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I am wondering how conductors are counted for box fill calculations. In the illustration, the box on the left has 4 conductors, hot/neutral coming in and hot/neutral wires carrying power out to the next receptacle. In the box on the right, the wires are pigtailed, yet still two wires bringing power in and two carrying power out to the next thing downstream. But, there is also two short wires connected to the receptacle itself. Does this count as 6 conductors for box fill calculations?


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    You did not state the wire size so see my answer for a better explanation if you don’t have a ground , only hot and neutral your box fill for 14 awg would be 4 x 2 or 8 for the conductors a double for the receptacle or 4 more for 12 and last + 2 for the clamp or a total box fill of 14 cu in the pigtails and wirenuts are not included in the calculation. – Ed Beal Dec 9 '19 at 15:09

Box fill is covered in article 314.16 of the NEC. The wires are counted by size and type for example 14 awg wire is 2 cubic inches for each hot , neutral and only counted once for the ground, if you have a pig tail that doesn’t add to the count, if you have 12 awg wire it is 2.25 for each with a mix of 14 & 12 the 1 ground wire counted will be the largest or 2.25. The device or switch/ receptacle counts 2x the cu inches of the largest wire connecting to it so if a 12 gauge wire connects the receptacle takes 4.5 cu inches. If the boxes have clamps those count 1 time also for the largest wire size . That’s the basics the short pigtails and wirenuts are not added in the amount of the fill.

  • Okay, I am trying to follow. So, if this circuit was wired with 12 awg wire, the box on the left would be: 2.25 (neutral) 2.25 (hot) 2.25 (neutral) 2.25 (hot) 2.25 (ground) and 4.5 (receptacle) for a total calculation of 15.75? – clwhoops44 Dec 9 '19 at 15:09
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    Plus the clamp at 2.25 but yes you have it. Grounds, clamps studs and hickeys are all only counted 1 time even if multiple ground wires. Receptacle boxes won’t normally have a stud or hickey, but do have clamps normally but not always. – Ed Beal Dec 9 '19 at 15:13
  • Ed, on the blue plastic boxes with the "integrated" clamps (basically just tabs) do those count as a clamp? – clwhoops44 Dec 9 '19 at 15:15
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    It depends on the inspector, some do and some don’t if the wire is stapled within 8” of the box my jurisdiction doesn’t count single gang but a double gang they do. 314.16.2 clamp fill on a integrated plastic box the clamps not required to be added. – Ed Beal Dec 9 '19 at 15:26

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