Minimum circuit ampacity (MCA / MOP)
From the compressor RLA, fan FLA, and accessory loads of an HVAC unit, the calculator returns the Minimum Circuit Ampacity per NEC 440.32, the Maximum Overcurrent Protection per NEC 440.22 (175 % primary rule, 225 % fallback), the standard breaker size, and the minimum copper wire size. With PDF report. Reviewed by a licensed PE.
Use the calculator
Add each device on the HVAC unit (compressor RLA, fan FLA, heater amps), pick voltage and phases, and the calculator returns the MCA, the MOP at 175 % and 225 % rules, the recommended standard breaker, and the minimum copper wire size with NEC 240.4(D) small-conductor cap auto-applied.
The two NEC Article 440 rules
- MCA
- minimum continuous ampacity for the supply conductor, A
- I_motor,max
- RLA / FLA of the LARGEST motor on the equipment, A
- I_motor,other
- sum of all other motor RLA / FLA, A
- I_load
- sum of non-motor loads (heaters, controls), A
The 1.25 factor only applies to the single largest motor — not to every motor. This makes the MCA roughly 25 % above the steady-state running current of the equipment, sized for continuous duty per NEC 210.19(A).
- MOP
- maximum allowable breaker rating, A
- 1.75
- primary multiplier for largest motor, —
- ⌊·⌋_std
- round DOWN to nearest standard breaker per NEC 240.6(A), —
Round DOWN — not up. NEC 440.22 explicitly chose this direction so the breaker stays at or below the calculated maximum. The breaker must also be ≥ MCA to actually protect the conductor; if rounding down drops below MCA, switch to the 225 % fallback rule.
- 2.25
- fallback multiplier when 175 % fails to permit start, —
Use this only when the 175 % primary breaker won\'t allow the compressor to start without nuisance tripping — typically in high-inertia or low-temperature systems where starting current is sustained for several seconds. The manufacturer must support the larger breaker selection in their installation manual.
How to size an HVAC branch circuit, step by step
- Read the equipment nameplate. Modern HVAC nameplates list the MCA and MOP directly — those values are mandatory per NEC 440.4(A) and override anything you calculate. Use this calculator when the nameplate is missing, illegible, or for design when only component RLA / FLA values are known.
- Identify each motor and load. Hermetic compressors have RLA (Rated Load Amps); separate fan motors have FLA (Full-Load Amps from NEC Table 430.248 / 430.250). Heaters and accessories are listed in pure amps. Each gets a row in the calculator with its type and current draw.
- Find the largest motor. NEC 440.32 multiplies the LARGEST motor (compressor or fan, whichever has the highest amps) by 1.25. All other motors and loads add at 100 %. The single-largest rule applies even if there are several similar-size motors — only one gets the 1.25× factor.
- Compute MCA per NEC 440.32. MCA = 1.25 × largest motor amps + Σ(other motor amps) + Σ(non-motor loads). This is the minimum ampacity the conductor must have after all derating factors. Use it to look up wire size in NEC Table 310.16.
- Compute MOP per NEC 440.22. MOP = 1.75 × largest motor amps + others, then round DOWN to the nearest standard breaker size. If the rounded-down breaker is smaller than the MCA, NEC Exception lets you increase to 2.25 × largest motor + others, again rounded down. The MOP is the maximum allowable breaker rating — not a recommended size, but a ceiling.
- Pick the wire and verify the small-conductor cap. Smallest 75 °C copper wire whose ampacity ≥ MCA. NEC 240.4(D) caps the breaker for 14, 12, and 10 AWG copper at 15, 20, and 30 A regardless of insulation rating — if your MOP exceeds this, you must use larger wire. The calculator handles the bump automatically.
