ARTICLE 378. NONMETALLIC WIREWAYSARTICLE 378. NONMETALLIC WIREWAYSThis article covers nonmetallic wireways. As given in 378.22, the maximum conductor fill is determined in the same manner as for metal wireways, but support requirements are slightly different. 378.30 requires support at 3-ft (0.9-m) intervals, instead of 5-ft (1.5-m) spacing. The most significant difference is found in 378.22 with respect to mutual conductor heating derating factors. Because nonmetallic wireways are not efficient heat sinks and transfer heat comparatively poorly, the 30-conductor allowance prior to the imposition of derating factors that pertains to metal wireways does not apply to nonmetallic wireways. The derating factors begin to apply with the fourth current-carrying conductor, just like ordinary tubular raceways.
Frederic P. Hartwell; Joseph F. McPartland; Brian J. McPartland: McGraw-Hill's National Electrical Code 2017 Handbook, 29th Edition. ARTICLE 378. NONMETALLIC WIREWAYS, Chapter (McGraw-Hill Professional, 2017), AccessEngineering