As is the case with many installations, a solar system is only as strong as its weakest link. One link that should certainly not be overlooked is the cable between the inverter and the meter box. After all, incorrect sizing of this cable can lead to yield loss, faulty equipment or, in the worst case, short circuiting or fire.

A solar system generates energy that is delivered to the grid when it is not being consumed directly on site. But if the cable connecting the inverter to the grid does not have sufficient capacity, the inverter will not be able to deliver the generated energy resulting in a loss of yield. The inverter then no longer functions optimally and the entire system becomes less efficient.

The inverter also always has a slightly higher voltage than the grid to which it is connected, so the generated electricity flows to the grid. This voltage rises if the cables are not properly dimensioned. However, there is a maximum to this voltage; the Dutch standards in this area prescribe 253V, to ensure that devices connected to the same grid are not damaged.

In addition, the cables will generate heat when overloaded, which in extreme cases can lead to fire or short circuit. Especially when several cables are installed close together, this is a risk.

In addition to choosing the correct values for fuses and circuit breakers, it is therefore important to check that all cables are suitable for feeding back the power before they are installed. To avoid the above risks as much as possible, a maximum voltage drop in the cable between the inverter and the meter box of 1% is assumed. In the case of a 230V grid connection, this would therefore be 2.3V.

How do we put this into practice?

To illustrate this, let’s take a 1000W inverter as an example. At maximum power and a voltage of 230 V, the inverter delivers a current of: 1000 W/230 V = 4.35 A.

If we want to use a cable of 2 x 2.5 mm², what is the maximum length of this cable where the voltage loss remains under 1%, or 2.30V?

Based on current and voltage, we can calculate the maximum resistance for this cable. This is in this case: R = V / I = 2.30 / 4.35 = 0.53 Ω

So for a single conductor in the cable, the resistance should not exceed 0.53 / 2 = 0.265 Ω. The formula for calculating the resistance of a cable of a given length, thickness, and material is as follows: R = L x ⍴ / A. From this we can deduce that L = R x A / ⍴ = 0.265 x 2.5 / 0.0175 = 37.85 m.

Therefore, if you want to reduce the loss in the cable, or if the length between the inverter and the grid connection is greater than 37.85 m, a thicker cable must be connected.

More examples

In the following table, the maximum cable length is calculated for a number of different inverters and cable diameters. Here we also assume a mains voltage of 230V and a maximum voltage drop of 1%.

wdt_ID Aderdikte (mm²) 1 kW 3 kW 5 kW 5 kW (3-fase) 6 kW (3-fase)
1 2.5 38 13 8 23 19
2 4 60 20 12 36 30
3 6 91 30 18 54 45
4 10 151 50 30 91 76

This table clearly shows the beneficial effect of using a 3-phase inverter. After all, for the same power of 5kW, the current is divided over three phases. A lower current means less voltage loss, which allows you to use a thinner cable, or to bridge longer lengths with the same cable diameter.

Need more information?

The above calculations are intended as a guide to provide insight into sizing the AC cabling in a PV system. In individual cases, calculations may turn out differently or there may be other requirements for the cabling. We strongly recommend that you pay attention to this when designing a PV system, because good preparation can prevent high costs later. You can of course always contact Autarco for advice and suggestions in this area.

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