Mini Split & AC Breaker Size Calculator
This free mini-split breaker calculator works the way the code actually works for HVAC: the nameplate governs. The MCA (Minimum Circuit Ampacity) sets the wire — it already includes the 125% compressor factor — and the MOP (Maximum Overcurrent Protection) sets the breaker ceiling. Enter both, and the calculator returns the conductor for NM-B or THHN-in-conduit with the Article 440 rules applied.
It also answers the number-one forum question honestly: yes, a 35 A breaker over wire rated 25 A is legal on an HVAC circuit. Article 440 (via 240.4(G)) lets compressor starting current set the protection while the MCA sizes the conductor for running load — a naive reading of the generic tables calls that a mistake; on this circuit it is the design.
Presets carry published MCA/MOP examples from MRCOOL DIY and Goodman spec sheets across the common BTU classes, but your unit's nameplate always wins — read it off the outdoor unit before buying anything. Reference only; verify with a licensed electrician and your local AHJ. Free, no signup.
Mini Split Breaker Calculator
What breaker and wire for your mini-split or central AC? Enter the nameplate MCA and MOP — the two values that size the whole circuit — and see why the breaker can legally exceed the wire's ampacity. Free, no signup.
Your unit's nameplate
Where these two numbers live on the unit
Both inputs come straight off the data plate on the outdoor unit. The MCA already includes the 125% compressor factor — you never multiply it again — and the MOP is the largest breaker the manufacturer allows, not a target to hit.
Calculation Formulas
The Minimum Circuit Ampacity already includes the 125% factor on the largest motor (440.32/440.33) — no further uplift.
Example:
MCA 21 A → 12 AWG Cu THHN (25 A at 75°C).
The Maximum Overcurrent Protection (440.22(A), up to 175–225% of RLA) is a ceiling — never exceed it; smaller is permitted if it holds.
Example:
MOP 35 A → up to a 35 A breaker.
HVAC conductors are protected per Article 440, not the general rules — compressor inrush legitimately sets protection above the conductor rating.
Example:
35 A breaker over 12 AWG (25 A) wire: legal and normal on this circuit.
NEC 440.14; the 2023 edition adds the 110.26(A) working-space requirement (not in 2017/2020).
Example:
The pull-out disconnect box beside every condenser.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| MRCOOL DIY 12k | MCA 11.5 / MOP 20 | Published spec — 14 AWG legal at this MCA. |
| Goodman GSXN4 3-ton | MCA 21 / MOP 35 | Summer 2023 master spec sheet. |
| Goodman GSXN4 5-ton | MCA 33.3 / MOP 50 | Summer 2023 master spec sheet. |
| Surge protection | NEC 230.67 | Dwelling services require a Type 1/2 SPD (2020 NEC, expanded 2023) — a service rule, not an Art. 440 one. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC Article 440 — HVAC equipment(NFPA 70 (2023) 440.4(B), 440.22, 440.32/33)
View StandardNameplate MCA/MOP govern conductor and protection; branch-circuit selection current basis.
Key Requirements:
- •Wire ≥ MCA
- •Breaker ≤ MOP
NEC 240.4(G) — Specific-circuit protection(NFPA 70 (2023) 240.4(G))
View StandardExempts Art. 440 circuits from the small-conductor caps — why the breaker can exceed the wire.
Key Requirements:
- •Protection per Art. 440, not 240.4(D)
NEC 440.14 — Disconnecting means(NFPA 70 (2023) 440.14)
View StandardWithin sight (≤50 ft), readily accessible, not on access panels; 2023 adds 110.26(A) working space.
Key Requirements:
- •Within sight of the unit
Manufacturer spec sheets(MRCOOL / Goodman (Daikin))
View StandardPreset MCA/MOP examples come from published sheets; the installed unit's nameplate always governs.
Key Requirements:
- •Nameplate over generic tables
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
NEC edition differences
2017 / 2020 / 2023
The 110.26(A) working-space sentence in 440.14 is 2023-only; the 230.67 dwelling SPD requirement began in 2020.
Regional Examples:
DIY boundary
Pre-charged mini-split kits
DIY-marketed kits (MRCOOL) make the refrigerant side homeowner-feasible; the panel-side breaker and whip termination remain electrician work in most jurisdictions.
