Just to throw another log on the fire:
What is the basis for the prohibition against using flexible cords as building wiring?
Clearly there are good historic reasons for doing so; cloth covered rubber insulated wires bundled in a cord have no comparison to conductors protected in raceway, or physically separated as in knob and tube.
But compare modern flexible cords to, for example, modern non-metallic cable types. The flexible cord has thicker insulation, and more finely stranded conductors. The insulation material is different, but has similar temperature rating. The insulation is softer, and probably more prone to damage, but the sheath is much thicker, and can 'absorb' small nicks and tears without exposing the inner conductors.
I see no reason _in principal_ that flexible cords could not be safe for permanent installation as building wiring. The cords would need to be suitably tested and listed for long term hidden stationary use as well as flexible use, so not all cord insulation would be suitable. The cords would probably need different support means, different clamps at junction boxes, and possibly different termination techniques to use with the finely stranded wire. The conductors would need to be sized to article 310 rather than article 400, and there are probably a number of other issues.
I bet that there are situations where it would be quite useful to have 'dual rated' cable that was both one of the 'SO' family of cords, and _also_ listed as a type NM cable.
-Jon
