Rooftop Unit: Nameplate MCA/MOP Versus the Reused Circuit
A 3-ton packaged rooftop unit is being replaced on a 208/230 V single-phase system. The new nameplate reads: minimum circuit ampacity (MCA) 26.4 A; maximum overcurrent protective device (MOP) 40 A; compressor rated-load current 16.5 A; outdoor fan FLA 2.2 A. The panel is 45 ft away. The installer ran 10/2 Type NM-B through an attic that reaches 48 °C (118 °F) in summer, protected by a 30 A breaker, and terminated at a non-fused disconnect mounted on the roof within sight of the unit. A 1/2 in. liquidtight whip connects the disconnect to the unit. The previous unit also had a 30 A breaker and 10 AWG, so the installer reused the existing breaker 'to keep it simple.' Before energizing, the journeyman must sign off on the installation and is asked whether the reused breaker and the NM cable are acceptable.
1. Evaluate the 10 AWG Type NM-B in the 48 °C attic against the new unit's nameplate MCA.
Read the worked answer
Type NM-B must be sized from the 60 °C column (334.80). The 60 °C ampacity of 10 AWG copper is 30 A (Table 310.16). The attic ambient then requires correction: at 48 °C, the 60 °C correction factor is 0.61 (Table 310.15(B)(1)).
30 A × 0.61 = 18.3 A.
That is far below the 26.4 A MCA. The branch-circuit conductors for this listed unit must have an ampacity at least equal to the nameplate MCA (440.32/440.33 and 110.3(b)), so the installation is not acceptable even though 10 AWG 'worked' on the old unit.
Options that would work:
- 6 AWG NM-B: 60 °C ampacity 55 A × 0.61 = 33.6 A, above 26.4 A.
- 8 AWG NM-B: 40 A × 0.61 = 24.4 A, still below 26.4 A, so not acceptable.
- 8 AWG THHN in conduit: 90 °C ampacity 55 A × 0.82 (90 °C correction at 48 °C) = 45.1 A, capped by the 75 °C termination value of 50 A; acceptable, but NM cable cannot use the 90 °C column.
2. Is the 30 A breaker acceptable, and what does the MOP rating mean for this circuit?
Read the worked answer
The MOP of 40 A is the maximum overcurrent device the listing permits for this unit (440.22 and 110.3(b)). The breaker must be at least the MCA and no larger than the MOP, and it must also protect the conductors.
With 6 AWG NM-B at an adjusted 33.6 A, a 30 A breaker protects the conductor and falls inside the 26.4 A to 40 A band, so 30 A is acceptable. A 40 A breaker would also be inside the nameplate MOP band, but it would exceed the 33.6 A adjusted ampacity of 6 AWG NM, so 40 A could not be used with that cable; it would require a conductor with at least 40 A adjusted ampacity.
What is not acceptable is the existing arrangement: 10 AWG NM at an adjusted 18.3 A on a 30 A breaker is both an unprotectable conductor and below the MCA. The reused breaker is only part of the problem; the cable is the larger failure. Also confirm the breaker is rated for the available fault current and is listed for the panel.
3. What needs checking at the disconnect and whip before you sign off?
Read the worked answer
The disconnect is within sight of the unit, which satisfies the location intent of 440.14, but its rating must be checked. For a hermetic refrigerant motor-compressor, 440.12(A)(1) requires the disconnecting means to have an ampacity at least 115% of the rated-load current. Using the MCA as the basis, 26.4 × 1.15 = 30.4 A, so a 30 A disconnect is marginally undersized; a 60 A non-fused disconnect is the usual safe choice. A non-fused disconnect is generally acceptable where the branch-circuit overcurrent device is at the panel and the installation instructions do not require fusing, but the nameplate and instructions govern.
The 1/2 in. liquidtight whip is likely too small. Three 6 AWG THHN conductors have a combined area of about 0.152 in², while 1/2 in. liquidtight flexible metal conduit at 40% fill is only about 0.12 in². Check Chapter 9, Table 1 and Table 4, and expect to need 3/4 in. Confirm the whip conductors are copper and rated for wet locations, the equipment grounding conductor is sized per 250.122 and bonded to the unit, and the unit is listed for the application.