Kitchen Multiwire Branch Circuit Red-Tagged at Rough-In
A two-gang box in a kitchen remodel is fed by 12/3 NM-B from a 200-ampere, 20-space panel. The intent is a multiwire branch circuit serving eight countertop receptacles around a peninsula and a sink. The crew landed the two ungrounded conductors on two separate single-pole breakers in adjacent spaces with no handle tie, and installed one single-pole GFCI receptacle at the first outlet, feeding the remaining receptacles from its LOAD terminals. Two 12/2 NM-B cables leave the box to downstream receptacles, one from each circuit; all equipment grounding conductors are twisted together with a crimp; the shared neutral is looped through the GFCI and through both receptacle yokes. The box is a 4-11/16-inch square, 2-1/8-inch deep, with two internal cable clamps. Because the panel had no open spaces left, the crew added four tandem breakers in the bottom four spaces. The subcode official red-tagged the rough-in and cited the multiwire branch circuit and panel fill.
1. List the code violations in the box and breaker arrangement and describe the correction, citing the rules you rely on.
Read the worked answer
1. No simultaneous disconnection (210.4(B)). A multiwire branch circuit must be disconnected simultaneously at the panelboard by a two-pole breaker or by single-pole breakers with an identified handle tie. Two unconnected single-poles is a violation regardless of whether the circuits are GFCI-protected. Correction: install a two-pole device, which is also required for the next two items.
2. GFCI cannot be a single-pole receptacle on a shared neutral (210.8(A)(6)). Kitchen countertop receptacles need ground-fault protection. A single-pole GFCI receptacle senses the net current of its own circuit only; with a shared neutral, the return current of the other circuit passes through the device's sensing circuit and causes nuisance tripping, and the LOAD-side neutral cannot be shared because the device would break continuity for the other circuit. Correction: protect the whole multiwire circuit at the panel with a two-pole GFCI breaker, or run separate 12/2 home runs if the owner will pay for the cable.
3. AFCI is required (210.12(A)). Kitchen branch circuits in a dwelling unit must be arc-fault protected in the editions adopted since kitchen areas were added to the scope. Use a two-pole combination AFCI/GFCI breaker, and verify the manufacturer lists the device for shared-neutral multiwire circuits. A single-pole AFCI cannot be used here.
4. Shared neutral must be pigtailed (300.13(B)). The grounded conductor of the multiwire circuit must be spliced with a pigtail so removing one receptacle does not interrupt the neutral for the other circuit. Looping the neutral through the yokes violates this, and 210.4(D) also requires the conductors of the multiwire circuit to be grouped and identified.
5. Receptacles. Confirm tamper-resistant receptacles are installed in the dwelling (406.12) and that countertop spacing and peninsula requirements in 210.52(C) are met after the correction, since relocating the first outlet to fit a two-pole breaker's needs may shift the layout.
2. The inspector also flagged box fill. Verify the fill calculation for the 4-11/16-inch box and state whether the box is actually adequate.
Read the worked answer
Available volume. Table 314.16(A): a 4-11/16-inch square box, 2-1/8 inches deep, has 42.0 cubic inches.
Per-conductor volume. Table 314.16(B): 12 AWG = 2.25 in³ per conductor equivalent.
Count the equivalents (314.16(B)):
- 12/3 incoming: 3 conductors = 3
- Two 12/2 outgoing cables: 4 conductors = 4
- Equipment grounding conductors: three EGCs count as one equivalent = 1
- Two internal cable clamps count as one equivalent (314.16(B)(2)) = 1
- Two receptacle yokes, two equivalents each (314.16(B)(4)) = 4
Total = 13 equivalents x 2.25 = 29.25 in³.
29.25 in³ is well under 42.0 in³, with about 12.75 in³ to spare, so the box is compliant and does not need to be enlarged. Pigtails that originate inside the box are not counted as additional conductors, so the required neutral pigtail in 300.13(B) does not change the result. If a plaster ring is added for the finished wall, use the volume marked on the ring and add it to the 42.0 in³; if the ring is unmarked, it contributes nothing and the calculation above is the one to submit. Show this arithmetic to the inspector rather than replacing boxes to satisfy a citation that is not supported by the calculation.
3. The four tandem breakers are the second citation. Are they permitted, and what are the options?
Read the worked answer
The controlling rule is 408.54, which prohibits a panelboard from containing more overcurrent devices than the number for which it is listed; 110.3(b) requires the equipment to be installed in accordance with its listing and labeling. A 20-space panel is only a 20/40 panel if its labeling and wiring diagram specifically show which spaces accept tandem (half-size) breakers. On a straight 20-space panel, snapping tandems into the bottom four spaces is a violation even though they physically fit, and the branch-circuit breaker fingers of a tandem do not land on the bus the way the panel was evaluated for.
What to check on the panel label. Look for a diagram showing tandem positions, a maximum circuit count, and the specific tandem breaker types the manufacturer lists. If the label shows tandem-rated spaces, tandems are permitted only in those spaces and only with the listed breaker type, and the total connected load and busbar rating must still be respected.
Practical options, in order of preference:
- Replace the panel with a larger enclosure. A service that is adding kitchen circuits, AFCI/GFCI two-pole devices (which consume two spaces each), and future load is going to be short of spaces permanently.
- Add a subpanel fed by a properly sized feeder with a grounding conductor sized per 250.122, and move existing circuits to it.
- If the existing panel is listed for tandems, use the listed tandem types only in the marked spaces - never in spaces reserved for the main or for two-pole devices.
Do not solve the space problem by double-tapping the bus, using a listed piggyback only where the label permits it, or by installing a smaller main breaker. Also confirm with the subcode official whether the panel change triggers the service calculation in the first case study, because the added EVSE and hot tub loads are what actually drove the panel to run out of spaces.