Rooftop unit branch circuit: MCA, MOP, and ambient derating
A contractor is replacing a 5-ton rooftop package unit on a 208/230 V, three-phase, 60 Hz building. The new unit nameplate lists a minimum circuit ampacity of 32 A and maximum overcurrent protection of 40 A. The existing branch circuit is 10 AWG copper THHN in 3/4 in. EMT, 115 ft from the panel, protected by a 40 A three-pole breaker. The conduit contains three current-carrying conductors and has 12 ft exposed to a rooftop ambient of 110°F; terminals are rated 75°C. The unit has a factory-installed disconnect, but the old disconnect is a 60 A non-fused disconnect at the curb. The contractor proposes to reuse the 10 AWG and 40 A breaker because the MCA is only 32 A and the old unit ran fine.
1. Is the 10 AWG THHN acceptable for the branch circuit? Show the ampacity checks, including termination and ambient correction.
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Check the weakest link. At 75°C terminals, 10 AWG copper THHN has an ampacity of 35 A. The MCA is 32 A, so the conductor is acceptable only if the corrected ampacity remains at least 32 A. For the 110°F rooftop exposure, use the 90°C conductor rating for derating but do not exceed the 75°C termination limit. 10 AWG THHN at 90°C is 40 A; a typical 110°F correction is about 0.87, giving about 34.8 A. That is above the MCA and below the 35 A termination limit, so it may be acceptable if the actual installation conditions match the assumptions.
However, the 115-ft run is marginal for voltage drop. At 32 A, 208 V, and about 1.24 ohms per 1,000 ft for 10 AWG, the drop is roughly 8 V, or nearly 4%. That is not automatically a code violation, but it is poor design for a motor load. I would verify the actual terminal ratings and rooftop temperature, and I would consider upsizing to 8 AWG for voltage drop and reliability.
2. What overcurrent device and disconnect arrangement is required? Can the existing 60 A non-fused disconnect remain?
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The branch-circuit overcurrent device must not exceed the nameplate MOP of 40 A, and it must be at least the MCA. The existing 40 A breaker is within that range, so it is acceptable if it is the correct type, has adequate interrupting rating, and is not tripping from the old unit. A 60 A breaker would not be acceptable.
The 60 A non-fused disconnect can remain only if it is rated for the application, in good condition, and acceptable to the authority having jurisdiction. A non-fused disconnect does not provide overcurrent protection; the branch breaker does. The disconnect must be within sight of the unit or permitted by code, properly mounted, and rated for the conductor and load. I would inspect it for corrosion, missing deadfront, and proper grounding, and replace it if damaged. The factory disconnect does not eliminate the need for the branch-circuit protection.
3. The rooftop ambient and 115-ft run raise voltage-drop and termination concerns. What would you do before energizing?
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Before energizing, verify the actual conductor insulation type and temperature rating, terminal ratings at both ends, conduit fill, and the true ambient temperature in the rooftop conduit. If the conduit is in direct sun or the 12-ft exposure is hotter than assumed, recalculate derating and possibly upsize the conductors. Confirm the breaker is the correct HACR type and has an interrupting rating adequate for the panel.
Then measure voltage at the panel and at the unit under load, check phase balance, and confirm the control transformer tap matches the supplied voltage. Verify the equipment grounding conductor is correctly sized and bonded, the disconnect is properly labeled, and all terminations are torqued. If voltage drop exceeds about 3% at the unit, I would upsize to 8 AWG rather than rely on the minimum code ampacity. Document the readings on the commissioning sheet.