You must size pull boxes and junction boxes, as well as conduit bodies (enclosures), so you don’t damage the conductor insulation. For conductors No. 4 and larger, size pull boxes, junction boxes, and conduit bodies according to the National Electrical Code (NEC) Sec. 370-28. Straight pull calculation [Sec. 370-28(a)(1)]. A straight pull calculation applies when conductors enter one side of an enclosure
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