NEC 314.16: Number of Conductors in Outlet, Device, and Junction Boxes
What This Article Covers
NEC 314.16 is the box fill calculation rule. It tells you how many cubic inches of capacity a box needs based on what's inside it — conductors, devices, internal clamps, and grounding wires.
Key Requirements
- Each conductor passing through or terminating in the box = 1 conductor of its own size
- Each grounding conductor (or bundle of grounding conductors) = 1 conductor of the largest grounding conductor present
- Each yoke = 2 conductors of the largest size connected to it
- Internal cable clamps (one or more) = 1 conductor of the largest size in the box
- Multiply the conductor count by the volume allowance in Table 314.16(B)
Common Field Applications
- Sizing a switch box that contains 12 AWG, two cables, and a single switch
- Verifying a junction box has capacity for a 6-cable splice
- Sizing a deep box for a multi-circuit switch installation
Common Mistakes & Inspection Failures
- Adding individual ground wire fill instead of one largest
- Forgetting yoke fill counts as 2
- Using the wrong volume allowance row
Related NEC Articles
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What does NEC 314.16 cover?
NEC 314.16 is the box fill calculation rule. It tells you how many cubic inches of capacity a box needs based on what's inside it — conductors, devices, internal clamps, and grounding wires.
What are the key requirements of NEC 314.16?
Key requirements include: Each conductor passing through or terminating in the box = 1 conductor of its own size; Each grounding conductor (or bundle of grounding conductors) = 1 conductor of the largest grounding conductor present; Each yoke = 2 conductors of the largest size connected to it. See the full requirements list on this page.
What are common mistakes with NEC 314.16?
Adding individual ground wire fill instead of one largest Forgetting yoke fill counts as 2 Using the wrong volume allowance row
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Michael B.
IBEW Local 134 Journeyman Electrician · Licensed Electrical Contractor
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