What Size Wire for an AC Condenser? (Use MCA, Not Tonnage)

Every condenser data plate carries two electrical numbers, required by NEC 440.4(B):

• MCA (Minimum Circuit Ampacity) — the smallest conductor ampacity the branch circuit may have. The manufacturer has already done the math per NEC 440.33: compressor rated-load amps × 125% plus the fan motor and any other loads. Do not add another 125% on top — the factor is baked in.
• MOCP (Maximum Overcurrent Protection) — the largest fuse or breaker allowed, sometimes printed as "Max Fuse/Ckt Bkr." If it says fuse only, you need fuses; if it lists an HACR-type breaker, a standard residential breaker qualifies (virtually all modern ones are HACR-rated).

This is why tonnage rules of thumb fail. Two 3-ton condensers can carry different MCAs depending on compressor type, efficiency rating, and fan motor — one may call for 12 AWG on a 20A MCA, the other for 10 AWG. A "3 tons = 10 AWG" habit either wastes copper or, worse, leaves an undersized conductor protected only by the generosity of Article 440's breaker allowance. Thirty seconds at the data plate replaces the guess.

With the MCA in hand, select the copper conductor from NEC Table 310.16 (allowable ampacities, not more than three current-carrying conductors, 30°C ambient):

Copper AWG	60°C column	75°C column
14	15 A	20 A
12	20 A	25 A
10	30 A	35 A
8	40 A	50 A
6	55 A	65 A
4	70 A	85 A

Which column? NEC 110.14(C) limits circuit ampacity to the lowest temperature rating of any termination — breaker lugs, disconnect lugs, the unit's contactor terminals. THHN wire is 90°C insulation, but that rating never applies at terminations. The safe field default is the 60°C column; use 75°C only when the breaker, disconnect, and equipment terminations are all marked 75°C. NM cable (Romex) is capped at the 60°C column regardless, per NEC 334.80.

Worked example: nameplate reads MCA 17.8A, Max Breaker 30A. From the 60°C column, 14 AWG (15A) fails, 12 AWG (20A) clears the MCA — that's the conductor. The breaker is 30A because the nameplate says so, and that pairing is compliant (next section).

Two adjustments can push you up a size: more than three current-carrying conductors in one raceway, or ambient well above 30°C (attic runs, rooftop conduit) — both trigger the correction factors referenced in NEC 310.15.

These pairings cover most residential condensers, straight from the 60°C column of Table 310.16. They are starting points, not a substitute for the nameplate:

Nameplate MCA	Copper conductor	Common nameplate MOCP
≤ 15 A	14 AWG (many installers run 12 AWG minimum)	15–25 A
15.1–20 A	12 AWG	20–30 A
20.1–30 A	10 AWG	30–45 A
30.1–40 A	8 AWG	40–60 A
40.1–55 A	6 AWG	60–70 A

The familiar combos — 12 AWG on a 20A-MCA unit, 10 AWG on a 30A, 8 AWG in the 40–50A class — fall out of this table, which is why they feel like rules of thumb. They aren't; they're just the most common nameplate values. A high-efficiency 4-ton unit can carry a lower MCA than an old builder-grade 3-ton. Read the plate.

One more check before pulling wire: voltage drop. NEC 210.19(A) Informational Note No. 4 recommends holding branch-circuit drop to 3%. On a long run — a condenser 100+ ft from the panel — a conductor that clears the MCA can still drop enough voltage under compressor load to warrant the next size up. Ampacity is code; voltage drop is craftsmanship.

On a general-purpose circuit, a 30A breaker on 12 AWG copper is a violation of the small-conductor rule, NEC 240.4(D). Air-conditioning circuits are the carve-out: NEC 240.4(G) sends motor-compressor circuits to Article 440, where the breaker's job is short-circuit and ground-fault protection only — it must ride through the compressor's locked-rotor inrush at every start. Overload protection for the conductor comes from the compressor's own internal overload device, not the breaker.

So the split of duties is:

• Conductor ≥ nameplate MCA (protects the wire under running load)
• Breaker or fuse ≤ nameplate MOCP, per NEC 440.35 (clears faults, tolerates inrush)

That's how a 12 AWG conductor lands on a 30A HACR breaker, or 10 AWG on a 40A, without a violation. Two hard limits: never exceed the nameplate MOCP, and never use the breaker allowance to shrink the conductor below MCA. Home inspectors flag these circuits constantly; the nameplate is your answer sheet if the pairing is questioned.

Most condensers need no neutral. A standard residential condenser is a straight 240V load — compressor and condenser fan both run hot-to-hot. The circuit is two ungrounded conductors plus an equipment grounding conductor: 10/2 with ground (or 12/2, 8/2 per your MCA), or two THWN conductors plus ground in conduit. Running 10/3 buys you a white wire with nowhere to land. The exception is rare — a unit whose wiring diagram shows 120V accessories (some package units, crankcase-heater arrangements). Check the diagram; if there's no neutral terminal, don't pull one. If you do run cable with a white conductor on a 240V circuit, re-identify it as a hot per NEC 200.7(C).

Disconnect: NEC 440.14 requires a disconnecting means within sight of and readily accessible from the unit. Outdoors that means a weatherproof (typically NEMA 3R) pullout or switch. A non-fused disconnect is fine when the panel breaker matches the nameplate MOCP and breakers are permitted; if the nameplate specifies maximum fuse only, use a fused disconnect.

Local code: jurisdictions on the 2023 NEC may enforce 210.8(F) GFCI protection on outdoor dwelling-unit HVAC circuits — adoption and amendments vary widely, so confirm with your AHJ before rough-in. UF cable or wet-rated conductors (THWN) for the outdoor whip, and physical protection for exposed runs, are standard inspection items.