Rooftop PVC Run to Condenser Disconnects
A 12-unit apartment building has a 60-foot rooftop conduit run from a house panel to four condenser disconnects. The electrician proposes 1-inch Schedule 40 PVC, supported every 10 feet, with three 8 AWG THWN-2 copper current-carrying conductors and a 40-ampere breaker. The rooftop is black membrane; summer design ambient is 40°C, and the conduit is mounted 1 inch above the roof. The run is straight with no expansion fitting, and the last 6 feet transitions to liquidtight flexible metal conduit at the disconnect. The building owner wants to avoid a new penetration and asks whether the existing 8 AWG is acceptable. The calculated load is 32 amperes continuous. There is no spare conduit, and the electrician has already pulled the conductors.
1. Identify the support and expansion problems with the PVC installation and explain the likely physical result.
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One-inch Schedule 40 PVC does not have a 10-foot support interval. Support spacing is much shorter - generally 3 feet for 1-inch Schedule 40 - with supports near boxes and terminations. More importantly, a straight 60-foot rooftop run with no expansion fitting has no way to absorb thermal growth. PVC expands roughly 3.4 × 10⁻⁵ inch per inch per °F. A 60-foot run is 720 inches; a 100°F swing produces about 2.4 inches of movement. Restrained at both ends, that movement pushes into fittings and boxes, bows the conduit, opens solvent joints, or cracks fittings. The electrician needs an expansion fitting and proper support, not just more straps. The transition to liquidtight must also be supported and listed for the location. Because the conductors are already pulled, correcting the raceway may require pulling them out and reworking the run.
2. Determine whether the 8 AWG THWN-2 conductors are acceptable for the 32 A continuous load, showing the adjustment steps.
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A 32 A continuous load requires conductors with an ampacity of at least 32 × 1.25 = 40 A before overcurrent protection is selected. 8 AWG THWN-2 copper has a 90°C ampacity of 55 A, but that is only a starting point. At a 40°C ambient, the 90°C correction factor is 0.91, giving 55 × 0.91 = 50 A. Because the conduit is exposed on a rooftop, the rooftop ambient adder also applies; for a 1-inch raceway, that adder is commonly 25°C, so the design ambient becomes 65°C and the 90°C correction is about 0.65. The adjusted ampacity is roughly 55 × 0.65 = 35.8 A. That is below the required 40 A. The 8 AWG fails once the rooftop adder is applied. The electrician should use a larger conductor - 6 AWG is the practical minimum - or change the raceway route or method so the rooftop temperature adder does not apply.
3. The electrician wants to keep the 8 AWG and simply change the breaker to 35 A. Evaluate that proposal.
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Changing to a 35 A breaker does not solve the continuous-load problem because the conductor ampacity after adjustment is about 35.8 A, and the load requires 40 A of conductor ampacity. A 35 A breaker is also below the 40 A continuous-load requirement, so it would likely trip under normal operation. The breaker protects the conductor, but upsizing or downsizing the breaker cannot create ampacity that the adjusted conductor does not have. The correct approach is to correct the installation: increase conductor size, reduce the rooftop temperature exposure, or provide a raceway method with a lower ambient adder. The electrician should also verify terminations are rated for the selected conductor and that the disconnect terminals accept the larger size. Simply resetting the breaker or ignoring the continuous-load factor is not acceptable.