The ability of a cord to release heat safely is the basis of the recommended current carrying capacity. Conductor sizes are selected on the basis of voltage drop over a given distance. The lost voltage shows up as heat. If you put multiple conductors in a single conduit the recommended current capacity decreases with the increase of the number of conductors in the conduit (there are charts for this). If you use a cable with conductors that are too small and coil the cable tightly you are duplicating the multiple conductor condition. You can generate enough heat to melt the insulation on the conductors. With properly sized conductors, as in the case of RVs, this is not an issue. However, if you try to power your RV with a 50' coil of 16 gauge wire, you can melt it.
