Conductor, Breaker, and Voltage-Drop Compliance for a 5-Ton Condenser
A 5-ton condenser nameplate lists 31.5 A minimum circuit ampacity and 40 A maximum overcurrent protection. The existing circuit is a 40 A breaker with 10 AWG copper NM-B cable, about 120 ft from the panel, run through a hot attic. The disconnect is a 60 A non-fused pullout within sight of the unit. Voltage at the panel is 240 V; voltage at the condenser under load is 228 V. The homeowner also added a 20 A convenience receptacle tapped from the same circuit at the disconnect. The condenser is a single-phase unit with a factory-installed contactor and no supplemental nameplate requirements.
1. Is the existing conductor acceptable for the condenser? Show the code reasoning.
Read the worked answer
No. The conductor must have an ampacity at least equal to the minimum circuit ampacity, and 10 AWG NM-B does not.
- NM-B is limited to the 60°C column. 10 AWG copper at 60°C has an ampacity of 30 A, which is below the 31.5 A MCA.
- The 40 A breaker is permitted because it does not exceed the 40 A maximum overcurrent protection, but the breaker size does not make the conductor legal. The conductor is protected against overload by the MCA sizing rules; it still must carry the MCA.
- A minimum replacement would be 8 AWG copper NM-B, which is 40 A at 60°C, or a conductor with an adjusted ampacity at least 31.5 A after any ambient or bundling correction.
- The 20 A convenience receptacle tapped from the condenser circuit is not acceptable as installed. It adds load to a circuit sized for the condenser and is not a recognized branch-circuit tap for this purpose; remove it or supply it from a separate circuit.
2. Evaluate voltage drop and attic ambient derating. What conductor would you specify?
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Voltage drop and attic temperature both matter here.
- Voltage drop for copper: VD = 2 x K x I x L / circular mils. Using 31.5 A and 120 ft, 10 AWG (10,380 cm) gives about 9.4 V, or 3.9% of 240 V. That exceeds a commonly used 3% branch-circuit target.
- 8 AWG (16,510 cm) gives about 5.9 V, or 2.5%, which is acceptable on voltage drop alone.
- The attic run derates the conductor. If the attic reaches 50°C, an 8 AWG NM-B conductor at 60°C starts at 40 A and is corrected to roughly 33 A, still above 31.5 A. If the attic reaches 60°C, the correction drops it to about 28 A, which is not enough.
- Specify 8 AWG copper NM-B only if the attic ambient is verified at or below the correction that keeps ampacity at 31.5 A or higher; otherwise use 6 AWG NM-B or a raceway method with conductors rated for the ambient. Confirm terminations are rated for the conductor and temperature.
3. List the corrections and inspection issues you would address before energizing.
Read the worked answer
Bring the circuit into compliance as a system, not just by upsizing wire.
- Replace the 10 AWG NM-B with a conductor whose corrected ampacity is at least 31.5 A, such as 8 AWG copper NM-B where attic temperature permits, or 6 AWG if the derated ampacity requires it.
- Keep the 40 A breaker because it is the maximum permitted and is not larger than the nameplate MOP. Do not install a larger breaker to compensate for voltage drop.
- Remove the convenience receptacle tap from the condenser circuit and provide a separate properly protected circuit if the homeowner wants a receptacle.
- Verify the 60 A non-fused disconnect is rated for the application and located within sight. Torque all terminals to the manufacturer's values and check the equipment grounding conductor is properly sized and connected.
- Obtain the required permit and inspection, and document voltage under load after the repair.