Overview
The lecture covers Article 314 of the NEC, focusing on box fill calculations for outlet, device, and junction boxes, with step-by-step instructions for counting conductor volumes and using NEC tables to determine minimum box sizes.
Box Fill Calculation Rules
- Article 314.16 outlines rules for box fill calculations and is found on NEC page 195.
- All applicable volumes from 314.16(B)(1)-(B)(6) must be added together for total box fill.
- Small fittings (locknuts, bushings) do not require volume allowance.
- Each conductor entering and terminating or spliced in the box counts once; those passing through also count once.
- Conductor loops longer than 12 inches count as two conductor volumes.
- Internal conductors (e.g., pigtails, bonding jumpers) do not count.
- Use Table 314.16(B)(1) for volume per conductor size.
Volume Allowances for Special Items
- Internal cable clamps: Allowance equals one conductor volume (use largest conductor in box).
- Supporting fittings (luminaire studs, hickeys): Each type gets one conductor volume, based on largest conductor.
- Devices (switches, receptacles): Each counts as two conductor volumes, per largest conductor connected.
- Devices wider than 2” count as four conductor volumes.
- Equipment grounding conductors: Up to four count as one conductor volume; each beyond four adds ¼ volume.
Using NEC Tables for Box Sizing
- Table 314.16(A) lists minimum volumes for different box types/sizes.
- For same-size conductors, use the table to match total number to box size.
- For mixed sizes, multiply quantity by volume per Table 314.16(B)(1), add, and match to box volume.
- Box volume is sum of box and any marked extension rings/plaster rings.
Example Calculations
- For six 12 AWG conductors: 6 × 2.25 in³ = 13.5 in³ needed.
- For a box with various conductors and devices, sum all volume allowances, then find a box that meets or exceeds that total volume.
Box Fill Calculation Process
- Count all conductors, devices, fittings, equipment grounding conductors, and internal clamps.
- Multiply by appropriate volume allowance.
- Add all volumes and compare to box size using NEC Table 314.16(A).
Key Terms & Definitions
- Box fill — The total space required for conductors and fittings inside an electrical box.
- Conductor volume — Volume allowance assigned to each conductor entering/passing through the box.
- Device volume — Volume counted for each installed switch or receptacle, usually double conductor volume.
- Equipment grounding conductor — Ground wire; up to four counted as one conductor volume.
- Table 314.16(A) — NEC table listing minimum box sizes and capacities.
Action Items / Next Steps
- Review Article 314.16 and Tables 314.16(A) and (B)(1) in the NEC.
- Practice box fill calculations with different scenarios.
- Prepare to answer calculation questions for contractor and electrician exams.