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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:

enter image description here

DerStrom8
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i-CONICA
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    Aluminum is cheaper. – Eugene Sh. Dec 22 '17 at 18:08
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    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
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    Why the downvote, downvoter? – i-CONICA Dec 22 '17 at 18:41
  • I have no idea why you were downvoted, I thought this was a great question and fits the scope of this site. I gave you a +1 – DerStrom8 Dec 22 '17 at 20:45
  • What material is the outer braid/shield? Is it copper or brass? Why not make that aluminum as well? – JS. Dec 22 '17 at 22:46
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    Sell it as a balanced interconnect wire to an audiophile. – pipe Dec 23 '17 at 11:36
  • Opposite charges attract like charges repel. Since electricity is the flow of electrons and electrons repel electrons very little current flows in the middle of a conductor. Electricity flows on the outer layer of the conductor. It appears the outer layer contains copper wiring in this picture. Just an educated guess – Pomagranite Dec 23 '17 at 16:54
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    @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](https://en.wikipedia.org/wiki/Skin_effect), but it's origin is distinctly different. – Neinstein Dec 23 '17 at 21:50
  • Still, it may be something to consider - the Wikipedia gives copper's skin depth to be 8.5mm at 60Hz - and the wire's diameter, though is's aluminium, seems to be bigger than twice of that. Opinions? – Neinstein Dec 23 '17 at 21:53
  • @old_timer implying? The question is about one section of cable in one part of the world, implying nothing – edc65 Dec 24 '17 at 07:38
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    That is a European (including the UK at the moment 8-) ) 3-phase (you can just see the brown/black/grey colours) distribution cable. The outer earth/neutral conductor can still be copper because the normal current flowing through it is very much smaller except in the sort of cases that the DNO is not going to tolerate (very out of balance loading on the phases). Given the answers seen below it is also interesting to note that there is a significant proportion of Copper-Coated-Aluminium (CCA) cables used for computer networking which are also used, one suspects, for reasons of economy... – SlySven Dec 24 '17 at 09:43
  • @edc65, and you are still assuming that in that part of the world the only ever have for the history of time bought and used one kind and will never change ever in the future. I appreciate the question being changed from unanswerable to answerable. which was the whole point. – old_timer Dec 24 '17 at 13:03
  • Also worth noting, perhaps, that copper & aluminium are not the only possible conductor types. The supply line running to my house, for example, has lead conductors, presumably because at the point in time that it was installed lead was a cheaper option (I don't believe it would be now, however). – Jules Dec 25 '17 at 07:57
  • It also doubles as the Mercedes Benz logo. – Tim Spriggs Dec 26 '17 at 20:57
  • Thanks for all the comments and answers. If it adds anything, this is from Manchester, UK. – i-CONICA Dec 26 '17 at 21:01

6 Answers6

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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
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Transistor
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    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
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    Aluminium wiring is now used in aviation (Airbus A380, and many others) principally to save weight. – TEMLIB Dec 22 '17 at 18:51
  • @Sparky256: What is "300 mcm"? – Transistor Dec 22 '17 at 19:08
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    @Transistor: "mcm" is short for "thousands of [circular mils](https://en.wikipedia.org/wiki/Circular_mil)", a unit of cross-sectional area – Dave Tweed Dec 22 '17 at 19:13
  • A fancy way of saying the wire is 300 milli-centimeters (3 cm) in diameter. It is sold with those markings on the wire once the size goes beyond the awg scale. –  Dec 22 '17 at 19:13
  • "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%." shouldn't the weight of alu be more for the same conductivity. still a saving –  Dec 22 '17 at 19:15
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    @JonRB: No, because the density of aluminum is so much less. – Dave Tweed Dec 22 '17 at 19:16
  • @Sparky256: Are you just making that up? (Hundreds of milli-centimeters would be mm, not cm.) – Dave Tweed Dec 22 '17 at 19:16
  • d'oh yer @DaveTweed –  Dec 22 '17 at 19:16
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    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](https://en.wikipedia.org/wiki/Circular_mil). What a mess! – Transistor Dec 22 '17 at 19:16
  • @Dave Tweed. No, but your answer may be the more accurate description. –  Dec 22 '17 at 19:17
  • I'd imagine that underground wiring also stays at a fairly constant temperature, reducing the risks associated with aluminum wire shrinkage and high-resistance connections. – rdtsc Dec 22 '17 at 21:52
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    @Sparky, DaveTweed: "thousands of circular mils", "milli-centimeters": With all due respect: WTF? – Volker Siegel Dec 22 '17 at 22:15
  • @VolkerSiegel. With all due respect, Transistor had the most accurate description of an awkward size to measure. How it originated I do not know. Must have been an engineer. –  Dec 22 '17 at 22:23
  • Weight is a huge factor. It effects shipping, availability, other "infrastructure" (like size of maintenance trucks and fuel used) I would expect weight to factor in more then raw material cost. – coteyr Dec 22 '17 at 23:58
  • It sounds about right. [This Siemens whitepaper](https://w3.siemens.com/powerdistribution/global/EN/tip/focus-markets/Documents/Siemens-Data-Center-Whitepaper-Aluminium-versus-Copper.pdf) says aluminium has "significantly lower material costs (up to 6 times lower than Copper per Ampere)." One thing not said there is that for cables, to get the same amperage as for copper you need a thicker cable, which does mean marginally more insulation (due to the increased diameter). I haven't (yet) found a published analysis taking this into account. – Fizz Dec 24 '17 at 12:01
  • Recently our neighborhood was re-wired. The utility company hired a contractor to do the job. Guess what kind of wire they installed. Guess what kind of wire they removed. I wonder what became of the old wire, and who profited from that treasure... I wonder what their business or family relationship might be to the managers in the utility company that awarded the contract. This illuminates the kind of conflicts that NSPE preaches about. – richard1941 Dec 29 '17 at 12:01
  • @Transistor I've learned about cmils last year when designing a transformer with a manufacturer from the US. Made me want to print t-shirts with the words "Go Metric!" – zebonaut Jan 11 '18 at 21:17
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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.

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    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. –  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 - Reinstate Monica 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. –  Dec 25 '17 at 23:07
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    Oh, you’re right btw. I’m in Manchester, UK. – i-CONICA Dec 26 '17 at 21:05
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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.

Peter Green
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    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 04:31
  • 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 04:33
  • Which leads on to the question "why is noone making it"...... – Peter Green Feb 17 '18 at 02:19
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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.

DerStrom8
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    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 06:07
  • @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
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    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
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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.

Some links demonstrating how big a problem is metal theft.

dotancohen
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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.

Guill
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