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 of the price of a copper cable of the same specification) and which can be found with the same coatings, 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 all medium-voltage public distribution lines are made of aluminum cables. If we already have the application of aluminum cable in some commercial and industrial installations, and if it has the advantages of weight and cost, 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 has 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. Since the voltage drop is directly proportional to the resistivity of the material, when using aluminum cables we must be more aware of this phenomenon.
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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 regarding aluminum cables is their connections. Aluminum, when exposed to air, forms a layer of aluminum oxide, which is an insulator.
As this insulating layer is on the order of a few micrometers, electrons can still through it, but this considerably increases the resistance of the connection, which can cause it to overheat.
In addition to the oxidation problem, since these are materials with very different electronegativities, 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 has other undesirable properties for connections: its coefficient of thermal expansion is higher than that of copper, and its malleability and yield strength are lower. Aluminum expands and contracts more easily with heat than copper, so there is a tendency for greater forces from the cable to act on the cable's fastening 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 s so that it can still return to its original shape.
If we bend 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, but will remain permanently dented, while the copper sheet will be able to return to being flat. 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 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 locations with a high density of people and/or with long and tumultuous escape conditions (BD4 classification), the use of aluminum conductors is strictly prohibited. In all scenarios, aluminum cables can only be spliced using compression connectors 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 made exclusively of copper, and there is no provision for the use of aluminum cables. An explanation for this prohibition can be found when we consider the direct current electric arc. 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.
Answers of 10
The bidirectional solar energy meter at my residence caught fire twice. I was informed that I have to change the input branch to the meter, which is made of aluminum, and replace it with copper, otherwise it will always burn.
Excellent article Matthew. The description allows easy understanding by laypeople and even more technical people.
It is important to add that the requirements of only copper cables in the DC only apply in Brazil. In many other countries, the use of aluminum cable is already applied in photovoltaic installations.
Hugs!
Great article. Very objective and timely at this time of many UFV assemblies with high costs, some cost reduction is always sought and cables would be a variable. Thanks to the excellent article and technical observations, this situation is ruled out.
Congratulations Mateus, very well prepared article!
I really liked the subject. Congratulations.
Good morning, gentlemen, images in figure 5 were services carried out in Paraná by the company HL Engenharia Elétrica!
We added the information in the image caption 🙂
Very good.
Congratulations on the article, very well explained technically!!
Excellent article Matthew!