Came across something unusual today. Home was built in 1966. Service panel had a stranded copper wire connected to the panel and it ran down through conduit into the floor. Could not locate other end. Any ideals.
Looks to me like could be the ground wire for the service .
Yep.
With what is visible in the picture, that would be my guess, too. 
That’s what I Thought. Never seen this done before around here.
Remember Green is always ground but Ground is not always green .
According to the NEC, a grounding electrode conductor can be any color.
Gee that’s what I thought I said .
Robert is 100% correct. The EGC is typically Green (notice I say typically, as most of the time it’s bare) as I think that was what Roy was referring too.
What is interesting is that we can’t see the upper and lower portion of this installation to see if it actually complies with Section 250.64(E) which requires bonding of each end of the EMT Raceway to the GEC, assuming we are dealing with a ferrous raceway. If not chances are it will be potentially useless when and if it is ever truly needed.
Again that’s assuming this is a GEC…that’s why I hate pictures…
However I would not suggest it to be Red.
I believe it’s bonded to the water or gas line, maybe both. Are there pipes or meters visible either inside or outside the house?