RBX-5 — Outlet, Device, Junction, and Pull Boxes

Calculate small-conductor box fill

Section 314.16 applies volume calculations to boxes and conduit bodies containing 4 AWG and smaller conductors. Use the box's marked volume or the volume assigned to a standard metal box by Table 314.16(A). A marked extension ring, dome, or cover can add its identified volume; an unmarked cover or unused space outside the box does not. Compare total required volume with total permitted volume.

Table 314.16(B) assigns each allowance by conductor size: 18 AWG = 1.50 in³, 16 AWG = 1.75, 14 AWG = 2.00, 12 AWG = 2.25, 10 AWG = 2.50, 8 AWG = 3.00, and 6 AWG = 5.00. Mixed-size boxes require multiplying each item by the allowance of the conductor specified by its rule, then adding the volumes.

Apply the five categories separately:

1. Conductor fill: each insulated conductor entering from outside and terminating or splicing in the box counts once; each unbroken pass-through conductor counts once. A loop or coil of unbroken conductor at least twice the 300.14 minimum free length counts twice. A pigtail that originates and remains entirely inside the box does not count.
2. Clamp fill: one or more internal cable clamps together take one allowance based on the largest conductor present. A connector whose clamping mechanism is outside the box takes no clamp allowance.
3. Support-fitting fill: one or more luminaire studs or hickeys take one allowance for each type of support fitting present, based on the largest conductor present.
4. Device or equipment fill: each yoke or strap takes two allowances based on the largest conductor connected to a device or equipment on that yoke. One duplex receptacle on one yoke is two allowances, not one per receptacle face.
5. Equipment-grounding fill: under the 2017 rule, all EGCs and equipment bonding jumpers together generally take one allowance based on the largest present. The additional isolated-ground set permitted by 250.146(D) receives the additional allowance specified by 314.16(B)(5). Do not import the later quarter-volume method into a 2017 problem.

Work a device-box example

Assume two 12/2 NM-B cables enter a metal box, their two black and two white conductors are spliced to pigtails feeding one duplex receptacle, both cable EGCs are joined to the device and box, and the box has an internal cable clamp. All conductors are 12 AWG.

• four insulated conductors entering from outside: 4 allowances;
• pigtails originating and ending inside: 0;
• all equipment grounding and bonding conductors together: 1;
• one or more internal clamps: 1; and
• one receptacle yoke: 2.

Total = 4 + 1 + 1 + 2 = 8 allowances. At 2.25 in³ each, required volume is 8 × 2.25 = 18.00 in³. A box marked 18.0 in³ meets the numerical fill exactly; a 17.5 in³ box fails. If all clamps are external, remove the one clamp allowance and the result is 7 × 2.25 = 15.75 in³. Actual device depth, conductor bending, installation damage, and manufacturer instructions can justify a larger box even when the minimum calculation passes.

Section 300.14 generally leaves at least 6 in. of free conductor measured from where it emerges from the cable sheath or raceway. If the box opening is less than 8 in. in any dimension, the conductor must extend at least 3 in. outside the opening. Conductors that neither splice nor terminate at the point have the stated exception. Do not shorten pigtails to solve an undersized-box problem.

Size boxes for large-conductor pulls

Section 314.28 applies minimum dimensions where raceways or cables contain insulated conductors 4 AWG and larger. Determine the pull direction and calculate each applicable row; box volume alone is not the test.

For a straight pull, the box length in the pull direction is at least eight times the trade size of the largest raceway. A straight pull through 3-in. raceways requires at least 8 × 3 = 24 in. from the entering wall to the opposite wall. Raceway entries should be arranged so the conductors follow that straight direction without a hidden angle pull.

For an angle or U pull, or a splice, the distance from each raceway entry to the opposite wall is at least six times the trade size of the largest raceway in a row plus the sum of the trade sizes of all other raceways in the same row on that wall. If one row contains 3-in. and 2-in. raceways, required distance is (6 × 3) + 2 = 20 in. Calculate other rows independently and use the controlling requirement.

For raceway entries enclosing the same conductor in an angle or U pull, their separation must also be at least six times the trade size of the larger raceway. Two 3-in. entries carrying the same conductors therefore need at least 18 in. separation. The opposite-wall dimension and entry-separation tests are both required. Listed boxes and conduit bodies of smaller dimensions can be used only within the specific conductor combinations, markings, and listing permission of 314.28(A)(3); field judgment alone cannot waive the calculation.

Finish the enclosure installation

Boxes must be rigidly and securely supported under 314.23 by a permitted building, framing, bracket, or raceway-supported method. A box intended to support a luminaire or ceiling-suspended fan must comply with 314.27 and its listing and weight marking; an ordinary lighting box is not automatically fan-rated. Metal boxes are bonded under 250.148, and removing a device cannot interrupt downstream grounding continuity.

Provide covers under 314.25 and keep wiring and pull boxes accessible under 314.29; a removable ceiling panel can provide access, but drywall or permanent finish cannot conceal the cover. Use wet-location boxes and covers where required, close unused openings, protect conductors at entries, and keep combustible wall surfaces within the edge and gap rules. Check box depth for projecting devices and large conductors. Final review includes calculated volume or directional dimensions, free conductor length, bend space, support, grounding, cover, environment, and accessibility.