Rooftop EMT Circuit for a Maintenance Receptacle
A 20 A, 120 V circuit is to feed a maintenance receptacle on the roof of a big-box store. The route is 85 ft of 3/4 in. EMT laid flat on the roof membrane, with a 3 ft rise to a disconnect and receptacle. The contractor proposes 12 AWG THHN copper, with three current-carrying conductors for a multiwire branch circuit, in the conduit. The design rooftop ambient is 38°C, and the raceway is less than 4 in. above the roof surface, so the 33°C rooftop temperature adder applies. The run crosses a building expansion joint and ends in raintight boxes. The inspector rejects the 12 AWG installation because of the corrected ampacity. The contractor argues that 12 AWG is protected by a 20 A breaker and that EMT is outside, so it runs cooler.
1. Calculate the corrected ampacity of the 12 AWG THHN conductors and determine whether the installation complies. Show the table values you used.
Read the worked answer
Ambient temperature
The design ambient is 38°C. Because the raceway is less than 4 in. above the roof, the rooftop temperature adder applies: 38°C + 33°C = 71°C.
Correction factor
For 90°C-rated THHN, NEC Table 310.15(B)(1) gives a correction factor of 0.50 for an ambient of 71–75°C.
Ampacity
- 12 AWG THHN at 90°C: 30 A
- Corrected: 30 A × 0.50 = 15 A
The circuit has three current-carrying conductors, so no 310.15(C)(1) adjustment applies for conduit fill. The corrected ampacity is still only 15 A.
Conclusion
A 20 A branch circuit requires conductors with an ampacity of at least 20 A after correction, and the terminations must also be suitable. The 12 AWG conductors do not comply. A 20 A breaker does not fix the problem: the breaker protects the circuit, but it cannot raise the conductor ampacity. The installation fails NEC 210.19(A)(1) and the termination-temperature rules in 110.14(C).
2. What minimum conductor size would comply, and what termination-temperature check is needed?
Read the worked answer
Try 10 AWG THHN copper:
- 10 AWG THHN at 90°C: 40 A
- Corrected: 40 A × 0.50 = 20 A
That meets the 20 A branch-circuit requirement after ambient correction.
Termination check
The 20 A overcurrent device and the equipment terminations are likely rated 75°C. At 75°C, 10 AWG copper has an ampacity of 35 A, which is greater than the 20 A circuit rating. NEC 240.4(D) limits 10 AWG copper to a 30 A overcurrent device, so a 20 A breaker is acceptable.
A 10 AWG conductor is therefore the minimum size that works by calculation. A better design may relocate the raceway to reduce the rooftop adder, use a wiring method with a higher temperature rating, or increase the size further for voltage drop, but those are design choices beyond the minimum code requirement.
3. List the EMT installation requirements the inspector will check on this roof run.
Read the worked answer
The inspector will check both the raceway and the wet-location details.
- Support: EMT must be supported within 3 ft of each box and at intervals not exceeding 10 ft under NEC 358.30.
- Expansion: The run crosses a building expansion joint. NEC 358.44 requires an expansion fitting where EMT crosses a structural expansion joint or where thermal expansion is expected.
- Wet-location fittings: The roof is a wet location. Couplings and connectors must be listed and raintight under NEC 358.42, and boxes must be listed for wet locations.
- Corrosion protection: EMT must be protected against corrosion. Fittings, straps, and supports must be suitable for the outdoor environment under NEC 358.10(B).
- Drainage and arrangement: The raceway must be arranged so water does not accumulate in boxes or equipment, and entries must be made from the side or bottom where required.
The corrected ampacity calculation is the first failure, but the raceway details are equally part of the inspection.