For a standard 3-ton 240V central AC unit with a Minimum Circuit Ampacity (MCA) of 22A and Maximum Overcurrent Protection (MOCP) of 35A, use 10 AWG copper THHN wire and a 35A double-pole breaker. This assumes copper conductors, the 75°C temperature column, 30°C ambient temperature, and EMT conduit.
- Conductor Material: Copper (Aluminum requires different sizing)
- Insulation Type: THHN / THWN-2
- Temperature Column: 75°C (per NEC 110.14(C) for standard residential terminations)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: EMT or PVC conduit, maximum 3 current-carrying conductors
Decoding the HVAC Nameplate: MCA vs. MOCP
When determining what size breaker for AC unit installations is correct, you must ignore the Full Load Amps (FLA) or Locked Rotor Amps (LRA) for your primary wire sizing. Instead, look for two specific values on the outdoor condenser's data plate:
| Nameplate Metric | Definition | What It Sizes | Example Value |
|---|---|---|---|
| MCA (Minimum Circuit Ampacity) | 125% of the largest motor FLA + 100% of all other loads. | Wire Size | 22A |
| MOCP (Max Overcurrent Protection) | The absolute maximum breaker size allowed to prevent nuisance tripping during compressor startup. | Breaker Size | 35A |
The manufacturer has already done the math required by NFPA 70 (NEC) Article 440. Your job is simply to match the wire to the MCA and the breaker to the MOCP.
Ampacity, the 75°C Rule, and Why We Don't Downsize
A common question on the jobsite is: "The MCA is 22A, and 12 AWG wire is rated for 25A at 75°C. Why can't I use 12 AWG instead of 10 AWG?"
While 12 AWG technically covers the 22A MCA, you must also protect the wire from the breaker. If you install a 35A breaker (the MOCP) on 12 AWG wire, a sustained fault of 30A could melt the 12 AWG insulation before the breaker ever trips. 10 AWG copper wire is rated for exactly 35A in the 75°C column of NEC Table 310.16. This perfectly aligns the wire's ampacity with the 35A breaker, ensuring the overcurrent device protects the conductor while still accommodating the compressor's massive inrush current.
Voltage Drop Verification at 80 Feet
Wire sizing isn't just about ampacity; it's also about voltage drop. The NEC recommends a maximum 3% voltage drop on branch circuits. Let's verify our 10 AWG choice for an 80-foot one-way run from the panel to the disconnect box.
- Formula: VD = (2 × K × I × L) / CM
- K (Copper at 75°C): 12.9 ohms per mil-foot
- I (Current/MCA): 22A
- L (Length): 80 feet
- CM (Circular Mils for 10 AWG): 10,380
Calculation: (2 × 12.9 × 22 × 80) / 10,380 = 4.36 Volts.
Percentage: 4.36V / 240V = 1.81%.
At 1.81%, we are well under the 3% threshold. If your run exceeds 130 feet, you must upsize to 8 AWG copper to maintain acceptable voltage drop, regardless of what the nameplate says. For deeper dive into the math, reference this voltage drop calculation guide.
Variables That Force an Upsize (Decision Tree)
The baseline answer assumes ideal conditions. Real-world installations often require derating or upsizing. Use this decision matrix to adjust your materials list.
| Condition | Impact on Circuit | Required Action |
|---|---|---|
| Using Aluminum Wire | 10 AWG Al is only rated 30A at 75°C. It cannot safely terminate on a 35A breaker. | Upsize to 8 AWG Aluminum (rated 40A at 75°C) and use anti-oxidant paste. |
| Bundling in Conduit | Pulling 4 to 6 current-carrying conductors in one raceway requires an 80% derating factor. | Upsize to 8 AWG Copper (40A × 0.8 = 32A, which covers the 22A MCA). |
| High Ambient Heat | Attic or rooftop conduit runs exceeding 110°F (43°C) require temperature correction factors. | Upsize to 8 AWG Copper to compensate for thermal derating. |
| Long Wire Runs | Distances over 130 feet push voltage drop past the 3% NEC recommendation. | Upsize to 8 AWG Copper strictly for voltage drop mitigation. |
When the AHJ or an Engineer Must Step In
While the MCA/MOCP method covers 95% of residential split-system air conditioners, there are scenarios where you must stop and consult the Authority Having Jurisdiction (AHJ) or a licensed mechanical/electrical engineer:
- Missing or Faded Nameplates: If you cannot read the MCA and MOCP, you cannot legally size the circuit. An HVAC technician must clamp the lines and calculate the load, or the unit must be replaced.
- Commercial VRF Systems: Variable Refrigerant Flow systems often have complex power factor corrections and multiple compressor stages. The branch circuit sizing for VRF often requires proprietary software and engineered single-line diagrams.
- Inverter-Driven Mini-Splits: Some modern hyper-heat mini-splits draw massive inrush currents for defrost cycles that don't align with standard Article 440 tables. Always defer to the manufacturer's specific installation manual, which supersedes general NEC tables when tested and listed by UL/ETL.
- Local Code Amendments: Some municipalities mandate a 20% safety margin on top of the manufacturer's MCA. Always check local AHJ bulletins before pulling wire.
Frequently Asked Questions
What size breaker for a 2-ton AC unit?
A typical 2-ton 240V central AC unit has an MCA of around 14A to 16A and an MOCP of 20A or 25A. For a unit with a 15A MCA and 20A MOCP, you must use 12 AWG copper wire and a 20A double-pole breaker. If the MOCP is 25A, you must upsize the wire to 10 AWG copper to safely match the breaker, even though 12 AWG covers the MCA. Always read the specific data plate on your condenser.
Can I use a 40-amp breaker on a 30-amp AC circuit?
No. If the nameplate specifies an MOCP of 30A, installing a 40A breaker is a direct violation of NEC 110.3(B), which requires you to follow the listing and labeling instructions of the equipment. A 40A breaker will not trip fast enough to protect the internal contactor and compressor windings during a short circuit, creating a severe fire hazard. If a 30A breaker constantly trips on startup, the compressor is likely failing or the breaker is worn out—do not oversize the breaker to mask a mechanical fault.
Does an AC unit need a dedicated circuit breaker?
Yes. NEC Article 440 and general appliance rules dictate that central air conditioning equipment must be on a dedicated, individual branch circuit. You cannot share this double-pole breaker with any other receptacles, lights, or appliances. The only exception is that a 120V service receptacle (for HVAC technician tools) can sometimes be tapped on the load side of the disconnect switch, provided it is GFCI protected and does not violate the dedicated circuit's load calculations, though running a separate 120V circuit for the service receptacle is the much cleaner, preferred jobsite practice.
