The wire size for an air conditioner disconnect is determined by the Minimum Circuit Ampacity (MCA) value printed on the AC unit nameplate, not by the unit wattage or BTU rating. NEC Article 440 governs air conditioning and refrigeration equipment wiring, and it requires conductors sized to at least the nameplate MCA. If the nameplate MCA is 20 amps, use 12 AWG wire. An MCA of 25 to 30 amps requires 10 AWG wire. An MCA of 35 to 40 amps requires 8 AWG wire. An MCA of 45 to 55 amps requires 6 AWG wire.
Why MCA Rather Than Wattage
Air conditioning compressor motors draw varying current depending on suction pressure, outdoor temperature, refrigerant charge, and compressor age. The MCA value on the nameplate is the manufacturer engineering figure that accounts for these variables and represents the minimum conductor ampacity required for safe continuous operation under all rated conditions. Using a calculated wattage figure (watts divided by volts equals amps) is not the correct method for AC wiring because it does not account for the power factor of the inductive motor load or the derating factors built into the MCA figure.
Maximum Overcurrent Protection (MOCP)
The nameplate also lists the Maximum Overcurrent Protection (MOCP) value, which is the maximum breaker size permitted for the circuit. The circuit breaker must not exceed the MOCP. These two nameplate values together define the acceptable range for the circuit: the breaker must be at or below the MOCP, and the wire must be rated for at least the MCA. A common configuration is MCA 23A / MOCP 40A, which means the wire must handle at least 23 amps (use 12 AWG minimum, or upsize to 10 AWG for better voltage drop performance) and the breaker must not exceed 40 amps.
Cable Type for AC Disconnect Runs
The run from the electrical panel to the outdoor disconnect box uses NM-B (Romex) for the interior concealed portion of the run. The transition from the interior wall to the outdoor disconnect box requires weatherproof conduit fittings and, for any exposed exterior portion, either UF-B cable or THHN conductors in weatherproof conduit (EMT or PVC). The disconnect box itself must be a listed disconnect switch rated for the MOCP of the equipment and rated for outdoor use. A short flexible liquid-tight conduit connects the disconnect box to the AC unit service panel.
Common Wire Sizes by AC Tonnage
These are typical wire sizes for common residential AC configurations, though always verify against your specific unit nameplate. 1.5-ton to 2-ton central AC: typically MCA 14 to 18A, use 12 AWG. 2.5-ton to 3-ton central AC: typically MCA 18 to 26A, use 10 AWG. 3.5-ton to 4-ton central AC: typically MCA 24 to 35A, use 10 AWG or 8 AWG based on exact MCA. 5-ton central AC: typically MCA 30 to 45A, use 8 AWG or 6 AWG.
IB Lighting carries 12/2, 10/2, 8/2, and 6/2 NM-B alongside THHN and UF-B in all standard gauges. Browse our complete wire and cable selection or call (800) 674-9019 for guidance. Free Freight Insurance Included on every order.