Rooftop Unit: Nameplate MCA/MOP Versus Field Conditions
A 208 V, three-phase packaged rooftop unit with hermetic refrigerant compressors has a nameplate listing minimum circuit ampacity (MCA) 42 A, maximum overcurrent protective device (MOP) 60 A, and rated-load current 38 A. It replaces an older unit of similar size. The existing branch circuit is 8 AWG copper THWN in 1 in. EMT, about 110 ft one-way from a panel, protected by a 60 A three-pole breaker. The conduit runs exposed on a flat roof where the design ambient is 100°F; the run is in full sun, roughly 1 ft above the roof surface. The only disconnecting means is a nonfusible switch on a wall at grade level, out of sight of the unit; roof access is through a hatch.
1. Determine whether the existing 8 AWG THWN conductors have adequate ampacity for this unit after temperature correction, and state what you would install if they do not.
Read the worked answer
Step 1 - Required conductor ampacity
For hermetic refrigerant motor-compressors, the branch-circuit conductors are sized to the nameplate MCA (the 125% factor is already built into the MCA). Required ampacity = 42 A.
Step 2 - Nameplate ampacity of 8 AWG THWN
- 8 AWG Cu at 75°C = 50 A (30°C ambient) → looks adequate at first glance.
- Rooftop ambient correction (310.15(B)(1)) plus the rooftop sunlight adder for raceways exposed on or above a roof (310.15(B)(2)) changes the answer.
Design ambient 100°F = 38°C. With a rooftop adder on the order of 22°C for a raceway about 1 ft above the roof, the effective ambient is roughly 60°C.
- 8 AWG THWN (75°C conductor) at 60°C: 50 A × 0.58 ≈ 29 A → fails 42 A
- Even if the conductor is THWN-2 (90°C): 55 A × 0.71 ≈ 39 A → still fails 42 A
- With no adder at all, 8 AWG THWN at 40°C = 50 × 0.88 = 44 A, a razor-thin margin that disappears with any bundling adjustment.
Step 3 - What to install
- 6 AWG Cu THWN-2: 75 A at 90°C × 0.71 = 53 A ≥ 42 A; termination limit at 75°C is 65 A (110.14(C)), so the usable ampacity is 53 A.
- Verify 110.14(C): the corrected ampacity must also not exceed the 75°C column value for the terminations, which it does not.
- Check voltage drop while you are there: at 42 A, 208 V three-phase, 110 ft of 8 AWG (≈0.778 ohm/1000 ft) gives about 6.2 V ≈ 3.0%; 6 AWG (≈0.491 ohm/1000 ft) gives about 3.9 V ≈ 1.9%. Voltage drop is advisory under 210.19, but the 6 AWG leaves a much better margin for a rooftop run.
Note: 6 AWG THWN-2 in 1 in. EMT is fine for fill - confirm with Chapter 9 Table 1 and Table 4 for the conductor count and size you actually pull.
2. Is a 60 A breaker permitted to protect 8 AWG conductors on this circuit? Explain the motor-circuit rules and where the conductor ampacity check still applies.
Read the worked answer
Yes - up to a point, and the point is often missed.
- The nameplate MOP of 60 A is the ceiling. You may protect a hermetic refrigerant motor-compressor branch circuit at up to that marked maximum. 440.22 permits the branch-circuit short-circuit and ground-fault protective device to be sized above the conductor ampacity for these circuits, and 240.4(G) lists motor-compressor circuits as an exception to the small-conductor rule.
- So a 60 A three-pole breaker on 8 AWG copper is permitted by the overcurrent rules, provided the conductors have adequate ampacity for the load.
The catch
The overcurrent exception does not excuse inadequate conductor ampacity. The conductors must still have an ampacity of at least the MCA (42 A) after ambient correction and adjustment. As shown above, 8 AWG THWN on that roof corrects to roughly 29 A (39 A if THWN-2), so the conductor - not the breaker - is the failure point.
Practical rule
- Size the OCPD to the nameplate MOP (never above it; 60 A here is acceptable).
- Size the conductor to the nameplate MCA, then apply 310.15 correction and adjustment factors and 110.14(C) termination limits.
- Check that the equipment grounding conductor is sized per 250.122 off the 60 A device (10 A Cu minimum).
With 6 AWG Cu, the 60 A breaker, the MCA of 42 A, and the corrected ampacity of about 53 A all line up.
3. What are the disconnecting means and servicing-receptacle requirements for this unit, and is the grade-level switch acceptable?
Read the worked answer
Disconnecting means - 440.14
A disconnecting means must be located within sight of and readily accessible from the air-conditioning or refrigerating equipment. A wall switch at grade level, out of sight of a rooftop unit accessed by a hatch, does not satisfy the general rule. Two compliant paths:
- Install a disconnect on the roof, within sight of and readily accessible from the unit (nonfusible switch, or a circuit breaker installed as the disconnect where permitted), sized per 440.14 at not less than 115% of the rated-load current (38 A × 1.15 = 43.7 A → a 60 A device is fine).
- Rely on the exception only where the disconnect can be locked in the open position with a lock provision at the disconnect, and the arrangement is acceptable to the AHJ. If the grade-level switch cannot be locked open, this option is unavailable, and lockout at the panel is not a substitute for a lockable disconnect.
Servicing receptacle - 210.63
A 125 V, 15 or 20 A receptacle outlet must be provided for servicing and located within 25 ft of the equipment and accessible from it without a ladder or scaffolding gymnastics. On a rooftop this usually means a dedicated rooftop receptacle. Because it is outdoors, it needs GFCI protection for personnel (210.8), and the circuit must be arranged so it is not the unit's own circuit.
Also confirm the unit's overcurrent device is accessible and labeled for the unit, and that the rooftop conduit and fittings are listed for wet locations and sunlight exposure.