What is the best copper for mains cables ?

With Acrolink Oyaide, Furutech, Nanotec, Neotec all claiming the best purity of copper for mains cable, which company is actually correct ?
If i have not mentioned any other company claiming their copper is best to provide electricity to our equipment please bring some more companies to the discussion.
Is there a best copper ?
Or are they all good just different ?
I am not talking cable geometry, filtering or insulation, just the best quality copper for mains cable duty.
Even if you do not buy into the marketing of these mentioned companies, please offer your opinion, Thank you.
I'm really not sure that there's such a thing as "best copper." Even if there is, I'd think other factors, such as geometry, would likely be more critical once a certain quality level of copper is reached.
If I recall correctly only the Japanese claimed eight 9s copper. Six 9s is probably ubiquitous. Continuous cast copper is also common or so it would appear. Of course direction trumps purity. 
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I would never have the budget to spend more than $300 on a power cable, but i find it interesting that there are Audiophiles that swear by spending vast amounts of money on power cords is beneficial. I believe Elizabeth has hit the nail on the head with, if amazing copper does in fact exist, how will anyone ever be able to confirm this fact, all we have to go on is the marketing by the big cable manufacturing companies as Geoff states in Japan. DIY power cords seems to be possibly the best option for me, but as already stated which cable manufacturer do i go with Oyaide, Furutech, Nanotec, Neotec, Acrolink ? They are all claiming super duper copper for mains cable. I have not got the budget to try them all, Chris at VH Audio claims Furutech is the way to go, but again i am still not certain.
We know about the claimed effects of geometry and insulation, i am probably asking an unanswerable question,going by the manufacturing claims, What is the best quality copper for a power cable ?
Most likely 6 nines cryo’d copper solid core unshielded 12 gauge that had been controlled for directionality would sound pretty darn good. You might not be able to bend it around corners but what the hell.
Where would i get cryo solid 7N copper, Furutech and Acrolink state their copper is 7N stranded from the Mitsubishi Labs.I have never seen anyone offering solid copper, only the REL Baseline Blue.
Acoustic Revive has solid copper ac cable labeled EE 2.6 which I use as a main cable powering my digital components from breaker to inlet. I believe there are other offerings.
Many people swear by the ultra pure ohno continuous cast mono crystal  or up occ copper
The real question is how many manufacturers actually make their raw wire. Probably most buy it from industry as there are only.so many wire manufacturers out there. I'm willing to  bet many cable companies buy their raw wire from the same places.
I'm willing to bet that Glen is right.
Based on a lot of testing and experimentation, the combination of OCC copper and solid-core is really the best in any situation (mains power cord, interconnect, speaker, digital, etc.).  The only company that does this combination in a manufactured power cord is Audioquest.  The OCC ohno continous cast offerings from Neotech and Furutech might be slightly better copper than Audioquest, but they really only have stranded cable offerings, and much of it is silver-plated as well.  Alternatively, you can build your own cable to your own specifications.
Select-Hifi I am currently trying to source cable for my own DIY job based on https://www.tnt-audio.com/clinica/ttse.html
Amazing how expensive cable is and European suppliers mostly sell full reels at many hundreds of euro upwards.   For availability reasons I'll probably get a cable made by Lapp.   Lapp Olflex classic 100 CY.   Belden do a quality copper  screened Teflon insulated cable 83702 or 83321E which is available cut in the US.
The link I gave uses an earthed screen with a full regular cable for each indepent live, neutral and earth core.  They are plaited too.   Very easy to make also. 

Cost is tiny and whilst I am a total sceptic on cable hype, the cost means I'm going to try it and see.   Sourcing issues mean I have to buy material for 5 but still so cheap that it's worth the risk. 
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A reasonable, if irrelevant, question.

Of course nobody makes their own wire. Nor do they make their own dielectric, nor the mesh wrapper. I seriously doubt any of them even makes their own terminals, plugs or sockets. It's simply way too inefficient, costly and counterproductive. Oh a few things here and there. You need to experiment and that at some point involves making something never made before. But even then, once you know what you need, its almost always more efficient to go to a manufacturer and have them make it to your spec. Because that level of manufacturing skill is not where the expertise that matters is anyway.

Way back in the early 1990's Ted Denney III told me he noticed the reels of wire made to his spec did not sound the same. Drilling down he had to learn a lot about how the wire was made, to eventually discover what is basically like the peanut allergy problem. Those signs warning "equipment used to process peanuts"? Because even a tiny amount can be a problem for some people. Well the same equipment is used to process different metals. The numbers we pay so much (way too much) attention to say how many nine's there are of copper, but say nothing about what the 0.001 is. What Ted figured out is that tiny trace actually makes a difference.

Ted did not find this out by testing. His supplier swore everything was to spec. Passed all the tests. Precision product. Only Ted with his ears could hear the difference. Some things you would never know or learn any other way than by listening. 
That is why all these debates bore me to tears. Total waste of time. Worse than a waste, because it distracts from time being spent on something productive, like what does it sound like? Yet it is what the vast majority keeps wanting to talk about.

Its just nonsense. The question of what is the right copper is really only one question out of dozens. What is the best wire diameter? How many strands? In what configuration? What is the best geometry? What is the best insulator? What is the best termination? What is the best way of fastening the wire to the terminal? On and on.

