The use of aluminum conductors in photovoltaic installations

Copper is not the only conductive material that can be used in electrical installations. The market is full of offers for aluminum cables, which are significantly lighter and cheaper (up to two thirds the price of a copper cable of the same specification) and which can be found with the same coverings, insulating properties and application voltages as traditional copper cables.

Therefore, it is not uncommon to find this type of cable in commercial and industrial installations. In fact, practically the entire public medium voltage distribution line is made up of aluminum cables.

If we already have the application of aluminum cable in some commercial and industrial installations, and if it has weight and cost advantages, the question arises: can we use aluminum cables in photovoltaic solar energy systems?

Limitations of Aluminum Cable

Aluminum has a density of 2.7 g/cm³, while copper has a density of 8.89 g/cm³ – a big difference, even more so if we consider the use of cables in a suspended overhead line.

Regarding resistivity, aluminum presents much higher numbers than copper: 28.2 Ω.mm²/km versus 17.2 Ω.mm²/km. Therefore, for the same current carrying capacity, the aluminum cable will have to have a larger cross-section than the copper cable.

As the voltage drop is directly proportional to the resistivity of the material, when using aluminum cables we must be more attentive to this phenomenon.

Understand more

Case study: voltage drop in the DC circuit according to NBR 16690

Photovoltaic inverter shutdown due to voltage variation

Below is Table 36 of the standard ABNT NBR 5410, in which it can be seen that, for the same cable section and installation method, aluminum conductors have a lower current carrying capacity.

Even with such economic attractiveness, we cannot use aluminum cables in any installation. The first and main limitation in relation to aluminum cables is their connections.

O Aluminum, when exposed to air, forms a layer of aluminum oxide on itself, which is insulating. As this insulating layer is on the order of a few micrometers, electrons can still pass through it, but this considerably increases the resistance of the connection, which can cause it to heat up too much. 

In addition to the problem of oxidation, as they are materials with very different electronegativity, we cannot make direct connections between aluminum cables and copper components. The two metals are galvanically incompatible.

Over time, the copper component, which is a more noble material, will corrode the connection with the aluminum, causing the same problems mentioned above.

Aluminum material has other undesirable properties for connections: its coefficient of thermal expansion is greater than that of copper and its malleability and yield limit are lower. 

Aluminum expands and contracts more easily with heat than copper, so there is a tendency for greater forces coming from the cable acting on its fixing system, which over time can cause the connections to loosen. 

Aluminum is also less malleable, that is, it breaks more easily and has a lower yield point. The yield point is related to the deformation that a material supports so that it can still return to its original shape. 

If we flex an aluminum sheet and a copper sheet with the same forces, there will come a time when the aluminum sheet will not return to its original state, remaining permanently dented, while the copper sheet will be able to return to being flat. A The same idea applies to cables.

Adding the effects of temperature to malleability and yield strength, we have that for some aluminum alloys (mainly older ones or those out of the norm for use in cables), natural heating causes the conductor to gradually lose its strength. original format, opening space for more oxidation attack, which worsens the connection and causes even more heat, forming an expansion cycle that ends with a loosening of the connection or, in extreme cases, even the cable breaking.

Aluminum cable installation

The facts shown above show that we must take special care when working with this type of cable. To prevent oxidation of connections, even those between aluminum and aluminum, we must use a specific anti-oxide paste for this purpose.

This paste contains flakes of other metals which, when pressed, break the aluminum oxide layer, thus improving their connection. Anti-oxide pastes also reduce contact with air and the cable's oxidation rate.

When connecting different metals, we need to use a special connector for this purpose, known as a bimetallic connector. This connector will reduce bimetallic corrosion between elements, as it is constructed with a metal compatible with copper and aluminum simultaneously.

Aluminum connections must also be mechanically reinforced so that there is no slack in the tightness or material leakage over time.  The connection with aluminum cables must always be carried out using suitable materials and by a qualified professional, given the specific characteristics of aluminum compatibility with other materials.

Application of aluminum cables according to standards

The NBR 5410 standard, which governs low voltage circuits, allows the use of aluminum cables, including making some adjustments to the current carrying capacity tables, given that the material is less conductive than copper.

The standard is also very specific when citing the limitations of aluminum connections. As a poor connection can lead to serious accidents and even major fires, and as previously seen that the aluminum connection has important installation features, the application of these cables is limited.

In industrial environments we can only use aluminum cables of at least 16 mm² and it is mandatory that the installation and maintenance of the cable is carried out by qualified people (electrical technicians, electrical engineers).

Aluminum cables can also be applied in commercial installations, but with greater restrictions such as: section equal to or greater than 50 mm², installation and maintenance carried out by qualified people and a location with easy escape conditions for people (BD1) – according to Table 21 BD classification of NBR 5410.

This excerpt from the NBR 5410 standard shows that the limitations of using aluminum cables are well defined:

Table 21 of the ABNT NBR 5410 standard, shown below, lists the conditions for people to escape with reference codes and examples of these situations. Several items in the standard refer to this table.

In residential installations or in places with a high density of people and/or with long and tumultuous leakage conditions (classification BD4), the use of aluminum conductors is strictly prohibited. In all scenarios, the splicing of aluminum cables can only be done using a compression connector or welding.

What do aluminum cables look like in photovoltaic systems?

Although it is possible to use aluminum cables in low voltage electrical installations, according to NBR 5410, the NBR 16690 standard, which governs photovoltaic systems, says that all direct current cables must have at least the same characteristics as solar type cables, which follow the NBR 16612 standard. 

Solar cables are exclusively made of copper, with no opening for using aluminum cables.  An explanation for this prohibition can be found when we think from the point of view of the electric arc in direct current. Direct current electric arcs are quite intense and dangerous. 

Separations between conductors that may occur due to inappropriate connections are even the biggest source of failures that cause electric arcs. Therefore, adding a risk of poor cable termination to a system that is especially sensitive to this problem becomes very dangerous.

Below is an excerpt from the ABNT NBR 16690 standard with the specifications of the conductors of photovoltaic arrays (direct current side). In this item there is a requirement that all cables meet the requirements of the NBR 16612 solar cable standard, which in turn only allows copper conductors.

In short, we can only use aluminum cables in the AC part of the photovoltaic system, and it is necessary to check whether the inverter and other connection points have an aluminum-compatible terminal internally or whether it will be necessary to use a bimetallic terminal. The anti-oxide paste recommendations remain unchanged.