Box Fill

he number of conductors in boxes and conduit bodies is regulated by Section 314.16.  Conductors that run through a box are counted as one conductor.  Each conductor originating outside of the box that terminates inside the box is counted as one conductor.  All the grounding conductors for each grounding system, usually the equipment grounding system and the isolated grounding system, are counted as one conductor, at the size of the largest contained equipment grounding conductor for the system.  Devices on a single yoke or strap mounted in or on a box are counted as two conductors.  One conductor is counted for each set of hickeys, fixture studs, or cable clamps inside the box.  Exterior cable clamps and interior locknuts are not counted.  Conductors that do not leave the box, such as pigtails, are also not counted.  A looped, unbroken conductor not less than twice the minimum length required for free conductors in 300.14 shall be counted twice.  This basically says that if you leave 12 inches or more of free conductor in a box then it counts as two conductors.