Why aren’t the main conductors in this underground power cable made from copper?

Anixter says:

Even though copper has a long history as the material of choice for conducting electricity, aluminum has certain advantages that make it attractive for specific applications.

Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Aluminum is generally more inexpensive when compared to copper conductors.

Fastmarkets says (and I don't claim to understand any of it):

Copper     LME Averages Cash Ask ($/MT)    Sep 2017     $6,583.19
Aluminium  LME Official 3M Ask ($/MT)      21 Dec       $2,125.50

So, if we need 1 MT (metric ton?) of copper it will cost $6.5k and we need half a ton of aluminium for the same conductivity that will cost $1.1k. That's a saving of > 80%.

The reason you don't see more copper may be for reasons explained by the continuation of the first article:

Aluminum conductors consist of different alloys known as the AA-1350 series and AA-8000 series. AA-1350 has a minimum aluminum content of 99.5 percent. In the 1960s and 1970s, due to the high price of copper relative to aluminum, this grade of aluminum began to be popularly used for household wiring. Due to low-quality workmanship at connections and the physical differences between aluminum and copper, high-resistance connections formed and became a fire hazard.

As a response, aluminum alloys were developed to have creep and elongation properties more similar to copper. These AA-8000 series alloys are the only solid or stranded aluminum conductors permitted to be used according to Article 310 of the 2014 National Electric Code*. AA-8000 series alloys meet the requirements of ASTM B800, Standard Specification for 8000 Series Aluminum Alloy Wire for Electrical Purposes–Annealed and Intermediate Tempers.

• American (USA).

For completeness, the densities are:

metal        g/cm³
copper       8.96
aluminum     2.70

Aluminum is often used for underground power feeds as it bends much easier around 45 degree turns in the PVC pipes. Only 45 degree turns are allowed, as the effort to make a 90 degree turn would snap the 'fish' wire or crack the PVC pipe. It still takes a steel 'fish' line and 'soap' and a motorized puller to help out, as they pull 4 wires through a 4" to 6" diameter pipe with wires of 300 mcm to 500 mcm in size, for 3-phase power, 5 wires if they have a double neutral.

I should clarify two things. A) The OP is showing a custom-made cable used outside of the USA, though the OP did not specify his location. In the USA the good ole NEC book and a long-existing industry still use individual cables in a thick PVC pipe. Resistance to being damaged by tree roots is one reason. B) The steel 'fish' line is used to start with by pulling a thick rope though the pipe that can tolerate 2,000 pounds of pull.

The wires are pulled from the service entrance panel (the insides are not yet installed) out to the concrete transformer pad. The transformer is dropped in place and the service entrance panel breaker rack are installed after the cable pull is completed, with several feet of wire coming out of each end. This is an all day task with several people on both sides working in unison. It is rough, dirty back-breaking work. Been there, done that.

It may take several pipes to supply power to a large office complex or manufacturing plant. The wires are tied parallel and phase matched at both ends of the pipes. For residential homes the wire is seldom over 4 awg in size with 3 wires (120/240 split-phase) so they use copper.

Aluminium has a lower conductivity than copper but it also has a much lower lower density and cost per ton. So (for a given resistance) while a copper cable will be smaller the aluminium cable will be lighter and cheaper.

The main downside of aluminium cables is that they need special termination practices to make reliable terminations and the exact details depend on the particular aluminium alloy. Termination reliability is a concern in electrical wiring because bad terminations can get very hot and pose a fire risk.

Operators of electricity distribution networks can tightly control the materials and practices used to terminate their wiring, the ratio of terminations to amount of wiring is relatively small and for the long large cables they use the cost savings of aluminium are too great to pass up.

OTOH in the building environment things are typically much less tightly controlled. There are many more terminations, a wide variety of different accessories are likely to be used, accessories frequently have designs where terminations have to be made before the wires are pushed into their final locations, many different people/companies are likely to work on the installation including some who are not really qualified to do so. In this environment the termination reliability issues are much harder to mitigate.

This is why Aluminium dominates distribution wiring but copper dominates building wiring.

Some standards bodies ban small Aluminium conductors because they are seen as too much of a liability. AIUI the US NEC sets a minium size of 8AWG ( ~8mm² ) for Aluminium (in new work) and BS7671 sets a minimum size of 16mm².