Heat Pump & HVAC Circuit Calculator
Size the conductors and breaker for central air conditioning or a heat pump from its nameplate MCA and MOCP, under NEC Article 440 — where the numbers come off the equipment rather than out of a motor table.
Last checked against the code
Heat Pump & HVAC Circuit Calculator
NEC (US)HVAC branch circuit
The window
- MCA — floor
- —
- Conductor
- —
- Breaker
- —
- MOCP — ceiling
- —
At the ordinary 175%
How this is calculated
440.4(C) makes the manufacturer do the calculation and print the result.What you get is not one number but two, and neither is a target — they are the edges of a range, and your job is choosing hardware that lands inside it.
The two round in opposite directions, on the same circuit. The conductor rounds up until Table 310.16 gives an ampacity that reaches the MCA. The device rounds down to the largest standard rating that stays under the MOCP. Doing both correctly requires holding two contradictory habits at once, which is why the round-down half is the mistake this page leads with.
The device usually ends up rated well above the conductor, and that is by design rather than an oversight. Like a motor circuit, the branch-circuit device here covers short circuits and ground faults only — it has to survive compressor inrush without tripping. Overload protection is inside the equipment. So a 45 A breaker on a 10 AWG conductor is ordinary, and the 240.4(D) small-conductor rule that would forbid it on a general circuit does not reach this equipment.
Where there is no plate to read, 440.32 and 440.22 rebuild both figures from component ratings — useful before equipment arrives, and no substitute for the plate once it has.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
A condensing unit whose data plate reads MCA 28 A andMOCP 47 A — an ordinary residential three-ton machine.
The conductor, rounding up. It has to carry at least 28 A. Table 310.16 at the 75 °C copper column gives 12 AWG 25 A, which is short, and 10 AWG at 35 A, which clears it by 7 A. So 10 AWG it is.
The device, rounding down. 47 A is not a standard rating — the 240.6(A) series goes 40, 45, 50 — so the answer is the largest one that stays under it: 45 A, leaving 2 A of the ceiling unused.
This is the whole point of the page. A general continuous load of 47 A would round up to 50 A, and that is what the breaker size calculator would tell you. Here it would exceed the manufacturer's stated maximum and void the listing. Same arithmetic, opposite direction, and the difference between a compliant installation and one an inspector will fail.
Note the finished circuit: a 45 A breaker on 10 AWG conductors, rated 35 A. The device is 17 A above the conductor's required floor and 10 A above its actual ampacity, which looks wrong on paper and is exactly right in Article 440.
Now the same machine without a plate to read. Say the label is painted over and the specification sheet gives a compressor RLA of 20 A and a condenser fan at 1.5 A. 440.32 puts the conductor floor at 1.25 × 20 + 1.5 = 26.5 A — which still lands on 10 AWG, the same conductor.
The device is where the estimate gets soft. At the 175% that 440.22(A) normally uses, the ceiling is 36.5 A and the breaker 35 A. At the 225% the section permits only where the equipment will not start on the lower figure, it is 46.5 A and 45 A — which happens to match what the real plate said. That spread, a whole two device sizes wide, is why the estimate is for planning and the plate is for installing.
Visual comparison
| Quantity | Bar | Amperes (A) | Role |
|---|---|---|---|
| MCA — floor | 28 | Floor — the conductor must reach it | |
| 10 AWG | 35 | 10 AWG at 35 A | |
| 45 A breaker | 45 | 45 A device | |
| MOCP — ceiling | 47 | Ceiling — the device must stay under it |
Source: NEC 2023 440.4(C), 440.22, 440.32 — conductor from Table 310.16
Reference tables
One table, and it is a comparison rather than a lookup. Both this page and the Motor FLA calculator size circuits for compressor motors, and readers move between them expecting the same conventions. Almost none of them carry over.
Read the second and third rows together — they are the reversal that catches people. Article 430 forbids using the nameplate current; Article 440 makes the nameplate the entire answer. Both are right about their own equipment, and the reason is in the last row of the first half: you cannot look up a motor that has no horsepower rating.
| HVAC equipment — Article 440 | General motors — Article 430 | |
|---|---|---|
| What it covers | Hermetic refrigerant motor-compressors — central AC, heat pumps | General-purpose motors — pumps, blowers, machine tools |
| Where the current comes from | The nameplate. 440.4(C) has the manufacturer compute MCA and MOCP | A code table. 430.6(A)(1) requires Table 430.248 or 430.250 |
| Is the nameplate used? | Yes — it is the whole answer | No — only for the overload device under 430.32 |
| Why the difference | A sealed compressor has no independently rated horsepower to look up | A motor has a horsepower rating, so the circuit can be standardised |
| Conductor basis | MCA directly — already includes the 125% | 125% of the table full-load current, per 430.22 |
| Device basis | MOCP directly — a stated ceiling | A percentage of table FLC, by device type, per Table 430.52 |
| Rounding direction | DOWN — MOCP is a maximum | DOWN — the table percentage is a maximum |
Notes and exceptions
Single-compressor equipment only. Two-stage and multi-compressor systems are sized under 440.33, which takes the sum of all the compressors with the largest at 125% — the same shape as a motor feeder under 430.24. Adding a second compressor to this calculator's model is a planned addition rather than something to approximate from the single-compressor figures.
