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314.16(B)(2) Clamp Fill

Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest conductor present in the box. This means that one or more internal cable clamps are counted as one conductor. If one size conductor is in the box, the cable clamp(s) counts as one of those conductors. For example, a metal device box is equipped with two internal cable clamps. One 14-2 with ground nonmetallic sheathed cable is installed in the box. Although there are two cable clamps, they only count as one conductor. Instead of the box containing three 14 AWG conductors, four 14 AWG conductors must be counted (see Figure 4).

https://www.ecmag.com/magazine/articles/article-detail/codes-standards-box-fill-calculations-part-vi

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