Heat Pump Changeout: Ampacity, Disconnect Location, and a Long Thermostat Run
You are wiring a new 3-ton R-410A heat pump condenser on a residential remodel in Oregon. The nameplate reads RLA 14.2 A, LRA 79 A, Minimum Circuit Ampacity 20.0 A, Maximum Fuse or HACR Breaker 30 A. The existing circuit is 10 AWG copper NM-B, about 85 ft, mostly through an attic that reaches 130°F in summer, terminating at a 30 A single-pole breaker in the panel. The existing non-fused pull-out disconnect is within sight of the unit but 55 ft away, behind a hinged gate that is normally latched. The thermostat is 140 ft away in a hallway and the existing 18/4 thermostat cable is being reused. The new unit needs R, C, Y, O, and W2, and the auxiliary heat strip control pulls 0.6 A at 24 V through W2.
1. Is the existing 10 AWG NM-B on a 30 A breaker acceptable for this unit, including the hot attic? Show the check.
Read the worked answer
Start with the nameplate, not the breaker. The Minimum Circuit Ampacity of 20.0 A is the minimum the branch-circuit conductors must be able to carry; the Maximum Overcurrent Protective Device of 30 A is the ceiling for the breaker. Those two numbers intentionally do not match, which is normal for hermetic motor-compressor equipment.
10 AWG copper NM-B is a 60°C conductor and carries 30 A before adjustment. The attic at 130°F is a 55°C ambient, and the correction factor for 60°C insulation at that ambient is roughly 0.71. That gives about 21 A, still above the 20.0 A MCA, so the conductor passes - but just barely. The 30 A breaker is acceptable because it equals the nameplate maximum and motor-compressor circuits are permitted overcurrent protection above conductor ampacity. Confirm the NM-B is not in contact with the flue, is supported and protected from damage where it crosses the attic, and check for any bundled or insulation-covered sections that add another derating factor. If the run were 12 AWG, or if two circuits were bundled in that attic, the calculation would fail. Note the marginal result in your paperwork so the next technician does not assume the circuit is oversized.
2. Evaluate the disconnect and describe what has to change.
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Two problems, not one. First, the disconnect must be within sight of the equipment and within 50 ft of it; this one is 55 ft away, which fails on distance alone. Second, a disconnecting means must be readily accessible, and a pull-out behind a latched gate that the technician has to open is not readily accessible in the code sense - the intent is that someone can shut the unit down quickly without hunting for a key or a gate latch.
Relocate the disconnect to the wall within sight of the condenser and inside 50 ft, in a location reachable without opening a gate or climbing. Keep it at a working height, not buried behind landscaping. Because the unit's overcurrent protection is the 30 A breaker at the panel, a non-fused pull-out is still acceptable at the unit; you do not need to add fusing, but you do need to confirm the pull-out is rated for the circuit and labeled for the equipment it controls.
Also check the equipment grounding conductor and the bonding of any flexible connections, and confirm whether the unit's location triggers any Oregon Electrical Specialty Code requirements for a receptacle outlet at the equipment. Then call for the rough and final electrical inspections the jurisdiction requires.
3. The reused 18/4 cable cannot carry R, C, Y, O, and W2. What do you install, and does length matter here?
Read the worked answer
Five functions need at least five conductors, and in practice you pull six so there is a spare and so future additions do not require a re-pull. Use 18/6 thermostat cable at minimum, and step up to 18/5 or 18/6 with a heavier gauge if the load and distance demand it. Do not splice the fourth wire into two functions; the common and the W2 circuit cannot share a conductor.
Length does matter. 18 AWG copper is roughly 6.4 ohms per 1,000 ft. Over a 140 ft run the round trip is 280 ft, so resistance is about 1.8 ohms. At 0.6 A through the W2 circuit, voltage drop is roughly 1.1 V - about 4.5% of 24 V. That is on the edge before you add contactor inrush and transformer tolerance, and the resulting voltage at the unit may fall below the 21–22 V the controls need to hold reliably. Options are to move up to 16 AWG, relocate the thermostat, or add a relay at the air handler.
Before you button up, measure 24 V at the unit with the thermostat calling for auxiliary heat and confirm the transformer has capacity for the added load. Label both ends of the cable and leave the spare conductor landed and tagged.