Question about assembling my own DIY AC Cable

The specific answer to your question is that with Furutech plugs and IECs, the connector consist of a clamp with center set screw, as shown below.  Therefore, best practice is to split the wire so that half goes into each side of the clamp.

@audphile1 made a good suggestion to coat the exposed conductors contact enhancer before clamping.  

Pure copper is a soft metal and does not have any memory retention properties. Copper contacts of the IEC will not hold the female contacts tightly against the IEC inlet male plug blades...

The above quote is correct and is the primary reason that many AC plugs and IECs actually do not use pure copper, although some have figured out a way to use pure copper contacts in some of their products and still provide the necessary gripping tension, like this product from Furutech. This link to Oyaide indicates their use of phosphor bronze and beryllium copper contacts, which is actually common.

I agree with @audphile1 @erik_squires ​​​​@jea48 - don’t overthink it.

You are almost surely plugging the plug into a wall receptacle or power conditioner with brass, phosphor bronze, or beryllium copper contacts, and depending on what was used in your amplifier, you are probably plugging the IEC into an inlet with one of those same materials, unless either the designer used a higher level Furutech IEC inlet or you upgraded it. Furutech apparently uses copper connectors in their upper level products, as shown here.

You can start with the exposed, stripped ends of the wire being a bit long (like 1/4 inch or 2mm longer than you need) and then use a pliers to grip them at the end and twist them together. The very end where you gripped the wires with the pliers will be screwed up so you cut the ends off so that all three sets of twisted wires are equally long and the ends look tidy.

I have done this many times so I probably have a better feel for what the wires can take before breaking than others who haven't made a lot of cables.  The idea is to twist them together just tight enough so they act like a single twisted wire and then you can feed them into the connector easier - they don't need to be any tighter.  Seven wires equaling an aggregate 5.53 mm square is just smaller than about 18 awg per strand.  Wires that size are more resilient than you might think, especially copper.  Just go slow and easy and you shouldn't break the individual wires.

A wire stripping tool has always worked best for me, with thin coverings and even with foamed Teflon.  You should know the gauge of your wire and be careful because you can break the wires if not careful.

I never had a wire that couldn’t be stripped using a tool like that, with the exception of cotton or silk covered wires. However, it is important to use the correct hole size matching the wire diameter. Another trick if you are having difficulty is to carefully give the tool a twist to create a cut in the insulation as a starting point, and then pull outward. You need to be really careful doing this with small diameter wires as it can be easy to break or cut through an individual wire.

In your case, with 7 wires having an aggregate area of 5.53 mm squared, your individual wires are likely somewhere between 18-19 awg. I have quite a bit of experience re-terminating Harmonic Technology speaker cables, which are similar in that they are made from multiple small gauge (20-24 awg in the HT cable) wires encased in Teflon. In the case of HT wire, I found the fingernail approach to actually be the easiest way to remove the insulation. I was usually able to strip one side of each individual wire and then pull the remaining insulation away from the wire and then use a small snippers to cut it off. It can wear out your fingers if you have a lot of wires to strip. One difference between your wire and the HT wire is that the insulation on the HT wire was foamed Teflon. Your situation may be more difficult. Good luck.

Correction, HT speaker cables actually use PE (air-foamed polyethylene with air bubbles as the main dielectric), not PTFE, as the insulation.