Rooftop Unit Nuisance Tripping
A packaged rooftop HVAC unit on a 208/230 V single-phase supply has a nameplate reading: Minimum Circuit Ampacity 41.2 A, Maximum Overcurrent Protection 60 A, compressor RLA 26.4 A, outdoor fan FLA 2.8 A. The crew installs 8 AWG copper THWN-2 in EMT from a 60 A breaker in the building panel to a 60 A non-fused pullout disconnect on the roof, within sight of the unit. One-way length is 140 ft. Summer roof ambient is 40 C and all terminations are 75 C. On the hottest afternoons the unit trips, and the maintenance tech proposes swapping the 60 A breaker for a 70 A.
1. Is 8 AWG copper acceptable on a 60 A breaker for this unit? Justify your answer, including the ambient correction.
Read the worked answer
Yes, it is acceptable.
- 440.35 requires branch-circuit conductors to have an ampacity not less than the nameplate Minimum Circuit Ampacity, which is 41.2 A. The MCA already includes the required 125% factor for the largest motor-compressor, so it is not multiplied again.
- 8 AWG copper THWN-2 at 75 C = 50 A. Ambient correction at 40 C from Table 310.15(B)(1) is 0.88: 50 A x 0.88 = 44 A, which still exceeds 41.2 A.
- The apparent conflict with the 60 A breaker is resolved by 240.4(G): conductors for hermetic refrigerant motor-compressors are protected under Article 440, not the general rules. The 60 A device provides ground-fault and short-circuit protection per 440.22; overload protection is provided by the unit's internal compressor and fan overloads.
The 8 AWG is therefore code-legal, even though a casual glance at a 60 A breaker suggests it should be 6 AWG.
2. Evaluate the technician's plan to install a 70 A breaker, and outline the correct troubleshooting approach.
Read the worked answer
The 70 A breaker is not permitted.
The nameplate MOP of 60 A is the maximum rating of the branch-circuit protective device. Installing a 70 A device:
- Exceeds the listing and the MOP, voiding the equipment listing (110.3(A)).
- Removes the intended short-circuit/ground-fault protection level for the 8 AWG conductors and the compressor.
- Hides a real problem instead of correcting it.
Correct approach - find the cause:
- Verify actual supply voltage at the unit under load. 140 ft of 8 AWG at about 41 A drops roughly 8 to 9 V (about 4% at 208 V); low voltage raises compressor current and can cause overload trips.
- Measure compressor running amps against the 26.4 A RLA and check start-up current.
- Check capacitor(s), contactor pitting, and lug torque; a loose lug heats the breaker and causes thermal trips.
- Check refrigerant charge, condenser coil condition, and fan operation.
- Confirm the breaker is not in a hot, sun-baked enclosure; a breaker in a high-ambient enclosure trips below its rating.
If voltage drop is the issue, upsize the conductors - the breaker stays at 60 A maximum.
3. What size copper equipment grounding conductor is required in the EMT, and does the disconnect location meet the code?
Read the worked answer
Equipment grounding conductor:
- 250.122 sizes the EGC by the rating of the overcurrent device ahead of the circuit: 60 A requires 10 AWG copper.
- If the circuit conductors are upsized for voltage drop, 250.122(B) requires the EGC to be increased proportionally. Going from 8 AWG to 6 AWG roughly doubles the circular-mil area, so the EGC would go from 10 AWG to 8 AWG.
- The EMT itself is a permitted EGC, but if a wire-type EGC is installed it must be the correct size and the conduit must still be made up tight.
Disconnect location:
- 440.14 requires a disconnecting means within sight from and readily accessible from the air-conditioning equipment. 'Within sight' means visible and not more than 50 ft away. A 60 A non-fused pullout on the roof beside the unit satisfies this, and a non-fused device is permitted because the branch breaker provides the short-circuit and ground-fault protection.
- The disconnect must be capable of being locked open only if it is also the means for disconnecting for maintenance; otherwise lockout is done at the panel.