Reference values
Standard NEC 240.6(A) breaker sizes
The MOP must be one of these standard ratings. Smaller increments (15, 20, 25, 30) are 1-pole or 2-pole; larger sizes are typically frame breakers or molded-case switches.
| Breaker (A) | Common use |
|---|---|
| 15 | Lighting, small AC ½ ton |
| 20 | Receptacles, very small AC |
| 25, 30 | 2-ton AC, small dryer, heat pump 1-stage |
| 35, 40 | 3-ton AC, mid-size heat pump, EV charger 32 A |
| 45, 50 | 4-ton AC, EV charger 40 A, cooktop |
| 60 | 5-ton AC, EV charger 48 A, large heat-pump auxiliary heat |
| 70, 80, 90, 100 | Subpanel feeder, large commercial RTU 7.5–10 ton |
| 110, 125, 150 | Commercial RTU 12.5–15 ton, small distribution |
| 175, 200 | 20-ton RTU, distribution feeder |
| 225, 250, 300, 350, 400 | Commercial chiller branches, distribution mains |
Typical HVAC equipment MCA / MOP
Approximate values for common residential and small-commercial equipment. Always confirm with the actual nameplate.
| Equipment | Voltage | Compressor RLA | Fan FLA | MCA | MOP breaker | Min wire |
|---|---|---|---|---|---|---|
| 2-ton split AC | 208 V 1φ | 12 A | 1.0 A | 16 A | 25 A | 10 AWG Cu |
| 3-ton split AC | 208 V 1φ | 18 A | 1.5 A | 24 A | 30 A | 10 AWG Cu |
| 4-ton heat pump | 208 V 1φ | 22 A | 2.0 A | 29.5 A | 40 A | 8 AWG Cu |
| 5-ton heat pump + 10 kW aux | 208 V 1φ | 28 A | 2.0 A + 41.7 A heat | 78.7 A | 90 A | 3 AWG Cu |
| 10-ton RTU | 480 V 3φ | 19 A | 3.5 A | 27.3 A | 40 A | 8 AWG Cu |
| 20-ton RTU | 480 V 3φ | 38 A | 7.0 A | 54.5 A | 70 A | 4 AWG Cu |
Worked example: 5-ton heat pump with 10 kW auxiliary heat
A 5-ton air-source heat pump on 240 V single-phase: compressor RLA 28 A, outdoor fan FLA 2 A, indoor blower FLA 4.6 A, 10 kW resistive auxiliary heat (41.7 A). Compute MCA, MOP, and minimum wire.
| Step | Calculation | Result |
|---|---|---|
| Identify largest motor | compressor 28 A > blower 4.6 A > fan 2 A | 28 A |
| Other motor amps | 4.6 + 2 | 6.6 A |
| Non-motor loads | 10 kW heater = 41.7 A at 240 V | 41.7 A |
| MCA = 1.25 × 28 + 6.6 + 41.7 | 35 + 6.6 + 41.7 | 83.3 A |
| MOP 175 % candidate | 1.75 × 28 + 6.6 + 41.7 = 97.3 → round DOWN | 90 A breaker |
| Check: 90 A > MCA 83.3 A? | Yes | use 90 A |
| Wire from NEC 310.16 (75 °C Cu) | 3 AWG: 100 A ≥ 83.3 A | 3 AWG Cu |
| Verify NEC 240.4(D) cap | 3 AWG isn\'t small-conductor; no cap applies | no bump |
The auxiliary heat dominates the MCA — without it, MCA would be only 41.6 A. This is why heat-pump branch circuits are usually sized for the worst-case "heat strip on" condition and not for normal compressor-only operation.
Variants and special cases
Multi-circuit equipment
Some larger commercial RTUs use two separate branch circuits — one for the compressor with its own breaker, one for the auxiliary heat. The nameplate shows two MCA / MOP pairs. Run two cables to the equipment and provide two breakers. NEC 440.4(B) governs the marking and circuit separation.
Mini-splits and VRF systems
Variable-refrigerant-flow systems with multiple indoor heads behave like one equipment for sizing — the outdoor unit nameplate gives one MCA / MOP for the entire system, regardless of how many indoor heads. The indoor heads run on low-voltage control power supplied by the outdoor unit; no separate branch circuit needed.
Disconnect requirements (NEC 440.14)
Within sight of and not more than 50 ft from the equipment, an HVAC disconnect is required. For residential single-package units, a fused or non-fused disconnect at the outdoor unit is standard. For roof-mounted commercial RTUs, the disconnect is mounted on or adjacent to the unit. The disconnect ampacity must be ≥ 115 % of the equipment nameplate amps.
EV chargers vs HVAC sizing
EV chargers are continuous loads (NEC 625.41) and use the 1.25× rule on the rated charger current — same form as the MCA single-largest-motor rule but applied to the entire load. A 48 A continuous EV charger needs MCA = 60 A and a 60 A breaker, plus 6 AWG Cu wire. Conceptually identical math; different code section.
