Condenser Replacement on Existing 10 AWG Circuit
A 3-ton air-conditioner condenser is being replaced with a 4-ton unit. The new nameplate reads minimum circuit ampacity 27.5 A, maximum overcurrent protective device 45 A, rated-load current 22.4 A, and locked-rotor current 112 A. The existing branch circuit is 10 AWG NM-B, 60 feet long, run through an attic that reaches 130°F, protected by a 30-ampere breaker. The disconnect is 8 feet from the unit. The installing technician wants to reuse the existing circuit because the nameplate MCA is only 27.5 A and the breaker is 30 A. The homeowner reports the old unit tripped the breaker occasionally on hot afternoons. The new unit has a crankcase heater and a factory-installed hard-start kit. The attic access is tight, and the technician proposes to leave the circuit as is and replace the breaker with a 45-ampere unit if nuisance tripping occurs.
1. Is the existing 10 AWG NM-B acceptable for the new condenser, considering the attic ambient?
Read the worked answer
The nameplate MCA of 27.5 A is the minimum conductor ampacity, and 10 AWG NM-B has a 60°C ampacity of 30 A. At first glance that appears acceptable. But the attic reaches 130°F, about 54°C. The ambient correction factor for 60°C conductors at 54°C is roughly 0.58. The adjusted ampacity is 30 × 0.58 = 17.4 A, well below the 27.5 A MCA. The conductor is not acceptable in that attic unless the actual ambient along the run is lower or the conductor is rerouted. Even 8 AWG NM-B at 40 A × 0.58 = 23.2 A would fail. A 6 AWG NM-B at 55 A × 0.58 = 31.9 A would pass, or the electrician could use a 90°C-rated conductor in conduit with the correct correction. The technician cannot rely on the breaker size alone; the conductor ampacity after adjustment controls.
2. The technician wants to install a 45 A breaker on the existing 10 AWG. Is that permitted, and is it wise?
Read the worked answer
For a motor-compressor, the branch-circuit short-circuit and ground-fault protective device is sized from the nameplate maximum or from 440.22, not from the conductor ampacity in the same way as a general-purpose circuit. A 45 A breaker is the nameplate maximum and can be permitted with conductors having an ampacity at least equal to the MCA, provided the installation meets the motor-compressor rules. However, the existing 10 AWG fails the attic ambient correction, so the 45 A breaker cannot rescue it. Even if the ambient were acceptable, a 45 A breaker is at the top of the allowed range and may increase the available fault energy at the compressor; it should be used only when the nameplate and starting current require it. The technician should first correct the conductor ampacity, then select the breaker within the nameplate MOP. Upsizing the breaker to stop tripping without checking conductor temperature is a dangerous shortcut.
3. Evaluate the disconnect location and the proposed response to nuisance tripping.
Read the worked answer
The disconnect is 8 feet from the unit, which is within sight and within the 50-foot limit for a motor-compressor disconnect, so the location is acceptable. The proposed response - wait for nuisance tripping and then install a 45 A breaker - is not acceptable. The old unit’s tripping on hot afternoons suggests the existing circuit may already be marginal because of attic temperature, loose terminations, or a failing compressor. The new unit’s locked-rotor current of 112 A and hard-start kit affect starting, but they do not correct an undersized conductor. The electrician should perform a load and temperature check, verify terminations, megger the compressor, and correct the conductor size or route. The breaker must be selected within the nameplate MOP, and the conductor must still have adequate ampacity after ambient correction. A judgment call to reuse a marginal circuit because the attic is difficult to access is exactly the kind of shortcut that leads to overheating and callbacks.