Rooftop Feeder: Ambient Derating, Cable Substitution, and Thermal Expansion
A 40 A minimum-circuit-ampacity, 480 V three-phase rooftop package unit with a 50 A maximum overcurrent device is fed from a panelboard 140 ft away. The contractor pulls four 8 AWG THWN-2 copper conductors (three ungrounded plus a neutral for the unit's 120 V controls) in 3/4 in. EMT. Fifteen feet of the run crosses the roof at 4 in. above the TPO membrane in full sun, where the design ambient is 100 F (38 C); the jurisdiction's plan reviewer applies the rooftop temperature adder to that section and, for the run as installed, the effective ambient used for derating is 48 C. Terminals are rated 75 C. The mechanical contractor wants to swap the rooftop portion to NM-B because 'it is under a cover board and it is dry up there,' and wants to omit expansion fittings because the run is short and the conduit is strapped tight at both ends. The panel breaker is 50 A. Roof surface temperatures range from about minus 20 F in winter to 150 F in summer.
1. Determine the permitted ampacity of the 8 AWG conductors and state whether they are adequate. Show each adjustment.
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Start from the conductor's own insulation column and then apply every correction, because the lowest result governs.
8 AWG THWN-2 in the 90 C column is 55 A. Ambient correction at 48 C for a 90 C conductor is 0.82, giving 45.1 A. Four current-carrying conductors (the neutral counts because of the VFD's harmonic content) require 80 percent adjustment under 310.15(C)(1), giving 36.1 A. The 75 C terminal limitation for 8 AWG is 50 A, so it does not control. Permitted ampacity is 36.1 A.
The unit's minimum circuit ampacity is 40 A, so 36.1 A is short by about 4 A and the 8 AWG fails. Move to 6 AWG: 75 A in the 90 C column, 61.5 A after ambient, 49.2 A after the 80 percent adjustment, and 65 A at the 75 C terminals. That exceeds the 40 A MCA and remains protected by the 50 A device within the nameplate maximum. Document the calculation on the plans, because the inspector will ask which ambient and which adder you used.
2. Evaluate the proposed NM-B substitution for the rooftop section.
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It has to be rejected on at least three independent grounds, any one of which is enough.
A raceway run outdoors on a roof is a wet location under 300.9, and 334.12(B) does not permit NM cable in wet locations, cover board or not; condensation and driving rain both reach the interior of that raceway. Second, 334.80 limits NM-B ampacity to the 60 C column regardless of the 90 C conductor rating, which alone would drop 8 AWG to about 26 A after adjustment. Third, NM cable is not sunlight resistant and is not listed for the ambient temperature swings of a roof assembly, and substituting it also breaks the continuity of the equipment grounding path that the EMT provides.
If the intent was to save labor, the better economy is to leave the EMT intact, use THWN-2 throughout, and only change the conductor size to 6 AWG. The rooftop section is 15 ft of a 140 ft run, so upsizing the entire run costs very little and removes an argument at inspection.
3. Is the decision to omit expansion fittings defensible? Support it quantitatively.
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No. Steel expands about 6.5 x 10^-6 in/in per degree F, and the roof surface range in the setup is roughly 170 F from winter low to summer high. For 140 ft, 1680 in, the movement is 1680 x 6.5 x 10^-6 x 170, about 1.9 in. A run strapped tight at both ends has nowhere to put that movement, so it buckles, pulls couplings apart, or opens at the curb penetration.
300.7 requires provisions for thermal expansion in raceways, and 358.44 requires expansion fittings where EMT is subject to large temperature changes. Provide an expansion fitting (or a listed slip joint) in the rooftop section, set the EMT with the proper support spacing and allow the fitting to take up the calculated travel, and use listed wet-location fittings throughout. At the curb penetration, use a listed flashing and a sealing method rated for the membrane. Adding one fitting is inexpensive compared with a callback to repair a separated coupling after the first cold snap.