For a standard 3-ton (36,000 BTU) 240V central air conditioner with a Minimum Circuit Ampacity (MCA) of 20A, use a 25A double-pole breaker and 10 AWG THHN copper wire. Always size the breaker to the manufacturer's Maximum Overcurrent Protection (MOP) and the wire to the MCA.
Baseline Assumptions for This Guide
- Conductor Material: Copper (AA-8000 aluminum requires upsizing, detailed below).
- Temperature Column: 75°C column per NEC Table 310.16 (standard for modern breakers and AC disconnects).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Individual THHN/THWN-2 conductors in EMT or PVC conduit, with no more than three current-carrying conductors bundled together.
The Direct Answer: Breaker and Wire Sizing for Common AC Units
Central air conditioners do not use standard receptacle wiring rules. Because they are motor-driven, they fall under NEC Article 440. The manufacturer provides two critical numbers on the outdoor condenser nameplate: MCA (Minimum Circuit Ampacity) and MOP (Maximum Overcurrent Protection).
| Unit Size (Tons / BTU) | Typical MCA (Amps) | Typical MOP (Amps) | Min. Wire Size (THHN Cu) | Required Breaker Size |
|---|---|---|---|---|
| 1.5 Ton (18,000) | 9.5A | 15A | 14 AWG (12 AWG rec.) | 15A Double-Pole |
| 2.0 Ton (24,000) | 12.5A | 20A | 12 AWG | 20A Double-Pole |
| 3.0 Ton (36,000) | 19.2A | 25A | 12 AWG (10 AWG rec.) | 25A Double-Pole |
| 4.0 Ton (48,000) | 25.5A | 40A | 8 AWG | 40A Double-Pole |
| 5.0 Ton (60,000) | 31.0A | 50A | 8 AWG | 50A Double-Pole |
Decoding the Nameplate: Why We Size to MOP and MCA
A common mistake is calculating the breaker size using basic Ohm's law (Watts ÷ Volts = Amps) and sizing the breaker to match the running current. Do not do this. If you install a 20A breaker on a unit with a 20A running draw, the breaker will trip instantly when the compressor kicks on.
Compressor motors experience Locked Rotor Amps (LRA) during startup—a massive inrush current that can be 5 to 7 times higher than the running current, lasting for a fraction of a second.
- Why we size wire to MCA: The MCA represents the maximum continuous running current plus a safety margin (typically 125% of the largest motor load plus 100% of other loads). The wire only needs to handle the continuous running heat, so sizing it to the MCA ensures the insulation won't melt during normal operation.
- Why we size the breaker to MOP: The MOP is deliberately set higher than the MCA to allow the startup inrush current to pass without tripping the magnetic mechanism. The breaker protects the wire from catastrophic short circuits, while the AC unit's internal overload protector guards the motor windings from sustained, low-level overloads.
- Why not one size smaller? If the nameplate specifies a 30A MOP and you install a 25A breaker to "be safe," you risk nuisance tripping on hot days when the compressor works hardest and the startup surge peaks. You also violate NEC 440.22, which requires the overcurrent device to match the manufacturer's listed rating.
Decision Tree: Sizing Your Exact Breaker and Wire
Use this decision path to finalize your materials list before heading to the supply house.
| Step | Action | Result / Rule |
|---|---|---|
| 1. Read Nameplate | Locate MCA and MOP on the outdoor condenser data plate. | Example: MCA = 24A, MOP = 40A. |
| 2. Size the Wire | Match MCA to the 75°C column in NEC 310.16. | 24A requires a wire rated for at least 24A. 10 AWG (35A) is the correct pick. |
| 3. Size the Breaker | Match MOP to standard breaker sizes (NEC 240.6). | 40A is a standard size. Buy a 40A HACR double-pole breaker. |
| 4. Check MOP vs MCA | What if MOP is not a standard breaker size (e.g., 45A)? | NEC 440.22(B) allows you to round down to the next standard size (40A). Never round up. |
| 5. Terminate | Tighten lugs to manufacturer torque specs. | Typical 10-8 AWG lug torque is 25-30 in-lbs. Verify with a torque screwdriver. |
What Changes the Answer: Distance, Bundling, and Insulation
The baseline assumptions above apply to short, standard runs. Real-world jobsite conditions frequently force you to upsize your wire.
