Well, not necessarily. Romex won't really pass for an external power cord because it really doesn't have enough insulation/covering for durability. It also is very thick solid core, which will actually break over time when you bend it back and forth many times (which is what happens with normal external power cords).
The Audioquest NRG really is solid core. It just uses much smaller 19awg and 21awg conductors. They are also wrapped together with a thick PVC insulation/sheath so it is impossible to bend the power cord to the point where you are stressing the solid-core. You can't bend the Audioquest cords at a 90 degree angle like you can Romex. It's more like a gradual curve. The cables are very stiff, so you need to allow room to gradually bend the cables around.
The NRG-4 uses 4 x 19awg for HOT and 5 x 21awg for NEUTRAL and another 5 x 21awg for GROUND. All solid-core.
Other companies make solid-core power cords, but in my opinion Audioquest is the only one who does it right.
If you look at VH Audio, they have a Neotech NEP-3200 that uses 4 x 17awg OCC solid-core conductors. It's excellent copper, but I think you may start losing high frequency.
Nordost uses 16awg solid-core conductors in their power cords and use silver-plating to increase the high resolution resoponse (large awg conductors will start to lose high frequency capability).
There's a company called Evidence Audio that uses 4 x 18awg conductors for hot/neutral, but I don't think the copper is anywhere as good as Audioquest.
