Packing a Box and Bundling Cables in a Kitchen Remodel
During a kitchen remodel, an electrician must install a receptacle in an existing 4 in. × 4 in. × 2-1/8 in. metal box, which has a volume of 30.3 cu in. Three 12/2 NM cables and one 12/3 NM cable enter the box through internal cable clamps. The 12/3 supplies two 120 V circuits on opposite legs of the 120/240 V service, protected by a 20 A two-pole breaker. A duplex receptacle is mounted on a single-gang plaster ring that is not marked with any additional volume. All conductors are 12 AWG. The contractor then wants to pull one more 12/2 home run through the same box, and points out that all four existing cables already lie bundled together for 3 ft inside a fire-blocked chase.
1. Perform a box-fill calculation for the installation as it stands. Is it compliant?
Read the worked answer
Volume allowance - Table 314.16(B): 12 AWG = 2.25 cu in per counted item.
Count per 314.16(B)(1)–(5):
| Item | Count | cu in |
|---|---|---|
| Insulated conductors: 3 × 12/2 = 6, plus 1 × 12/3 = 3 | 9 | 20.25 |
| Equipment grounding conductors (4 EGCs count as one) | 1 | 2.25 |
| Internal cable clamps (count as one, if present) | 1 | 2.25 |
| Device on a single yoke (counts as two conductors) | 2 | 4.50 |
| Total | 13 | 29.25 |
29.25 cu in ≤ 30.3 cu in → compliant, but with only about 1 cu in of margin.
Notes: the plaster ring counts only if it is marked with its volume; this one is not, so it adds nothing. The 12/3 neutral is counted for fill purposes even though it may not be counted for ampacity adjustment - box fill counts every conductor that enters and is spliced or terminated.
2. The contractor wants to add one 12/2 NM cable to this same box. What does the calculation show, and what are the practical options?
Read the worked answer
Adding one 12/2 (two insulated conductors plus a ground that merges into the single EGC count):
- Insulated conductors: 9 + 2 = 11 → 11 × 2.25 = 24.75
- EGCs (still counted as one) → 2.25
- Clamps → 2.25
- Device (2 conductors) → 4.50
- Total = 33.75 cu in vs. 30.3 cu in available → overfilled by 3.45 cu in. Not compliant.
Options:
- Replace the box with a 4-11/16 in. square, 2-1/8 in. deep box (42.0 cu in), which accommodates the added cable with room to spare.
- Use the existing box for the receptacle and make the new home run in a separate box or a junction point elsewhere.
- Reduce the count in the existing box (fewer cables entering, or a device with a smaller yoke allowance) - rarely practical in a remodel.
The error to avoid is assuming that because the box passed before, one more cable is fine. Fill calculations are cumulative and the margin here was already thin.
3. Assume the added cable is installed and all five cables lie bundled together for 3 ft in the chase. Determine the ampacity adjustment and whether the 20 A breakers can stay.
Read the worked answer
Count current-carrying conductors (310.15(C)(1) and 310.15(E)(1)).
- Three 12/2 cables: 2 conductors each = 6 CCCs.
- The 12/3 is a multiwire branch circuit feeding two 120 V circuits on opposite legs; 310.15(E)(1) says the neutral is not counted when it carries only the unbalanced current from the other conductors of the same circuit → 2 CCCs (black and red).
- Add the new 12/2: +2 CCCs.
- Total = 10 CCCs.
(Note: the original total was 8 CCCs, before the addition.)
Adjustment - Table 310.15(C)(1): 10–20 conductors → 50%.
Ampacity: 334.80 limits NM cable to the 60°C column, so 12 AWG NM starts at 30 A. After adjustment: 30 A × 0.50 = 15 A.
The bundling length of 3 ft exceeds the 24 in. threshold in 310.15(C)(1), so the adjustment applies.
Result: 15 A adjusted ampacity will not support a 20 A overcurrent device (210.20(B), 240.4). The 20 A breakers cannot stay. Corrective actions:
- Re-route so the cables are not bundled for more than 24 in., or space them (separate holes with maintained spacing), restoring 30 A ampacity for the 12 AWG; or
- Reduce the overcurrent protection to 15 A for all affected circuits, which may be unacceptable for the kitchen small-appliance circuits; or
- Use a different wiring method or larger conductors for the added circuit.
The design lesson: box fill and ampacity adjustment are two independent checks, and passing one does not imply passing the other.