EuroWire January 2023

Technical Article

the insulation, sheath and armour materials of the aluminium conductor weaken the price advantage of using the aluminium conductor. That is to say, the price advantage of aluminium over copper is not great, because the conductor is only a small part of the cost of the whole cable. Taking 70mm 2 copper core cable as an example, the cost of copper conductor accounts for about 65 per cent of the cost of the whole cable, while the cost of 120mm 2 aluminium core cable with equal current-carrying capacity can be saved by 23.5 per cent. If the cost of copper conductor accounts for 80 per cent of the cost of the whole cable, the cost savings of aluminium core cable with equal current-carrying capacity can reach 46 per cent. Tensile strength The tensile strength of copper is approximately twice that of aluminium, but it is worth noting that given the equivalent aluminium conductor is bigger and lighter it often does not require the same degree of tensile strength. Behaviour at short circuit When a short circuit occurs, copper conductors protect their mechanical strength better than aluminium conductors. The use of aluminium may cause pulling and twisting of the rigid conductors, and may cause bending, loosening or compressing of the flexible conductors. Aluminium conductors are not as durable as copper conductors to vibration, cracking and breakages. Mechanical fatigue occurs when exposed to short circuit, which can reach critical extent over time and can lead to large breaks. Thermal conductivity When we examine the thermal conductivity, the heat formed under the current is distributed much faster by the copper conductor. This is an important feature in the transfer of heat, for example during the overcurrent draw of any motor. Preferring a smaller diameter for the conductor allows the heat to be transferred quickly to the surface in a shorter way.

Due to low-quality workmanship at connections and the physical differences between aluminium and copper, high-resistance connections formed and became a fire hazard. As a response, aluminium alloys were developed to have creep and elongation properties more similar to copper. These alloys are the only solid or stranded aluminium conductors permitted to be used according to Article 310 of the 2014 National Electric Code. The alloys meet the requirements of ASTM B800, Standard Specification for 8000 Series Aluminum Alloy Wire for Electrical Purposes – Annealed and Intermediate Tempers. Aluminium (Al) and copper (Cu) Aluminium is a soft and light metal with a silvery colour. It is estimated that the earth is made up of about 8 per cent aluminium. It is very rare in nature. The manufacturing process is very complex and requires a high amount of energy. As a result of electrolysis, aluminium is obtained with a purity of 99.5 per cent. Copper is a metal that has a reddish-brown colour and is found in nature as free or compound. It has been one of the most used metals in the world since ancient times because of its soft structure and easy processing. Although it is 0.01 per cent in the earth, it is preferred by mining companies primarily. In the relative cost of copper and aluminium conductors, we need to consider the price, specific gravity, conductivity and other factors in order to get the real answer as to whether using aluminium conductors can save costs or not. The comparison between copper and aluminiumwill be under two main topics – physical and electrical properties for both metals. Specific gravity One of the most important points that shall be considered when comparing two metals is the weight. The specific gravity for copper is 8.89g/cm 3 , and the specific gravity for aluminium is 2.7g/cm 3 . That is to say, under the same conductor specification, the weight of copper is 3.3 times that of aluminium conductor. Cross-sectional area The conductivity of aluminium is only 61 per cent of that of copper, that is to say, the aluminium conductor cable cross section that can be used in the same system of copper must be approximately 1.6 times the cross section of the copper conductor. Price ratio The price ratio of copper to aluminium is about 3.5, so the cost of equivalent copper conductor is seven times that of aluminium conductor. Of course, this simple cost comparison does not explain the whole problem. As mentioned above, to make the conductor have the same current-carrying capacity of copper, the aluminium cross section must be increased by 1.6 times that of copper. Since the cross section of the aluminium conductor is greater than that of the copper conductor, the subsequent increase in

Metal

Thermal conductivity [BTU/(hr.ft.°F)]

Aluminium

136

Copper

231

Melting temperatures The melting temperature of copper is higher than that of aluminium, which means that the copper conductor can withstand the mechanical forces that occur in thermoactive processes (expansion, contraction, etc) without losing its mechanical properties.

Metal

Melting temperature

Aluminium

660.32°C / 933°K

Copper

1,084°C / 1,357°K

50

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January 2023