The NEC Article 440 mandate
Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.
Related calculators and references
Frequently asked questions
- What does MCA mean?
- MCA = Minimum Circuit Ampacity (NEC 440.32). It is the minimum continuous current the supply conductor and overcurrent device must carry without overheating. Calculated as 1.25 × the largest motor RLA / FLA + Σ(other motor amps) + Σ(non-motor loads). The 1.25 covers continuous-load duty (NEC 210.19(A) for general circuits, 440.32 for HVAC).
- What does MOP / MOCP mean?
- MOP = Maximum Overcurrent Protection (also MOCP). It is the maximum permitted breaker or fuse rating that protects the circuit. Per NEC 440.22(A), start at 175 % of the largest motor + others, round DOWN to the next standard breaker size. If that breaker is too small to allow the compressor to start without nuisance trips, NEC permits 225 %. The breaker rating must always be ≥ MCA but ≤ MOP.
- Why 175 %, then 225 %?
- A motor draws 5–8× FLA at start (locked-rotor current) for ~1 second. A breaker sized only for steady-state amps would trip on every start. NEC 440.22 picks 175 % as the standard sweet spot — large enough for most starts, small enough to still protect the conductor. The 225 % path exists for high-LRA or hard-starting compressors where 175 % isn't enough; it requires the manufacturer's starting-amp data to justify.
- What is the difference between RLA, FLA, and LRA?
- RLA (Rated Load Amps) — used only for hermetic and semi-hermetic refrigeration compressors. Established by the manufacturer per UL 1995 testing, not by NEC tables. FLA (Full-Load Amps) — used for separately-driven motors (fans, blowers, pumps), looked up in NEC Tables 430.247–430.250. LRA (Locked-Rotor Amps) — the current the motor draws at zero speed; only relevant when checking that the breaker won't trip on start.
- What size breaker do I need for a 3-ton AC?
- A typical 3-ton residential AC has compressor RLA 18 A, condenser fan FLA 1.5 A, no other loads. MCA = 1.25 × 18 + 1.5 = 24 A. MOP = 1.75 × 18 + 1.5 = 33 A → round DOWN to 30 A breaker. Minimum wire: 10 AWG copper (35 A ampacity at 75 °C). The nameplate values may differ slightly; trust the nameplate.
- Do I need a HACR-rated breaker?
- Yes for HVAC equipment. NEC 440.22(A) requires the breaker to be marked "HACR" (Heating, Air-conditioning, Refrigeration) or listed for "use with motor compressors." Standard residential / commercial 1-pole and 2-pole breakers from major manufacturers (Square D QO/Homeline, Eaton, Siemens) are HACR-rated by default. Verify the marking on the breaker.
- Can I use a smaller breaker than MOP?
- Yes, as long as breaker rating ≥ MCA. The MOP is a ceiling, not a target. A smaller breaker may trip more often on starts; a larger breaker (up to MOP) gives more starting margin but provides less short-circuit protection to the conductor. Most installations use the next standard size at or above MCA.
- Does the MCA include voltage drop?
- No — MCA only sets the conductor ampacity. Voltage drop (NEC 210.19 informational note: 3 % branch / 5 % combined) is a separate check. For long HVAC runs (over ~30 m), check voltage drop separately and step up wire size if needed. The Wire Size calculator embedded in our wire-sizing pages handles both checks at once.
Sources and methodology
- NFPA. National Electrical Code (NEC) NFPA 70, 2023 Edition. Article 440 — Air-Conditioning and Refrigerating Equipment, §§ 440.4, 440.14, 440.22, 440.32.
- NFPA. NEC NFPA 70, § 240.6(A) Standard Ampere Ratings.
- NFPA. NEC NFPA 70, § 240.4(D) Small Conductors.
- NFPA. NEC NFPA 70, Table 310.16 — Ampacities for Insulated Conductors in Raceway.
- NFPA. NEC NFPA 70, Tables 430.247 and 430.250 — Full-Load Currents for AC and DC Motors.
- UL. UL 1995 — Heating and Cooling Equipment. Defines RLA testing for hermetic compressors.
- ASHRAE. ASHRAE Handbook — HVAC Applications, 2019. Equipment selection and sizing tables.