DIY Power cord

I have some 4 core 10 gauge screened  cable with ptfe dielectric that I'd like to use for a power amplifier mains cable that will be terminated with an IEC plug. What should I do with the fourth core? Just leave it,  use it as an extra earth cable or perhaps an extra live wire? 
Looks like you are making a good electric chair!

Use only UL approved power cords IMO.
Who makes the cable, and what is its rating?  I wouldn’t bother on this cable for what you just described. My experience on DIY PC’s is the ends have as much or more to do on the sound so if you’re going to build your own, spend the money on the ends or don’t bother. There’s plenty of nice used PC’s out there so not sure buying ends at retail you can beat say a used Kimber PK10 Gold. Many known brand PC’s aren’t UL listed so don’t get too hung up there. If you were to use this cable, I’d float the fourth conductor, but I think I’d pass altogether anyway. If the cable is unshielded which is ok, it’s possible the fourth conductor could possibly pick up EM/RFI. If you want to build your own look at VHAudio for their least expensive PC and buy some decent ends. Make sure you terminate the ends correctly as well. Don’t burn your house down.

Double up on the neutral side and use opposite conductors not side by side...

  N       N

My own diy PC's have a dual neutral which seems to provide better performance.

Thanks adg101 and willywonka, the double neutral is a great idea,  It'll give it a try and let you know the results.  I try to get good terminations so thanks too for the advice.  I don't know the cable manufacturer but its milspec and appears to be of very good quality. 

I have made numerous DIY power cables with a range of connectors. I found that following Chris VenHaus' cable recipes and using at LEAST upper-medium quality connectors resulted in excellent power cables. The one drawback is that the specified Belkin wires are so stiff that the cable is very difficult to use in lengths under 2 or  3 feet. Great value and very good quality, but a problem in a tight space. (I also used DH Labs bulk power cable, and find no difference between it and the Belkin, could be me, just saying.)

I really have no way to judging if cryo treatment of copper wires or connectors produces discernible results (it could be me, it could be "their" claims...), so I don't bother ...but I do use at least "pretty" good quality parts. 

It should take you less than 30 minutes to make your first power cable from scratch, maybe 15 minutes for subsequent ones. You can make them as pretty as you want (shrink tubing and tec sheath), or you can switch out cable or connector to see if your ears can tell...
Read Chris VenHaus' recipe for pointers.

Infinityaudio, If you want a higher performing DIY cable, try this one...

Using a higher quality conductor like furutech will improve dynamic performance, but the helix geometry will get you better image, deeper, better controlled bass, more clarity, and smoother mid and highs

 I first tried one made from Romex house wire and it was better than my PC made from Furutech bulk cable. Both had the silver plated copper connectors.

I use them throughout my system and am yet to find a better cable.

The Connctors, being silver plated copper,  provide great bang for the buck

Something to try on the cold winter nights 😊