[TCML] Wire gauge for large diameter secondary?

[ConorPerry at comcast.net]

I'm trying to help someone with a 20" secondary which is starting to fall 
apart.  It's space wound, probably 20-22 gauge wire, and the windings are 
seperating from the PVC.  This has caused several locations of arcing 
between windings and carbon tracking.  All the modern coils that I've seen 
(12" and under) are tightly wound using the wire gauge size which will cause 
the correct winding lengths.  But when you get into a secondary this large 
what to do?  Is THHN, TFFN, or MTW suitable at all?  Is space-wound still 
and option to help save on copper costs?  If we do space-wound, what gauge 
should be used?

Conor