TryBuildCalcHeat Pump MCA & MOCP Calculator (NEC Article 440)
Size the circuit for a heat pump.
🕒 Last updated: September 1, 2026
Inputs
ℹ️Minimum Circuit Ampacity, printed directly on the equipment's nameplate — this sets the minimum required wire ampacity.
ℹ️Maximum Overcurrent Protection (sometimes labeled MOP or MFS for max fuse size), printed directly on the nameplate — this sets the maximum allowed breaker/fuse size.
ℹ️Copper and aluminum have different ampacity tables — aluminum needs a larger wire size for the same MCA.
ℹ️The wire's own temperature rating (e.g. THHN is 90°C).
ℹ️Per NEC 110.14(C), the equipment/breaker terminal rating governs if it's lower than the wire's own insulation rating — most residential equipment is rated 60°C or 75°C even when 90°C wire is used.
Derating
Cost
Wire Size (Copper)
10 AWG
Breaker / Fuse Size
40 A
MCA — Minimum Circuit Ampacity
MCA used: 24 A
Wire ampacity (base): 35 A
Entered directly from nameplate
MOCP — Max Overcurrent Protection
MOCP used: 40 A
Entered directly from nameplate
Assumptions Used
NEC Article 440 (440.32 MCA, 440.22 MOCP), NEC Table 310.16 conductor ampacity, NEC 240.6(A) standard overcurrent device sizes. Manufacturer nameplate MCA/MOCP values, where marked, take precedence over any from-scratch calculation — this reflects UL listing testing specific to that equipment. The 175%-to-225% MOCP rounding step has genuinely documented interpretation differences across published sources (see the note above when calculating from component ratings). This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed HVAC/electrical contractor before final installation.
Want the full walkthrough on MCA vs. MOCP vs. RLA/FLA, and the 175%/225% rounding rule? MCA & MOCP Sizing Guide →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete MCA & MOCP Calculator.
Heat pump condenser circuit sizing
A heat pump's outdoor condenser unit is sized the same way as a central AC condenser — this page focuses on that outdoor unit's circuit, separate from any indoor electric backup heat strip circuit.
This page is pre-set to a common residential heat pump's MCA/MOCP range — edit the inputs above to match your actual unit's nameplate.
- The outdoor heat pump condenser and an indoor air handler's electric backup heat strips are typically on two SEPARATE circuits with their own nameplate MCA/MOCP each — don't combine them into one calculation unless the equipment's nameplate explicitly does so.
- Electric heat strips add a real load consideration beyond this calculator's simplified from-scratch model — always use the nameplate value for equipment with integrated backup heat.
- Cold-climate heat pumps can have a different MCA/MOCP than a standard-climate model of similar cooling capacity — always check the specific unit's nameplate.
MCA & MOCP Formula: How Are They Determined?
Two independent NEC 440 calculations from the same motor/load data, followed by translating each into an actual standard wire and breaker size.
Step 1 — MCA (NEC 440.32/440.33)
MCA = (1.25 × Largest Motor RLA/FLA) + Other Motor Loads + Other Non-Motor Loads
Wire Size = smallest standard AWG/kcmil whose (derated) ampacity ≥ MCA
Only the single largest motor gets the 125% factor — every other motor and non-motor load on the circuit is added at 100%.
Step 2 — MOCP (NEC 440.22)
Base = (1.75 × Largest Motor RLA/FLA) + Other Loads, rounded UP to the next standard size
Cap = (2.25 × Largest Motor RLA/FLA) + Other Loads
MOCP = the rounded-up value, UNLESS it exceeds the cap — then round DOWN to the largest standard size within the cap
Breaker/Fuse Size = the resulting MOCP value directly
The 175%→225% range exists specifically to accommodate a motor-compressor's startup inrush current without nuisance tripping, while the 225% figure is a hard ceiling that can never be exceeded regardless of standard-size rounding. Published NEC sources genuinely disagree on whether rounding up to the next standard size is the routine default (used above) or should be reserved for a demonstrated starting-current problem, with a more conservative round-down-by-default reading otherwise — the calculator surfaces both values when calculating from component ratings; confirm the preferred interpretation with a licensed electrician or your local AHJ.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Input method | Nameplate values | Determines whether MCA/MOCP are entered directly or calculated |
| Wire material | Copper | Sets which NEC Table 310.16 ampacity column governs the wire size |
Step 1 — MCA
| Calculation | Result |
|---|---|
| MCA | 24 A |
| Wire size | 10 AWG (copper) |
Step 2 — MOCP
| Calculation | Result |
|---|---|
| MOCP | 40 A (from nameplate) |
| Breaker/fuse size | 40 A |
Therefore, for this equipment you need approximately 10 AWG copper wire on a 40 A breaker/fuse.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.