Replacement Rooftop Unit on an Undersized-Looking Existing Circuit
A three-ton rooftop package unit is replaced with a larger unit. Nameplate: MCA 41.2 A, MOCP 60 A, compressor RLA 28 A, LRA 145 A, fan FLA 2.2 A, 208/230 V single-phase. The existing branch circuit is a 50 A 2-pole breaker feeding three 8 AWG THHN copper conductors plus a 10 AWG equipment grounding conductor in 3/4 in EMT; the run is 60 ft and all terminations are 75 C. A non-fused pull-out disconnect is mounted on the roof curb about 40 ft from the unit, around a parapet. A 125 V receptacle on the roof is fed from the load side of that disconnect and sits 30 ft from the unit. On the first cool morning the new unit trips the 50 A breaker roughly one start in five, and the installer proposes to swap in a 70 A breaker because it only trips on startup.
1. Are the existing 8 AWG conductors and 50 A breaker acceptable for the new unit? Show the sizing logic.
Read the worked answer
Yes - the circuit is code-minimum acceptable as it stands.
Conductors: For a single hermetic refrigerant motor-compressor, branch-circuit conductors need ampacity not less than 125% of the rated-load current or branch-circuit selection current. The nameplate MCA already includes that 125% plus the other loads, so the conductor ampacity must be at least 41.2 A. 8 AWG Cu THHN at the 75 C termination rating = 50 A > 41.2 A. Compliant.
Overcurrent device: 440.22 permits the short-circuit and ground-fault protective device up to 175% (225% if needed for starting), but it must not exceed the equipment's marked maximum. The 50 A breaker is below the 60 A MOCP and above the 41.2 A MCA, so it is a permitted size.
Why a 50 A breaker on 50 A conductors is not the whole story: the unit's internal overload protection protects the conductors against overload; the branch breaker provides only short-circuit and ground-fault protection. That is why 240.4(G) directs motor-compressor conductors to Article 440, Part IV, and why #8 conductors on a 60 A breaker would still be permissible here.
What is not acceptable: 8 AWG on a 70 A breaker with a 60 A marked maximum. The MOCP is a listing limit (110.3(B)) - exceeding it voids the listing and the branch-circuit protection scheme.
2. Evaluate the disconnect and the roof receptacle locations.
Read the worked answer
Disconnect - 440.14
A disconnecting means must be located within sight from and readily accessible from the air-conditioning equipment. "Within sight from" means visible and not more than 50 ft away. This pull-out is 40 ft away around a parapet, so it is not visible from the unit. Distance passes, visibility fails - it is a violation.
Fix: install a disconnect within sight of the unit on the roof (non-fused is fine where the branch breaker provides the short-circuit protection), or relocate the existing one so it is visible from the service access panel.
Servicing receptacle - 210.63
- A 125 V, 15 or 20 A receptacle must be provided within 25 ft of the heating/air-conditioning equipment for servicing. At 30 ft, this one is out of range.
- It must not be connected to the load side of the equipment disconnecting means. Wired to the load side, it de-energizes with the unit, which defeats the purpose of servicing equipment - and it is a code violation independent of the distance.
Fix: refeed the receptacle from the line side (or from another circuit in the same structure), and locate it within 25 ft of the unit and accessible without a ladder from the service access point.
Note that 210.63 applies to the equipment, not to the convenience of the existing roof layout; a receptacle "close enough" on the other side of a parapet does not comply.
3. The unit trips the 50 A breaker on startup about one start in five. Evaluate the proposed 70 A breaker and give a diagnostic plan.
Read the worked answer
Reject the 70 A breaker. It exceeds the 60 A marked maximum. Installing it makes the equipment noncompliant, may void the unit warranty, removes the manufacturer's intended protection, and does not address the cause.
Permitted moves:
- Go to 60 A, not 70 A. A 60 A HACR-rated breaker is within the marked MOCP for a 41.2 A MCA unit. The 8 AWG conductors remain acceptable because overload protection is provided inside the unit and the branch device is providing short-circuit/ground-fault protection only (240.4(G), Article 440 Part IV). Verify the breaker is HACR listed.
- Confirm it really is a starting issue. LRA is 145 A; a healthy 50 A thermal-magnetic breaker normally rides through that. One failure in five suggests an intermittent condition, not a steady-state overload.
Diagnostic plan:
- Check actual supply voltage at the unit contactor under load - low voltage raises running and starting current. A 60 ft run of 8 AWG at 41 A drops several volts; measure at both ends.
- Inspect the start components: run capacitor, start capacitor/PTCR, contactor pitting, control wiring and the low-voltage thermostat circuit.
- Check refrigerant charge and head pressure; an overcharged or restricted system starts against high head.
- Check for short-cycling or a control that lets the compressor restart before the system equalizes.
- Verify terminations are tight and rated 75 C, and that the conduit is not carrying a second circuit that was missed in the fill/derating check.
Sequence: replace with the correct 60 A HACR breaker, then instrument the unit. If a 60 A device still trips on one start in five, the fault is mechanical or electrical inside the unit, not the breaker size.