermally insulated walls at 25°C). The cable reserve results from the difference between the current carrying capacity (Iz) of the cable with 16.5 A, minus the circuit breaker with 16 A (In). The cable reserve in thermally insulated walls is therefore 0.5 A. If the rated current Ig of the balcony power station exceeds the line reserve of 0.5 A, the line circuit breaker should be replaced with a smaller one in order to comply with the requirements of the DIN VDE 2948-4 standard. By replacing the 16 A circuit breaker with a 13 A circuit breaker, only 13 A can now be drawn from the public power supply network, resulting in a line reserve of 3.5 A and a balcony power station with up to 800 W generation power can be connected to the consumer circuit.