Regional Examples:
Climate sizing
Heat pumps in cold climates
Cold-climate heat pumps run larger MCA for the same tonnage — nameplate variance is the rule, not the exception.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
Related Calculators
Breaker Size Calculator
What size breaker do you need? Free NEC calculator from amps or watts — 125% continuous rule, standard 240.6(A) ratings, and the wire it must pair with.
Wire Size Calculator
What gauge wire do you need? Free NEC wire size calculator by amps, wiring method, and run length — with derating, 240.4(D), and appliance presets.
HVAC BTU / Manual J Calculator
Cooling + heating BTU, AC tonnage, furnace size, heat-pump balance point — per ACCA Manual J / S, ASHRAE 1%/99%. DOE 2023 SEER2. Free.
How to Use This Calculator
- Read the MCA and MOP (Max Fuse/Ckt Bkr) off the outdoor unit's data plate — or pick a published example preset for planning.
- Choose the wiring method (THHN in conduit is typical for condenser whips; NM-B for interior portions) and optionally the run length.
- Click Calculate for the minimum wire, the breaker ceiling, and whether the breaker-exceeds-wire exception applies to your unit.
- Verify against the nameplate at install — the sheet values are examples; the plate is the law.
Why HVAC Breakers Can Be Bigger Than the Wire
On a general-purpose circuit, protecting 12 AWG wire with a 35 A breaker is a violation. On an air-conditioner circuit it can be exactly what the manufacturer engineered: NEC Article 440 sizes the conductor to the MCA — the real running load plus the 125% compressor factor — while allowing the overcurrent device up to the MOP, typically 175% (up to 225%) of the compressor's rated-load amps, so the breaker rides through starting inrush without nuisance-tripping. 240.4(G) explicitly exempts these circuits from the small-conductor caps that would otherwise forbid it. The conductor is still protected: the compressor's overloads handle running protection, and the breaker handles faults. That division of labor is why the nameplate — not a generic chart — is the only correct source for HVAC circuit sizing, and why this calculator asks for MCA and MOP instead of BTU.
Frequently Asked Questions
What size breaker for a 24,000 BTU mini split?
Commonly 25–30 A — MRCOOL's DIY 24k lists an MCA around 22 A with a max fuse/breaker of 25–30 A depending on the model year — but the only correct source is your unit's nameplate MOP. HVAC circuits are sized from two nameplate values: the wire must meet the MCA, the breaker must not exceed the MOP (NEC Art. 440). BTU class predicts the ballpark; the plate decides the circuit.
Why is my AC breaker bigger than the wire's rating?
Because Article 440 allows it. The MCA sizes the conductor for running load (it already includes the 125% compressor factor), while the MOP may run up to 175–225% of the compressor's rated-load amps so the breaker rides through starting inrush. NEC 240.4(G) exempts HVAC circuits from the small-conductor caps that would otherwise forbid it — a 35 A breaker over 12 AWG wire (25 A at 75°C) is legal and engineered, not an error. The compressor's internal overloads provide the running protection.
What is MCA vs MOP?
MCA (Minimum Circuit Ampacity) is the smallest conductor ampacity the circuit may use — wire ≥ MCA. MOP (Maximum Overcurrent Protection, sometimes 'Max Fuse/Ckt Bkr') is the largest breaker or fuse allowed — breaker ≤ MOP. Both are computed by the manufacturer per NEC 440.4(B) and printed on the outdoor unit's data plate. Between those two numbers, the whole circuit is specified — which is why this calculator asks for them instead of BTU.
Can I use 14 gauge wire on a 20 amp HVAC breaker?
Yes — if the nameplate MCA is 15 A or less and the MOP allows 20 A. That combination is illegal on a general-purpose circuit (240.4(D) caps 14 AWG at 15 A) but legal on HVAC under Article 440's exemption. Many 9k–12k mini-splits carry MCA values of 10–15 A with MOP 20, making 14 AWG copper the code-minimum conductor. If that feels wrong, that's the point of this page — HVAC plays by nameplate rules.
Where does the outdoor disconnect have to go?
Within sight of the unit (visible and ≤50 ft), readily accessible, and not mounted on a panel that must be removed for service or where it blocks the nameplate (NEC 440.14). The 2023 edition adds that it must meet the 110.26(A) working-space rules — a sentence not in the 2017/2020 editions, so its enforceability depends on which NEC your state has adopted. The familiar pull-out disconnect box beside the condenser satisfies all of it.