Solar panels generate energy that is fed back into the grid. To ensure that this can happen, the inverter will always make the voltage a little higher than the connection to the grid so that the power goes into the grid. To make sure that the voltage does not get too high, it is important to use the right cables. There is a maximum voltage to prevent other connected devices from breaking down (in the Netherlands this is 253 Volt)..

Maximum voltage difference in a cable


A common value for the maximum voltage difference in a cable is 1%. With 230 volts, we therefore recommend that there should be no more than 2.3 volts difference. If this loss increases, the cable will become warmer and one must ensure that it is adequately cooled. If a cable cannot dissipate its heat, a fire or short-circuiting may occur.

In addition to selecting the correct value for the fuses and circuit breakers, it is therefore important to check that all the cables are suitable for feeding back the power before installation.

Calculation example of voltage drop with a 1000 watt inverter


At maximum power and a voltage of 230 V, the inverter delivers a current of:
1000 W/230 V = 4.34 A.
A cable with a length of 43 metres and 2.5 mm2 is connected.
The resistance of a wire in the cable is:
R = 0.0175 x 43 /2.5 = 0.3 ohms, total (for both wires) therefore 0.6 ohms.
The voltage drop in the cable is then:
V = I x R = 4.34 A x 0.6 ohm = 2.6 Volt

So the loss is: 2.6 V /230 V = 1.1%.
In this case, a 1000 W inverter can be connected with a maximum cable length of 43 metres and the voltage loss is acceptable at around 1%. If you want to reduce the loss in the cable, you must connect a thicker cable, e.g. 4 mm2.

Maximum length of a cable


The following table calculates the maximum cable length for a number of inverters (assuming a voltage drop of around 1%).

wdt_ID Dikte (mm2) 1 kW 3 kW 5 kW 5 kW (3 fasen) 6 kW (3 fasen)
1 2,50 43 15 7
2 4,00 69 24 12 53 43
3 6,00 105 36 18 80 65
4 10,00 171 57 28

It is clear from the example of the 5 kW inverter that a three-phase inverter is more favourable when an electrical system is already equipped with three phases.

All calculations above are intended as a guide. If, for example, the mains voltage supplied in a system is higher, it may be advisable to choose a thicker cable or to adapt the system.

You can always contact Autarco for advice and suggestions, if you are going to install a larger inverter in an existing installation. This can prevent high costs afterwards.