Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in Section E3905.12.1, be less than the box fill calculation as calculated in Section E3905.12.2. The minimum volume for conduit bodies shall be as calculated in Section E3905.12.3. The provisions of this section shall not apply to terminal housings supplied with motors or generators.
The volume of a wiring enclosure (box) shall be the total volume of the assembled sections, and, where used, the space provided by plaster rings, domed covers, extension rings, etc., that are marked with their volume in cubic inches or are made from boxes the dimensions of which are listed in Table E3905.12.1.
TABLE E3905.12.1
MAXIMUM NUMBER OF CONDUCTORS IN METAL BOXESa
| BOX DIMENSIONS (inches trade size and type) | MAXIMUM CAPACITY (cubic inches) | MAXIMUM NUMBER OF CONDUCTORSa | ||||||
| 18 Awg | 16 Awg | 14 Awg | 12 Awg | 10 Awg | 8 Awg | 6 Awg | ||
| 4 × 11/4 round or octagonal | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 × 11/2 round or octagonal | 15.5 | 10 | 8 | 7 | 6 | 6 | 5 | 3 |
| 4 × 21/8 round or octagonal | 21.5 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 × 11/4 square | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 × 11/2 square | 21.0 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 × 21/8 square | 30.3 | 20 | 17 | 15 | 13 | 12 | 10 | 6 |
| 411/16 × 11/4 square | 25.5 | 17 | 14 | 12 | 11 | 10 | 8 | 5 |
| 411/16 × 11/2 square | 29.5 | 19 | 16 | 14 | 13 | 11 | 9 | 5 |
| 411/16 × 21/8 square | 42.0 | 28 | 24 | 21 | 18 | 16 | 14 | 8 |
| 3 × 2 × 11/2 device | 7.5 | 5 | 4 | 3 | 3 | 3 | 2 | 1 |
| 3 × 2 × 2 device | 10.0 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 3 × 2 × 21/4 device | 10.5 | 7 | 6 | 5 | 4 | 4 | 3 | 2 |
| 3 × 2 × 21/2 device | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 3 × 2 × 23/4 device | 14.0 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 × 2 × 31/2 device | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 × 21/8 × 11/2 device | 10.3 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 4 × 21/8 × 17/8 device | 13.0 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 × 21/8 × 21/8 device | 14.5 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 33/4 × 2 × 21/2 masonry box/gang | 14.0 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 33/4 × 2 × 31/2 masonry box/gang | 21.0 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
For SI: 1 inch = 25.4 mm, 1 cubic inch = 16.4 cm3.
a. Where volume allowances are not required by Sections E3905.12.2.2 through E3905.12.2.5.
E3905.12.1.1 Standard boxes. The volumes of standard boxes that are not marked with a cubic-inch capacity shall be as given in Table E3905.12.1. E3905.12.1.2 Other boxes.
Boxes 100 cubic inches (1640 cm3) or less, other than those described in Table E3905.12.1, and nonmetallic boxes shall be durably and legibly marked by the manufacturer with their cubic-inch capacity. Boxes described in Table E3905.12.1 that have a larger cubic inch capacity than is designated in the table shall be permitted to have their cubic-inch capacity marked as required by this section.
The volumes in Section E3905.12.2.1 through Section E3905.12.2.5, as applicable, shall be added together. No allowance shall be required for small fittings such as locknuts and bushings.
E3905.12.2.1 Conductor fill.Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. Each loop or coil of unbroken conductor having a length equal to or greater than twice that required for free conductors by Section E3406.10.3, shall be counted twice. The conductor fill, in cubic inches, shall be computed using Table E3905.12.2.1. A conductor, no part of which leaves the box, shall not be counted.
Exception: An equipment grounding conductor or not more than four fixture wires smaller than No. 14, or both, shall be permitted to be omitted from the calculations where such conductors enter a box from a domed fixture or similar canopy and terminate within that box.
TABLE E3905.12.2.1
VOLUME ALLOWANCE REQUIRED PER CONDUCTOR
| SIZE OF CONDUCTOR | FREE SPACE WITHIN BOX FOR EACH CONDUCTOR (cubic inches) |
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
For SI: 1 cubic inch = 16.4 cm3.
E3905.12.2.2 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 E3905.12.2.1 shall be made based on the largest conductor present in the box. No allowance shall be required for a cable connector with its clamping mechanism outside the box. E3905.12.2.3 Support fittings fill.
Where one or more fixture studs or hickeys are present in the box, a single volume allowance in accordance with Table E3905.12.2.1 shall be made for each type of fitting based on the largest conductor present in the box. E3905.12.2.4 Device or equipment fill.
For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table E3905.12.2.1 shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap. For a device or utilization equipment that is wider than a single 2-inch (51 mm) device box as described in Table E3905.12.1, a double volume allowance shall be made for each ganged portion required for mounting of the device or equipment. E3905.12.2.5 Equipment grounding conductor fill.
Where one or more equipment grounding conductors or equipment bonding jumpers enters a box, a single volume allowance in accordance with Table E3905.12.2.1 shall be made based on the largest equipment grounding conductor or equipment bonding jumper present in the box.
Conduit bodies enclosing 6 AWG conductors or smaller, other than short-radius conduit bodies, shall have a cross-sectional area not less than twice the cross-sectional area of the largest conduit or tubing to which they can be attached. The maximum number of conductors permitted shall be the maximum number permitted by Section E3904.6 for the conduit to which it is attached.
E3905.12.3.1 Splices, taps or devices.Only those conduit bodies that are durably and legibly marked by the manufacturer with their cubic inch capacity shall be permitted to contain splices, taps or devices. The maximum number of conductors shall be calculated using the same procedure for similar conductors in other than standard boxes. E3905.12.3.2 Short-radius conduit bodies.
Conduit bodies such as capped elbows and service-entrance elbows that enclose conductors 6 AWG or smaller and that are only intended to enable the installation of the raceway and the contained conductors, shall not contain splices, taps, or devices and shall be of sufficient size to provide free space for all conductors enclosed in the conduit body.