Box Fill and Class 2 Separation in a 4-11/16 Square Junction Box
You are replacing a damaged junction box in a commercial office. The existing 4-11/16 in. square, 2-1/8 in. deep metal box has a listed volume of 42.0 cu in. It contains four 12 AWG THHN conductors (two ungrounded, one neutral, one equipment ground) for a 20 A, 120 V circuit, plus two 18 AWG CL2 conductors for a 24 V control circuit. One cable clamp is used for the Class 2 cable; two EMT connectors enter the box. No device or fixture stud is installed. The Class 2 cable enters through a knockout without a barrier, and the power conductors are spliced with wire nuts within 1/2 in. of the control conductors. The owner wants to keep the same box because it fits the wall cavity. You must determine box fill, separation, and whether a barrier or a separate box is required.
1. Calculate the box fill and determine if the 42.0 cu in box is adequate.
Read the worked answer
Use the volume allowances in Table 314.16(B). Four 12 AWG conductors at 2.25 cu in each = 9.0 cu in. Two 18 AWG Class 2 conductors at 1.5 cu in each = 3.0 cu in. One cable clamp counts as one conductor of the largest size in the box, so add 2.25 cu in. The EMT connectors are raceway fittings, not cable clamps, and are not counted as box-fill conductors. No device or fixture stud is present, so there is no additional allowance for those items.
Total = 9.0 + 3.0 + 2.25 = 14.25 cu in. The box has 42.0 cu in available, so box fill is adequate with a large margin. The fill calculation is not the reason to reject the installation. If a listed barrier is added, check its instructions and whether it reduces the usable volume of each compartment; box fill must still be satisfied for the conductors in each compartment and the barrier must not damage conductors.
2. Is the mixing of 12 AWG power and 18 AWG Class 2 conductors in the same box permitted as installed?
Read the worked answer
No. The installation has Class 2 conductors and 120 V power conductors in the same box with no barrier. Article 725 generally requires Class 2 circuit conductors to be separated from electric light and power conductors by a permanent barrier when they occupy the same box, raceway, or enclosure. The wire nuts and the 1/2 in. spacing do not satisfy that requirement. The 18 AWG Class 2 cable entering through an open knockout is not a barrier.
Acceptable corrections are to install a listed box divider or barrier that maintains permanent separation, or to use a separate box for the Class 2 circuit. For a retrofit in a wall cavity, a separate box is often simpler and less likely to be compromised. Do not simply wrap the control conductors with tape or push them to one side. After separation, recheck that the power conductors are spliced with listed connectors, the equipment ground is continuous, and the box cover is installed.
3. If a barrier is installed, what else must be checked?
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The barrier must be listed for the box and installed per its instructions. It must be rigidly secured and must maintain the required separation between the power and Class 2 conductors; it cannot be a loose piece of plastic or tape. Check that the barrier does not cut or pinch conductor insulation and that each compartment has enough wiring space for its conductors, splices, and any device.
If the barrier is metal, it must be bonded to the box or otherwise be part of the equipment grounding path; a floating metal barrier can become energized. Keep the Class 2 cable entry separate from the power entry if the barrier design requires it. Confirm box fill after the barrier is installed, because the barrier may reduce usable volume even though the total box volume is unchanged. If these checks cannot be met with the existing 42.0 cu in box, install a separate box.