Supplementary heat is a separate load and often the larger one. A heat pump with electric resistance backup brings 440.34 and 424.3(B) into the calculation, and strip heaters are frequently on their own circuits with their own nameplate figures. A unit sized from the compressor plate alone can be badly undersized once the heat comes on.
The 175% and 225% figures are not interchangeable. Where this calculator rebuilds MOCP from component ratings it reports both, because 440.22(A) sets 175% as the figure to use and permits 225% only where the equipment will not start on the lower one. Presenting the higher number as the default would turn a starting allowance into a routine oversize. As with Table 430.52, the permitted percentage also varies by protective device type, which this version does not model.
Not covered here. The disconnecting means and its required location within sight of the equipment under 440.14, the room air conditioner rules of Part VII, the 440.9 requirements where the equipment is on a rooftop, and the ampacity correction that a hot roof or attic run demands — for which the ampacity calculator applies the temperature factor to whatever conductor this page recommends.
Common mistakes
Rounding the breaker UP to the next standard size
The habit every other overcurrent calculation teaches, and here it is a violation. MOCP is a maximum: a nameplate reading 47 A takes a 45 A device, not a 50 A one. Exceeding the stated ceiling voids the equipment listing, and it is invisible on a drawing unless someone re-reads the plate.
NEC 2023 440.22
Recalculating from RLA when the nameplate is right there
The manufacturer already did this calculation, with knowledge of the specific compressor, its starting characteristics and any factory accessories. A figure rebuilt from two currents cannot see any of that. Where the plate is legible, the plate wins — the component mode here is for planning before delivery, not for second-guessing.
NEC 2023 440.4(C)
Reaching for the motor tables because it contains a motor
A hermetic compressor is sealed inside the refrigerant circuit and has no independently rated horsepower, so there is no row to look it up in. Article 440 exists precisely because Article 430 cannot describe this equipment, and using Table 430.250 on an air conditioner produces a confident wrong answer.
NEC 2023 440.6, 430.6
Sizing the conductor to the breaker instead of to the MCA
The two are not related here. A 45 A device routinely protects a 10 AWG conductor on this equipment, because the breaker covers short circuits and ground faults only — overload protection lives inside the unit. Sizing the wire to the breaker buys copper nobody asked for.
NEC 2023 440.32, 440.52
Forgetting the supplementary heat strips
A heat pump with electric backup heat is not sized by the compressor nameplate alone. 440.34 and Article 424 bring the resistance heaters into the calculation, and they are frequently the larger load — often on their own circuits with their own nameplate figures.
NEC 2023 440.34, 424.3(B)
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
How is this different from the Motor FLA Calculator?
They invert each other. Article 430 forbids using a motor’s nameplate current and sends you to a horsepower-indexed table. Article 440 has no table — a sealed compressor has no horsepower rating — so the manufacturer computes the circuit and the nameplate is the whole answer. Same kind of motor, opposite instruction, and the comparison table further down this page sets the two out row by row.
What breaker do I use for an MOCP of 47 amps?
45 A. The 240.6(A) series runs 40, 45, 50 — so 47 is not a size you can buy, and the answer is the largest one that stays under the ceiling. A 50 A device would exceed the manufacturer’s stated maximum and void the listing.
What do MCA and MOCP actually mean?
Minimum Circuit Ampacity is a floor for the conductors; Maximum Overcurrent Protection is a ceiling for the device. Between them they bound a window rather than naming a point, which is why one rounds up and the other rounds down on the same circuit.
Why is the breaker so much bigger than the wire?
Because it is not protecting the wire against overload — that job is done by devices inside the unit. The 240.4(D) rule that would cap 10 AWG at 30 A on a general circuit does not reach this equipment either: 240.4(G) hands air-conditioning and refrigeration circuits to Article 440 instead.
Can I calculate MCA myself from the compressor rating?
You can estimate it, and this calculator has a mode for it. But where the two disagree the nameplate is what the inspector reads, so treat a rebuilt figure as planning information for equipment that has not arrived yet.
Does a heat pump need a different circuit from an air conditioner?
The compressor circuit is sized identically — both are hermetic motor-compressors under Article 440, and both carry MCA and MOCP on the plate. The difference is supplementary heat: a heat pump usually has electric resistance strips for cold weather, and those bring 440.34 and Article 424 into play. They are frequently a larger load than the compressor and often run on separate circuits with their own nameplate figures.