1. Voltage Drop at Distance
NEC 310.15(B) information note recommends keeping voltage drop under 3% for branch circuits. Let's run the math for a 4-ton unit (MCA 25.5A, MOP 40A) using 8 AWG THHN copper.
- At 75 feet: Voltage drop is roughly 1.8%. 8 AWG is perfectly fine.
- At 150 feet: Using the formula
VD = (2 × Length × Current × Resistance) / 1000, and using the MOP (40A) for worst-case startup surge, the drop exceeds 3.5%. You must upsize to 6 AWG copper to prevent the compressor from struggling to start due to low voltage at the contactor.
2. NM-B Cable vs. THHN in Conduit
If you are running underground UF-B or indoor NM-B (Romex) instead of pulling THHN through conduit, NEC 334.80 mandates that you use the 60°C column for ampacity, regardless of the wire's 90°C insulation rating.
For example, 10 AWG THHN in conduit is rated for 35A (75°C column). But 10 AWG NM-B is legally limited to 30A (60°C column). If your AC nameplate has an MCA of 29A, 10 AWG NM-B is technically compliant but leaves zero margin for error or ambient heat. Upsizing to 8 AWG NM-B is the professional choice here.
3. Conductor Bundling (Derating)
If you are pulling multiple circuits through the same conduit (e.g., the AC disconnect feed plus a landscape lighting transformer feed), NEC 310.15(C)(1) requires derating. If you have 4 to 6 current-carrying conductors in a single raceway, you must multiply the wire's base ampacity by 80%. A 10 AWG THHN wire (35A at 75°C) derates to 28A. If your MCA is 26A, that 10 AWG wire is now too small; you must pull 8 AWG.
4. Aluminum Conductors
Copper and aluminum are not interchangeable. If you are using AA-8000 series aluminum wire (common for long underground feeder runs to save money), you must use the aluminum column in NEC 310.16. To replace 10 AWG copper, you must step up to 8 AWG aluminum. Always use anti-oxidant compound (like Noalox) on aluminum terminations and ensure your disconnect lugs are explicitly rated "AL/CU".
When to Call an Engineer or the AHJ
While 95% of residential split-system air conditioners follow the straightforward MCA/MOP logic above, modern HVAC technology introduces edge cases where you must consult the AHJ or a licensed electrical engineer.
- Inverter-Driven Mini-Splits and VFDs: Some high-efficiency variable-speed units have nameplates where the MOP is actually lower than the MCA, or where the manufacturer explicitly forbids standard thermal-magnetic breakers, requiring specialized ground-fault or arc-fault protection due to the sensitive Variable Frequency Drive (VFD) boards. Always read the installation manual, not just the sticker.
- Panel Busbar Capacity (The 120% Rule): If you are adding a 5-ton AC (50A breaker) to an older 100A or 150A main panel that is already heavily loaded, you may violate the NEC busbar capacity rules. If the sum of your breaker amp ratings exceeds 120% of your main busbar rating, the panel is unsafe. An engineer must perform an NEC Article 220 load calculation to determine if a service upgrade is required before the new AC circuit is energized.
- High Ambient Temperatures: If your conduit is routed across a flat, unshaded commercial roof in the Southwest US, the ambient temperature inside the conduit can easily exceed 110°F (43°C). This requires applying temperature correction factors from NEC Table 310.15(B)(1), which will almost always force you to upsize the wire by one or two gauges.
For more details on calculating whole-home electrical loads and ensuring your panel can handle new major appliances, refer to the U.S. Department of Energy's guidelines on central air conditioning and electrical infrastructure planning.
