According to NEC^® 300.14, at least 6 in. of free conductor, measured from the point in the box where it emerges from its raceway or cable sheath, shall be left at each outlet, junction, and switch point for splices or the connection of luminaires or devices. The 6 inches of free conductor shall be permitted to be spliced or unspliced. Where the opening to an outlet, junction, or switch point is less than 8 in. in any dimension, each conductor shall be long enough to extend at least 3 in. outside the opening.

Installing too many extension rings onto an existing 4 inch square box may result in the existing conductors entering from the back of the box being too short to extend at least 3 inches outside the opening unless a splice is made.

In North Dakota, the rules on the number of junction box extensions are amended. Refer to the Laws, Rules and Wiring Standards of North Dakota for additional rules pertaining to junction boxes.