This seems like a simple question, but I cannot seem to find the answer in the 2008 NEC.
There are references to "maintaining spacing" in many sections of the code such as the following:
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
For the life of me, I cannot find what "proper spacing" is for two wires run in parallel in free air. At the moment, I am most interested in the answer for NM-type wiring, but it would be good to have a reference for all types of wiring if possible.
Here is an example from the NEC for multiconductors in cable trays where I am hoping someone knows of an equivalent section that defines proper maintained spacing for free-air:
(c) Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperature-corrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).