Learn what the NEC requires for junction boxes, from box fill calculations and grounding to outdoor use and fire-rated wall installations. With regard to the ambient conditions, several factors and standardised specifica-tions must be taken into account, in order to select the right junction box for the intended place of use. Thus, with installations. Measure from where the wire comes out of the cable sheath or raceway. Leave at least 6 inches of free wire inside. By: Thor, Senior Electrical Engineer at Weisho Electric Co. Whether it's a home, office, or industrial site, NEC compliance is legally required in most states.