Twelve-Unit Apartment: Which Findings Block the Permit and Which Wait for the Field
A permit set for a 12-unit, three-storey wood-frame apartment building arrives for plan review. Each unit is fed from a 400 A house panel by a 100 A, 208Y/120 V feeder; the longest is 62 m of 3/0 AWG aluminum in PVC to a unit panel whose calculated load is 62 A. Type NM-B cable fills the corridor ceiling, which forms part of a 1-hour rated assembly; the cables cross a 2-hour rated demising wall through a 50 mm PVC sleeve shown without firestopping. A kitchen wall box is specified as 4 in square, 2-1/8 in deep (marked 30.3 in³), with three 12/2 and two 12/3 NM cables entering, one duplex receptacle and a plaster ring. Each 12/3 feeds two 20 A circuits sharing a neutral; no handle ties are shown. Protection is shown as AFCI in kitchens and bathrooms and GFCI in bedrooms. The panel schedule shows 12 × 100 A feeder breakers plus a 60 A house load on a 400 A panel; no load calculation is included.
1. Sort the findings into three buckets - must be resolved before permit issuance, verify at rough-in, acceptable as designed - and justify each placement.
Read the worked answer
Must be resolved before issuance (design defects, cannot be fixed by field judgment):
- Missing load calculation for the 400 A house panel. The panel rating cannot be verified from a breaker schedule; twelve 100 A breakers in a 400 A panel may be perfectly correct or badly wrong, and only the calculation says which.
- Firestopping of the 50 mm PVC sleeve through the 2-hour demising wall. An unprotected sleeve in a rated assembly voids the rating; a listed firestop system must be specified with the sleeve, the annular space and the cable type all matching the system.
- Multiwire branch circuits without simultaneous disconnection. Two circuits sharing a neutral need handle ties or a two-pole device so that working on one does not leave the other live through the shared neutral.
- Protection scope is inverted. Bedrooms and living areas need arc-fault protection; kitchens and bathrooms need ground-fault protection (and the kitchen small-appliance circuits need both where the code requires it).
- Box fill - see the calculation below.
Verify at rough-in: NM support intervals and protection where it passes through framing, actual fill as installed with the plaster ring volume, cable routing and the corridor ceiling assembly, torque on feeder terminations, and the unit feeder bending radius.
Acceptable as designed: the 3/0 AWG aluminum unit feeder. Its 75 °C ampacity is well above the 100 A device, and at 62 A over 62 m the voltage drop is only a few volts; neither the ampacity nor the drop governs here.
2. Show the box fill calculation for the kitchen box and give the remedies you would accept.
Read the worked answer
Count in 12 AWG conductor volumes (2.25 in³ each):
- Three 12/2 cables: 6 circuit conductors.
- Two 12/3 cables: 6 circuit conductors (the shared neutral counts as a conductor).
- Total circuit conductors: 12 → 12 × 2.25 = 27.0 in³.
- Equipment grounding conductors: four EGCs, all 12 AWG, count as one → 2.25 in³.
- One duplex receptacle on one strap: one conductor volume → 2.25 in³.
- Cable clamps: none shown; if the box has internal clamps, add 2.25 in³.
Total = 27.0 + 2.25 + 2.25 = 31.5 in³ against 30.3 in³ marked on the box. Overfilled by 1.2 in³ - a small margin, but over is over, and with clamps it is worse.
Accepted remedies:
- Substitute a 4-11/16 in square, 2-1/8 in deep box (42 in³) at the same location; or
- Add a listed extension ring whose volume is marked and add its volume to the assembly; or
- Split into two boxes; or
- Reduce conductor count (for example, feed the second circuit from a different box).
Do not accept a plaster ring whose volume is unmarked, and do not accept a field calculation that ignores the device or the EGC count.
3. The designer says the 400 A house panel is fine because the optional multifamily demand method applies. What must the submitted calculation show, and how would you sanity-check it?
Read the worked answer
Require a dated, signed calculation showing:
- Each unit's calculated load by the standard method first (62 A is asserted but not shown), including the range, dryer, water heater and any air conditioning.
- The qualifying conditions for the optional method - it is available only where the stated conditions for dwelling units are met; if any unit has a load profile outside those conditions, the optional factor cannot be used.
- The demand factor applied to the connected unit load, with the table reference and edition.
- The house load separately (corridor lighting, elevators or lifts, house panel receptacle circuits, fire alarm, emergency lighting), with its own demand treatment, not merely the 60 A nameplate.
- The largest motor at 25 %, and the service conductor and busbar rating that result.
Sanity check: 12 × 62 A = 744 A. A multifamily demand factor in the low 40 % range gives roughly 310 A; adding the 60 A house load gives about 370 A against a 400 A panel. That is inside the rating but only by about 7 %, so it is exactly the case where the details decide the outcome: a single unit with a larger load, a house load that is actually 100 A, or use of the standard method instead of the optional one will push it over. Do not approve on the basis that the arithmetic is close; approve on the basis that the calculation is complete and the correct method was applied. Also confirm that the service conductors and the panel busbar match the 400 A device.