The latest code I am aware of is to use thwn in an approved enclosure (conduit etc). That will come in stranded or solid and the insulation "should not" deteriorate.HTHMelbo
Technically speaking, is an "extension cord" still an extension cord if you cut the ends off and permanently wire it and fasten it in place? I suspect that it is not. At that point it becomes "wire" of a particular type and with whatever approvals it carries. I bet the rules related to extension cords don't cover the use of "extension cord wire". Brian