None of which is answered any way other than by listening. Nothing else matters. 

J Gordon Holt introduced listening as the evaluation standard more than 50 years ago. Would've thought it'd have caught on more by now. Oh well.
Audible differences between two ostensibly identical cables or wires can be explained simply by directionality. Cryogenic treatment is not outwardly visible, either. You can forget about everything else.
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Screened copper cable for IT is designed with screening and material designed to eliminate interference and loss of signal quality. e.g. Belden. Probably will be shot down for suggesting it but a fraction of the price of Audio cables.   

As suggested elsewhere I doubt the audio companies draw their own wire and extrude their own insulation.  What do they use? and who do they buy their raw material off? 

I took the pluge and went for 3 x 2M lengths of Nanotec 308 with Furutech NCF connectors, I must say on very few hours i am really pleased with my choice. Below is the description from the Nanotec website. I will be buying a 1M length of Nanotec 309 for my class D amplifier. I have also listed the description from their website of the Nanotec 309. My search is over for DIY off the reel copper power cable, the sound is already to my ears a step up from the kettle leads that came with the equipment, I paid $99 for 1M and various prices for the Furutech NCF range UK plugs and connectors.My belief is Nanotec manufacture their own cable, I could of course be wrong.

Wonderful 308 “Our First Power Cable with PC-Triple C Conductor Soaked with Gold/Silver Nano Colloidal Liquid”
PC-Triple C is the new name conductor, which uses forge process, well-known for high quality auto wheels. The combination of PC-Triple C and our original colloidal liquid process is the ideal noise-less, smooth electric current for audio/visual components

309 4 years since the release of POWER STRADA # 308. A new power cable [POWER STRADA # 309 “MISSION IMPOSSIBLE”] has been released. Adopted Japan original cable conductor [PC-Triple C] with diameter 0.32 mm × 70, cross section 5.63㎟ polyolefin with vibration damping characteristics for conductor insulation and outer sheath material. Damping performance and electromagnetic wave absorption characteristics are improved with cotton yarn and Japanese paper.
select-hifi writes:
I took the pluge and went for 3 x 2M lengths of Nanotec 308 with Furutech NCF connectors, I must say on very few hours i am really pleased with my choice....the sound is already to my ears a step up from the kettle leads that came with the equipment...
... I paid $99 for 1M and various prices for the Furutech NCF range UK plugs and connectors.

Kettle leads, eh? Never heard them called that before. We just called em patch cords back in the day. Whatever. Shakespeare said A rose by any name would smell as sweet. I say a patch cord by any name would sound as crappy. Whatever you call it, free is a pretty low standard. Go buy whatever $100 will get you in Synergistic Research. Not even the $150 you spent including the connectors, just the $100 you spent on the wire. My guess is you'll be surprised how much better it is even for less than you spent.
Be prepared for a long burn in on those Furutech NCF rhodium connectors.  A few hours will not get you into the "painful" area.  I would suggest leaving equipment on for 15 or so days (300-400 hours).  It will go through all sorts of changess but the Furutech rhodium are the absolute best in my opinion.
Every electron I ever interviewed expressed a preference for the Furutech ones, but there is a controversy brewing among them.  I will keep interviewing electrons to determine the copper they love the most.  Stay tuned!!

You interviewed the wrong particles. The electrons don’t have a horse in this race. 
European Kettles use IEC connectors very similar to standard IEC (C13 I think) connectors used on audio equipment hence we use kettle lead term a lot. 
you cannot go wrong choosing any of the Japanese big brands.
They have the resources and techniques for creating excellent copper cables, stranded or solid core. The best thing is that you can DIY and save some money. PCOCC, TripleC, a-OCC, OFC, Nano, Stressfree, DUCT, ...all sound different, but have the same sonic signature within the family (not bad). Which sounds best is only a matter of synergy and to enjoy the full potential of the copper material all cables in the chain should be the same.

I put a Russ Andrews £2200 power cable on my Esoteric K-01 and left it in the system for 2/3 weeks. I went back to my Nanotec 308 with Furutech NCF connectors and it made the Russ Andrews sound like it was holding back Dynamics and openness. The Russ Andrews cable (£2200) uses Kimber which is a multi individual insulated copper cable. 
For the money i highly recommend Nanotec 308 with good quality connectors.
I liked using Jena cables for my speaker wire. I could only afford the cheap one.  She says ALL COPPER is CONTINUOUS CAST and the wire buying public has had the wool pulled over their eyes. Don't argue with me argue with Jena Labs. I think they braid their own wire but doubt they make it or its dielectric, which is thin transparent plastic cover on the copper. 
@select-hifi - have you ever considered making the actual cable from individual strands of wire?

Take a look at this link for details...

It uses Duelund wire for the live conductor and silver plated Mil-Spec wire for the Neutral. The ground is 12 gauge wire from Home Depot.

I have tried many different wire combinations using the Helix geometry, but - the combination above seems to provide the best results.

The helix geometry eliminates almost all noise generated within the cable itself, which is common in all cables that opt for the more standard geometry of three wires of equal length with a slight twist - as with the Nonotec 308.