Nine Conductors Over a Hot Commercial Kitchen Ceiling
A restaurant remodel runs a 1 in. EMT 30 ft across the ceiling space above a commercial kitchen, from a 120/208 V panelboard to a junction box serving four 20 A, 120 V multioutlet receptacle circuits (cord-and-plug appliance loads) and one 20 A circuit to a hand-sink area. Nine 12 AWG THHN copper current-carrying conductors plus one 12 AWG equipment grounding conductor are pulled. A thermometer left on the raceway at midday reads 100 degrees F. Each circuit is on a 20 A plug-on breaker; all terminations are rated 75 degrees C. The foreman says 12 AWG is fine because 'the 90 degree column gives you 30 amps.' The inspector, walking the job the same day, also asks about GFCI protection for the counter receptacles.
1. Calculate the corrected and adjusted ampacity of the 12 AWG conductors and determine whether the 20 A breakers are acceptable.
Read the worked answer
Step 1 - start with the 90 degrees C column: 12 AWG THHN = 30 A.
Step 2 - ambient correction (Table 310.15(B)(1), 90 degrees C column): 100 degrees F = 37.8 degrees C, which falls in the 36-40 degrees C band = 0.91. 30 A x 0.91 = 27.3 A.
Step 3 - conductor adjustment (Table 310.15(C)(1)): 9 current-carrying conductors = 70 percent. 27.3 A x 0.70 = 19.1 A.
Step 4 - termination limit: 12 AWG copper on 75 degrees C terminations is limited to 20 A, so the governing ampacity is the lower value: 19.1 A.
Step 5 - overcurrent device: 240.4(B) does allow the next higher standard device above an ampacity that does not match a standard rating - but 240.4(B)(2) prohibits that allowance for multioutlet receptacle branch circuits supplying cord-and-plug loads. Four of the five circuits are exactly that.
Conclusion: 20 A breakers on 19.1 A ampacity are a violation. The foreman's '90 degree column' reasoning ignores both the ambient correction and the bundling adjustment.
2. If the run is split into two raceways, does the installation comply? Show the arithmetic and any other limits that must be rechecked.
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Splitting the raceway drops the number of current-carrying conductors per raceway to 4 and 5.
- Table 310.15(C)(1) adjustment for 4-6 conductors = 80 percent.
- 30 A x 0.91 (ambient) x 0.80 = 21.8 A of ampacity, which exceeds the 20 A circuit rating and is acceptable on a 20 A breaker with 12 AWG copper (termination limit 20 A).
Recheck before installing:
- Conduit fill for each new raceway (Chapter 9, Table 1 and Table 4).
- Ambient correction still applies at the new routing; do not assume the second raceway is cooler.
- Junction box fill (314.16) at both ends - nine CCCs, a grounding conductor, and splices in one box add up quickly.
- Voltage drop is not a Code requirement here but should be sanity-checked on a 30 ft run.
Alternative: upsizing to 10 AWG gives 40 A x 0.91 x 0.70 = 25.5 A in the original single raceway, which also resolves the problem but costs more copper than splitting the run.
3. Is GFCI protection required for the counter receptacles, and what is the basis?
Read the worked answer
Yes.
- Section 210.8(B) requires ground-fault circuit-interrupter protection for personnel on 125-250 V, single-phase receptacles in a list of non-dwelling occupancies that includes kitchens. A restaurant kitchen is a commercial kitchen, not a dwelling unit, so 210.8(A) does not apply but 210.8(B) does.
- The protection must be at the outlet or via a GFCI breaker, and it must be installed in a readily accessible location. GFCI receptacles downstream of a non-GFCI receptacle do not protect the upstream device.
- The hand-sink-area circuit should also be evaluated against the same list.
Practical note: the correction for the derating problem and the GFCI requirement are independent issues. Fixing the ampacity does not cure a missing GFCI, and vice versa.