Box Fill in a Tiled Kitchen: Does the Box Pass?
A 4 in. square, 2-1/8 in. deep steel box (Table 314.16(A) volume 30.3 cu in.) is set in a tiled kitchen backsplash for a countertop receptacle. Four 12/2 NM cables enter through the box's two internal cable clamps: two from the panel side, one feeding the next counter outlet, and one running down to a dishwasher switch. All conductors are 12 AWG; every splice is made with wire nuts, and the receptacle is a duplex on a metal yoke. The installer added a 3/4 in. raised plaster ring marked 5.0 cu in. to bring the device flush with the tile, and ran one 12 AWG grounding pigtail from the bundle of bare equipment grounding conductors to the receptacle. The inspector red-tagged the box for fill.
1. Perform the box fill calculation and state whether the box complies.
Read the worked answer
Count each item at the volume from Table 314.16(B): 12 AWG = 2.25 cu in. per conductor.
| Item | Count | Volume |
|---|---|---|
| Ungrounded + grounded conductors (4 cables x 2) | 8 | 8 x 2.25 = 18.00 |
| Equipment grounding conductors - all four bare EGCs count as one per 314.16(B)(5) | 1 | 2.25 |
| Internal cable clamps - one allowance per 314.16(B)(2) | 1 | 2.25 |
| Duplex receptacle yoke - counts as two conductors based on the largest conductor connected, per 314.16(B)(4) | 2 | 4.50 |
| Listed plaster ring, volume marked on the ring | - | 5.00 |
| Total required | 32.00 cu in. |
Available: 30.3 cu in. Required: 32.00 cu in. The box is overfilled by 1.70 cu in.
The red tag is correct. Note what does not count: the wire nuts, the splicing itself, and the 12 AWG grounding pigtail to the receptacle (it is part of the EGC allowance, not an added conductor). The two EGC items that are most often miscounted are the four EGCs - they do not count as four - and the device yoke, which does count as two.
2. The installer wants to fix it without disturbing the tile. Give the compliant options and rank them.
Read the worked answer
The available volume must reach 32.00 cu in. including the ring's 5.00 cu in.
- Listed extension ring with marked volume. Least destructive and compliant: add a listed extension ring whose marked cu in. plus 30.3 covers 32.00. The ring must be listed for the box, the device must still mount properly, and a cover must close it. Total volume is the sum of the box and the marked ring - the same rule that made the plaster ring count adds to the available volume here.
- Change the box to 4 x 4 x 2-1/2 (34.0 cu in.). Since 32.00 <= 34.0, it passes. Requires removing the tile or cutting from behind, so it is usually not the first choice in a finished backsplash.
- Change to a 4-11/16 x 2-1/8 box (42.0 cu in.). Comfortable margin, but the tile opening almost certainly will not accept it without rework.
- Reduce the conductor count. Re-route the dishwasher switch cable to its own box, or eliminate one of the through-cables with a splice elsewhere. This changes the design, not just the hardware, and is often the best long-term answer when a box is loaded with four cables.
What is not an option: forcing the conductors, trimming the EGC pigtail to a stub, or omitting the clamp. 110.12 requires the installation to be performed in a neat and workmanlike manner, and 314.16 exists because crushed conductors and damaged insulation are the failure mode.
3. The installer argues the plaster ring should be ignored because it is a ring, not a box, and that a shorter grounding pigtail would solve the problem. Evaluate both claims.
Read the worked answer
Claim 1: the plaster ring does not count - wrong.
314.16(A) allows the volume of a box assembly to include a listed extension ring or plaster ring where the volume is marked on the ring. This ring is marked 5.0 cu in., so its volume is part of the assembly's available volume and the conductors and device inside it consume volume. You cannot take the marking as a benefit only when it helps. If the ring carried no marking, it could not be used to add volume - and on some assemblies an unmarked ring changes nothing in either direction, which is why the calculation is done from the marked value.
Claim 2: shorten the grounding pigtail - irrelevant.
The pigtail is not counted as a separate conductor at all. 314.16(B)(5) gives one allowance for all the equipment grounding conductors, and the pigtail from that bundle to the device is part of that allowance. Cutting it shorter changes the required volume by 0.00 cu in. and does nothing about the 1.70 cu in. deficit.
The only things that change the number are: more available volume (larger box, marked extension ring), fewer counted conductors (route a cable out of the box), or a device that does not add a yoke allowance (not realistic for a receptacle). Anything else is a guess dressed up as a calculation, and the next inspector will red-tag it again.