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

I’ve found a piece of electrical cable left by the electricity company that supplies the area. They did a repair recently underground.

I’m curious why the three big triangular conductors aren’t made from copper. It looks like aluminium to me, but I’d have thought copper would be a much better conductor.

What am I missing? Is aluminium almost as good but far cheaper?

Photo:

• Aluminum is cheaper. – Eugene Sh. Dec 22 '17 at 18:08
• Just because one section of cable in one part of the world used something else doesnt mean it is a general practice which is what this question is implying. What makes one material "better" today may not make it better in the future or past. Cost of materials like these fluctuate. – old_timer Dec 22 '17 at 18:19
• Why the downvote, downvoter? – i-CONICA Dec 22 '17 at 18:41
• Sell it as a balanced interconnect wire to an audiophile. – pipe Dec 23 '17 at 11:36
• @Pomagranite Not quite. The electrons "flow" "between" the positively charged nuclear cores. There's no extra charge (which would indeed rest on the edge of the conductor), just an ordered movement of the othervise randomly moving electrons. There is indeed a so-called skin effect, but it's origin is distinctly different. – Neinstein Dec 23 '17 at 21:50

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

• Nice. I'd add another thought crossing my mind, reading this. If the weight per meter is half as much, it's less weight on the trucks (though perhaps more bulky) and then also certainly less man-handling work by the employees carrying and laying out the wire. – jonk Dec 22 '17 at 18:43
• Aluminium wiring is now used in aviation (Airbus A380, and many others) principally to save weight. – TEMLIB Dec 22 '17 at 18:51
• @Transistor: "mcm" is short for "thousands of circular mils", a unit of cross-sectional area – Dave Tweed Dec 22 '17 at 19:13
• @JonRB: No, because the density of aluminum is so much less. – Dave Tweed Dec 22 '17 at 19:16
• Thanks, guys. "A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch). It corresponds to 5.067×10−4 mm²." Source: Circular mil. What a mess! – Transistor Dec 22 '17 at 19:16

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.

• There's a lot of good information in this answer. Regarding terminations, the insulating aluminium oxide surface layer presents some problems - special work practices and materials are needed to compensate. Junctions between copper and aluminium wiring are also problematic - bi-metal connectors are needed to mitigate corrosion. As Peter says, it's easy for a utility company to enforce the correct work practices and materials - more difficult for domestic and commercial users. – Li-aung Yip Dec 25 '17 at 4:31
• Your last paragraph is actually wrong re: the NEC -- the 8AWG minimum size limitation is merely a practical one set by the current manufacture of Al THHN. The only limit on aluminum wire set by the NEC is the 310.106(B) requirement that Al wire for building use be made from an AA-8000 series alloy -- if someone was making 10/12AWG solid THHN in AA-8000, it'd be perfectly legal to wire your house with that. – ThreePhaseEel Dec 25 '17 at 4:33
• Which leads on to the question "why is noone making it"...... – Peter Green Feb 17 '18 at 2:19

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.

• This stuff didn’t run through PVC pipes. It’s the same thickness (50mm at a guess) as the PVC pipes that carry smaller utility cables like my cable broadband. This actually runs raw through the ground. It’s much bigger than it looks in the picture. I can only deflect it about 6 to 8cm with all my strength across a 1.5m length. – i-CONICA Dec 23 '17 at 15:00
• In the USA the wires are separate cables inside a thick PVC pipe. As is the power feed to my house. – Sparky256 Dec 23 '17 at 15:05
• It varies. Single wire THWN in conduit is acceptavle, but direct burial cable is also a perfectly common method of service to a residence. In that case, the entry/exit from the earth must be guarded by e.g. PVC pipe to protect it from physical damage, that is easily confused for "conduit the whole way". – Harper Dec 25 '17 at 15:40
• The power utilities decide what is best at the residential level. Ours is piped end-to-end only because tree roots were cutting the lines about 25 years ago, so the had to come in and put in pipe in our whole sub-division. They have the option of direct burial if there are no threats of a cut-through. – Sparky256 Dec 25 '17 at 23:07
• Oh, you’re right btw. I’m in Manchester, UK. – i-CONICA Dec 26 '17 at 21:05

Aluminum is indeed cheaper than copper, and that can be a significant factor in choosing the material for the cabling depending on the current market pricing for both materials. Additionally aluminum weighs significantly less than copper (~30%) so it is easier (and thus cheaper) to transport, move around, and install.

• A minor factor: the less conductive aluminum cable will have a larger cross-section than the equivalent copper cable) and thus will need about 25% more insulation (same thickness, larger area) with similar increase in insulation weight and material cost... – DJohnM Dec 23 '17 at 6:07
• @DJohnM Great point, I hadn't considered that. I only thought of the cable itself, not the insulation. – DerStrom8 Dec 24 '17 at 22:05
• In high voltage transmission, the larger diameter of the electrically equivalent aluminum conductor will lead to lower electric fields at the surface. This may allow thinner insulation. – richard1941 Dec 29 '17 at 12:13

I've no source but I've heard it mentioned that the lower cost of aluminum is not to save money on the cost of the cable, as the aluminum cable costs almost the same due to higher manufacturing costs. The metal aluminum is used to deter theft as copper wires are easily reformed and resold to recyclers, but aluminum is not.

This reasoning might be local, I live in Israel but I don't know if the engineer was discussing actual Israeli cable or not. The subject of discussion was theft of equipment, not the advantages and disadvantages of copper vs. aluminum.

An additional possible consideration is the contribution of the "skin effect." Because of the "skin effect" it matters little that aluminum is not as good a conductor as copper. However, since aluminum is cheaper and lighter, it is ideal for the construction of the center of the cable, and copper for the "skin" of the cable.

• I can’t see any copper skin on the alum conductors? – i-CONICA Dec 28 '17 at 18:42
• Do the math and calculate the skin depth for 60 Hz. – richard1941 Dec 29 '17 at 12:16

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