Rooftop Conduit, High Ambient, and Eight Current-Carrying Conductors
A set of four 20 A, 120 V general-purpose circuits (each carrying 16 A continuous load) is to run from a rooftop panel to equipment 90 ft away. The plan shows one 1 in. EMT run with eight 12 AWG THHN copper current-carrying conductors and one 12 AWG equipment grounding conductor. The raceway is supported on strut 4 in. above a flat roof; the outdoor design ambient is 35 °C. The installing contractor says 12 AWG is fine because it is 'good for 30 A at 90 °C' and the 20 A breakers protect it. The customer's equipment will not fit a larger conduit and wants the installation finished this week.
1. Apply the rooftop temperature adder and the conduit-fill adjustment and determine the allowable ampacity of the 12 AWG THHN conductors.
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Step 1 - start with the 90 °C column (THHN, 110.14(C) and 310.15): 12 AWG = 30 A.
Step 2 - rooftop adder. For a raceway mounted less than 7/8 in. above the roof with a design ambient in the 25–40 °C range, add 50 °F (28 °C). Effective ambient = 35 + 28 = 63 °C. Correction factor at 63 °C from the 90 °C column (Table 310.15(B)(2)(a)) = 0.71. 30 A × 0.71 = 21.3 A.
Step 3 - adjustment for current-carrying conductors. Eight current-carrying conductors in one raceway = 70% (Table 310.15(B)(3)(a)). 21.3 A × 0.70 = 14.9 A.
Step 4 - terminations. 110.14(C) caps the usable ampacity at the 60 °C/75 °C termination rating, but the adjusted value of 14.9 A is already the governing number.
Result: the 12 AWG conductors can carry only about 14.9 A, less than the 16 A load. The installation is non-compliant before any breaker discussion.
2. What is the largest overcurrent device permitted on this run, and what load does 240.4 allow?
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Protection cannot exceed the adjusted ampacity. 240.4 requires the overcurrent device to protect the conductor at its adjusted ampacity; 240.4(D) also caps 12 AWG at 20 A, but the lower of the two governs. With 14.9 A adjusted, the next standard device size at or below that is 15 A (240.6). A 20 A breaker over this run is a violation, regardless of the 90 °C column rating.
Separately, the 16 A continuous load itself requires 125% sizing per 210.19(A)(1): 16 × 1.25 = 20 A of circuit capacity - which 12 AWG in this raceway cannot deliver.
So there are two independent failures: the conductor cannot carry the load after adjustment, and the OCPD cannot be set to protect the conductor. The proper answers are to reduce the number of current-carrying conductors, raise the raceway, use a larger conductor, or re-route - not to keep 12 AWG and accept nuisance trips.
3. Propose two compliant fixes that work within the contractor's installation constraints, and show the numbers for each.
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Option A - split into two 1 in. EMT runs of four current-carrying conductors each (keep 1 in. EMT). Four CCCs = 1.0 adjustment factor (Table 310.15(B)(3)(a)).
- 12 AWG: 30 A × 0.71 (rooftop) × 1.0 = 21.3 A - still below 20 A after the 240.4 cap? 21.3 A exceeds 20 A, so a 20 A breaker is permitted and the 20 A continuous load sizing (16 A × 1.25 = 20 A) is met. Verify with the AHJ that the 21.3 A adjusted value is not rounded below 20 A at the termination.
- Safer: use 10 AWG in the two runs: 40 A × 0.71 × 1.0 = 28.4 A, comfortably above 20 A.
Option B - keep one conduit but upsize and reduce CCC count. Two 2-wire multiwire circuits sharing a neutral may reduce the count, but neutrals count as CCC on 4-wire circuits, so the clean fix is fewer circuits per raceway. If four CCCs remain and 10 AWG is used: 40 × 0.71 × 0.80 = 22.7 A → 20 A protection is compliant.
Option C - raise the raceway. If the raceway is supported more than 3-1/2 in. and up to 12 in. above the roof, the adder drops to 40 °F (22 °C): 35 + 22 = 57 °C, factor 0.76. With 8 CCCs: 30 × 0.76 × 0.70 = 16.0 A - still short of 20 A, so raising alone does not fix eight conductors of 12 AWG. The practical answer is two raceways or larger conductors.
Document the chosen option on the drawing with the ambient, adder, and adjustment factors shown, so the inspector can follow the math.