MATERIAL SUBSTITUTION

Switching Materials


When the price of a material rises, basic economic theory suggests that purchasers of the material will look to alternative materials that can deliver equivalent performance at a lower cost. Though the principle is simple, in practice material substitution is not straightforward, as users are not able to switch from one material to another without having to overcome some additional hurdles. For many types of cable there are substantial potential technical problems in switching between copper and aluminium conductors.

 

 

Copper Compared to Aluminium

Copper is currently much more expensive than aluminium in terms of US$/tonne. As the price of copper has recently moved ahead much faster than the price of aluminium, the ratio of copper and aluminium prices has increased. The simple comparison of the two metals in terms of price per tonne is in some respects misleading, since aluminium is a much lighter metal than copper (2.7 g/cm3 compared to 8.9 g/cm3), though its conductivity is lower (61% of copper). For many cable applications a more meaningful comparison of the metals is the cost per unit conductance; on this basis a tonne of aluminium is close to 2.0 times as effective as a tonne of copper. The price of aluminium, already lower than copper in terms of US$/tonne, needs to be divided by 2.0 to give a comparison on the basis of electrical performance, which makes the cost difference even wider.

 


But Comparisons Are Complex


This comparison based on conductance is not the whole story, however, as aluminium conductor is bulkier than its copper electrical equivalent. In a cable this means that the layers surrounding the conductor (insulation, shielding, jacket, armour, etc.) need to be more extensive in an aluminium cable than in its copper equivalent. The additional cost of these layers partially offsets the cost saving in the conductor gained through use of aluminium. In a power transformer similar considerations apply: while aluminium strip would have a lower cost than copper strip with equivalent electrical performance, the transformer would need to be larger, requiring more use of other materials (e.g. special steel for the laminated core and steel casing).

 


Copper v. Aluminium in Power Cables


Looking at the pattern of demand for power cables across the world, there are some clear examples of countries where power utilities make wide use of aluminium conductors, rather than copper in insulated power cables, for example in India and Brazil. The advantage of aluminium conductors over copper is particularly evident for overhead power distribution (bare or insulated cables), where the lighter weight of aluminium is a further benefit. From an historical perspective, it is interesting to note that, although aluminium conductor was used for overhead transmission as early as 1898, its widespread use did not occur until the 1940s, when copper was designated as a vital war material.

 


Utilities Continue to Prefer Copper


On the other hand, there are many other examples where utilities worldwide continue to prefer copper conductors to aluminium, despite the apparently lower cost of cables with aluminium conductors. In some cases there are clear reasons for the preference: for underground cables the greater bulk of cables with aluminium conductors can be a problem where there is only limited space to install cables in ducts, as in urban centres of Japanese cities. However, it often appears that the main reason that utilities use copper conductors in power cables in preference to aluminium is established custom and practice. In China, for example, copper conductors are widely perceived to be higher quality than aluminium. Power utilities are generally slow to change their technical preferences: favoured cable designs, once established, will be retained for many years.

 


Aluminium Used Mainly for LV Cables


Aluminium conductors are more likely to be used in LV power cables, and are least likely to be used in HV cables where the bulk of the aluminium conductor is a big disadvantage because more insulation and sheathing would be required. MV power cables may use either copper or aluminium conductors. Industrial users of power cables are more likely than utilities to prefer copper conductors, as contractors may not have the specialist installation skills needed for installing power cables with aluminium conductors.

 


Copper v. Aluminium in Building Wire


A simple cost comparison suggests that building wire with aluminium conductors would be substantially cheaper than building wire with copper conductors. In practice, however, copper conductors over aluminium are preferred in virtually all parts of the world. Experience with aluminium wires in the 1970s (e.g. in the US) highlighted potential serious problems with aluminium: fires were attributed to poor quality joints between aluminium wires. As a result, wires with copper conductors are required in most building codes, with the use of aluminium restricted (e.g. service entrance cables in the US). Despite improved jointing techniques, copper continues to be strongly preferred, and even a country such as India, where aluminium conductors were at one time widely used in building wire, now uses copper. Some companies have tried to promote composite copper-clad aluminium conductors for building wire, as these overcome the main potential problem of poor joints, but demand for this product is limited.

 


Faster Substitution in Non-Cable


These examples from wire & cable applications suggest that substitution of copper conductors by aluminium can happen only slowly, if at all, despite the major difference in price of the materials. There are, however, some markets outside wire & cable where material substitution is more obvious. An historical example is radiator material used in the auto industry: copper was once widely used, but aluminium is now preferred. More currently, there has been some switch away from copper in construction, as some countries that have traditionally used copper plumbing tube are now looking to plastic alternatives (“PEX” tube). Once initial doubts about the integrity of installation using the alternative material have been overcome, substitution can be quite rapid.

 

 

Technical Performance Drives Substitution in Communication Cables


In other cable applications it is really technical performance issues, not simple material cost comparisons, that have driven substitution. Fibre optic cables have already displaced copper telecom cables in some applications. For example, cables used in trunk telecom networks are now nearly always fibre optic, not multi-pair copper or coaxial cables. Similarly, in larger premises and campus networks, fibre optic cable is usually used in preference to multi-pair copper data cables for backbones or risers, while copper pair cables continue to be used in the horizontal structure. In these situations the cable (and the materials contained) is only one element of the overall system cost.

 


But Copper Fights Back


In telecom access networks, however, traditional copper pair cables continue to be widely used, though fibre optic cables are beginning to encroach in this area, as a result of major investment in fibre-to-the-home networks by telcos such as NTT in Japan and Verizon in the US. This substitution is driven by issues related to system performance and installed system costs, and has very little to do with a simple material cost comparison. More sophisticated electronics has enhanced the bandwidth that is available over copper pair cables. Thus in telecom access networks DSL techniques have allowed telcos to introduce enhanced services using existing copper networks, without major investment in fibre. In the premises network market suppliers of structured wiring systems are offering 10 Gbit/s solutions that use enhanced Cat 6 copper pair cables, not fibre, taking advantage of the highly sophisticated signal processing techniques now available.