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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:

310.15.B.3.a

(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:

392.80.A.1.c

(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).

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This is a "mike holt forum" depth of question. –  Bryce Aug 12 '13 at 18:44
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From Mike Holt Forum. "That is the million dollar question. I just keep them from touching. Keep in mind that you can have 4- 2 wire nm cables stacked without being hurt by the derating that is necessary." –  Tester101 Aug 12 '13 at 19:11
    
@Tester101 Thank you for the leg work on that. If you want to put your comment into an answer and explicitly indicate that it is not currently defined, I will accept your answer. –  user14416 Aug 12 '13 at 19:18
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The National Electrical Code (NEC) does not define the proper spacing that should be maintained.

A similar question was asked, and answered on the Mike Holt Forum.

That is the million dollar question. I just keep them from touching. Keep in mind that you can have 4- 2 wire nm cables stacked without being hurt by the derating that is necessary.

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