How to Size The Solar Cable for Your Photovoltaic System
How to Size The Solar Cable for Your Photovoltaic System

How to Size The Solar Cable for Your Photovoltaic System

When it comes to photovoltaic solar energy installations, one of the most common problems is inadequate solar wire sizing. This can lead to dangerous situations, such as overheating and burning solar wires in the electrical system.

In this article, I will show you how to correctly size the solar cables for the solar inverter, avoiding future problems. I will address the criteria for low-voltage electrical installations and provide a step-by-step guide for sizing alternating current cables.

The alternating current part of the photovoltaic system must meet the general conditions of low-voltage electrical installations.

The objective is to maintain the safety of people and animals, preserve material goods, and correctly function electrical installations, applying to all types of buildings with low voltage installations.

Solar Cable Sizing Step-By-Step

1. Inverter Choice

The first step to sizing the solar PV cables is to choose the inverter used in the system. It is necessary to know the nominal output power of the inverter, which will be used to determine the current that will circulate through the cables.

2. Minimum Section of Drivers

The alternating current solar PV cables must meet the general conditions of the standard. The section of the phase cables cannot be less than the value specified in Table 47. As with a photovoltaic system, the recommended minimum section is 2.5 mm² for power circuits.

3. Current Conducting Capacity

The next step is to determine the design current of the circuit, which corresponds to the rated output current of the drive. This information can be found in the equipment datasheet. In addition, it is necessary to know how the cables will be installed.

4. Number of Conductors Loaded

Based on the inverter’s output voltage, it is possible to determine the number of loaded conductors. For example, if the inverter output is 220V and the customer’s network is two-phase with a voltage of 220V between phases, two phases will be required to power the inverter.

5. Cable Insulation Type

For an indoor installation, it is possible to use solar cables with PVC/70ºC insulation, which comply with the standard. It is essential to consider the ambient temperature of the installation site to ensure adequate cable insulation.

6. Correction Factors

There are two correction factors that can influence solar wire sizing: the grouping correction factor and the temperature correction factor. These factors consider the number of circuits that pass through the same conduit and the ambient temperature of the installation site, respectively.

7. Voltage Drop Check

In addition to current dimensioning, checking the voltage drop along the circuit is essential. The maximum voltage drop allowed in a circuit must not exceed 4% for supply circuits and 6% for terminal circuits. To calculate the voltage drop, it is necessary to know the resistance of the cables, which can be found in the manufacturers’ tables.

8. Solar Cable Protection

Another aspect to consider is solar cable protection. They must be protected against overcurrents using suitable circuit breakers or fuses, according to the design current of the circuit.

9. Seek Qualification

Although this guide provides an overview of the solar cable sizing process for solar inverters, it is always recommended that a qualified professional perform a detailed and accurate analysis of the electrical installation.

A specialist electrician or electrical engineer can assess site-specific conditions and provide personalized advice. Therefore, if you work with photovoltaic installations and projects, keep updated and seek additional training whenever necessary.

Dimensioning example

Let’s consider an example of solar cable sizing. Let’s assume that the installation is carried out in a shed with an ambient temperature of 35ºC and that the inverter circuit is together with a socket circuit. That is, two circuits are running in the same conduit. In this installation, I will start with 4mm² cables.

Now let’s take the current that the 4mm² cable supports is 32A and apply the correction factors.

Consulting Table 42, the way of grouping the conductors in our case is in a closed conduit with 02 circuits within the same conduit. Therefore, the correction factor per grouping will be 0.8.

In Table 40, as we consider an ambient temperature of 35ºC and the solar wire insulation is PVC, the temperature correction factor will be 0.94.

To correct the current carrying capacity of the solar wire, multiply the current that the solar cable supports by Table 36 by the correction factor by grouping and by the correction factor by temperature. Therefore, the corrected current becomes 32 x 0.8 x 0.94 = 24A

Therefore, under these conditions, I cannot use the 4mm² cable because the inverter current is 27.3A and the cable only supports 24A.

The solution in this case, will be to increase the cable section. Checking Table 36, the 6mm² cable supports 41A for the B1 installation method and two conductors loaded. Making the correction calculations for the current carrying capacity, we will have a corrected current of 41 x 0.8 x 0.94 = 30.83A. Now yes!

The correct sizing of the solar cables for the solar inverter is essential to guarantee a safe and efficient electrical system. By following the general guidelines and considering factors such as inverter power, design current, cable section, and voltage drop, it is possible to avoid problems such as overheating and loss of system performance. Always remember to rely on the guidance of a specialized professional to ensure proper electrical installation.

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