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MCA & MOCP Sizing Guide
Air-conditioning and refrigeration equipment breaks the usual conductor-sizing pattern: instead of sizing wire and breaker from the same load current, NEC Article 440 gives you two separate numbers — MCA for the wire, MOCP for the breaker — calculated with different percentages specifically to handle a compressor motor's startup inrush current without either undersizing the wire or nuisance-tripping the breaker.
Last updated: September 1, 2026
Most branch circuits size the wire and breaker from the same load current. Air-conditioning and refrigeration equipment doesn't — NEC Article 440 deliberately splits it into two separate calculations, MCA for the wire and MOCP for the breaker, precisely because a compressor motor's startup surge would otherwise force an awkward compromise between an oversized wire and a breaker that nuisance-trips every time the unit starts.
This guide covers how NEC 440.32 (MCA) and 440.22 (MOCP) work, the 125%/175%/225% percentages, RLA vs. FLA terminology, and a full worked example.
MCA and MOCP, Side by Side
Both figures start from the same base — the largest motor's rated-load current plus any other loads sharing the circuit — but apply very different percentages to that base.
| Quantity | Formula | Code Reference |
|---|---|---|
| MCA (wire sizing) | 125% of largest motor RLA/FLA + 100% of other loads | NEC 440.32 / 440.33 |
| MOCP base (breaker sizing) | 175% of largest motor RLA/FLA + other loads, rounded up to next standard size | NEC 440.22(A) |
| MOCP ceiling | 225% of largest motor RLA/FLA + other loads — never exceeded | NEC 440.22(A) |
Only the SINGLE largest motor on the circuit gets the 125%/175%/225% multiplier — every other motor and non-motor load is added at 100% in both formulas.
Why the Breaker Can Be Rated So Much Higher Than the Wire
A compressor motor draws a large inrush current for a brief moment at startup — often several times its steady running current. NEC 440.22 permits the breaker to run up to 225% of the motor's rated-load current specifically to survive that startup surge without nuisance-tripping, while the wire itself only needs to handle the smaller, genuinely continuous MCA value. Seeing a 40A breaker on a wire sized for 27A isn't a mistake — it's exactly how Article 440 is designed to work.
Worked Example — Single-Compressor Condenser
20A Compressor RLA + 2A Condenser Fan FLA
Illustrative example
| Step | Calculation | Result |
|---|---|---|
| MCA | (1.25 × 20) + 2 | 27 A |
| Wire size | Smallest AWG ≥ 27A, 75°C copper | 10 AWG copper |
| MOCP base (175%) | (1.75 × 20) + 2 | 33.5 A |
| MOCP ceiling (225%) | (2.25 × 20) + 2 | 42.5 A |
| Final MOCP / breaker size | Round 33.5A up to next standard size (≤ 42.5A ceiling) | 40 A |
The 33.5A base rounds up cleanly to the 40A standard breaker size, which is comfortably under the 42.5A ceiling — no capping needed here. A slightly different RLA value could easily push the rounded-up size past the ceiling, forcing a round-down instead.
Common Mistakes
Using MCA to Size the Breaker, or MOCP to Size the Wire
MCA and MOCP answer two different questions with two different percentages — mixing them up either undersizes the breaker's short-circuit protection or oversizes the wire unnecessarily.
Recalculating When a Nameplate Value Already Exists
The from-scratch component-ratings calculation is a simplified single-largest-motor model. When the equipment has a combined nameplate MCA/MOCP, that value reflects actual UL listing testing and should always be used instead.
Forgetting the 225% Hard Ceiling
Rounding the 175% base MOCP up to the next standard breaker size is only permitted as long as that rounded value doesn't exceed 225% of the same base — past that point, the rule requires rounding DOWN instead, not up.
Treating MCA as a Simple Load Current
MCA already has the 125% continuous-load-style safety margin built in for the largest motor. Applying an additional continuous-load factor on top of MCA double-counts that margin and oversizes the wire.
Ignoring the Terminal/Equipment Temperature Rating
Per NEC 110.14(C), a 90°C-rated wire terminated on 60°C- or 75°C-rated equipment is still limited to that lower rating's ampacity — sizing off the wire's own insulation rating alone can understate what's actually required in practice.
Relevant Standards and References
HVAC/motor circuit sizing is code-specific — the underlying safety principle is universal, but the exact published formulas and rules vary by code and edition.
| Region | Relevant Codes / Guidance |
|---|---|
| United States | NEC (National Electrical Code) Article 440 for AC/refrigeration equipment circuit sizing — 440.32/440.33 for MCA, 440.22 for MOCP, 440.14 for the required disconnecting means |
| Europe / UK | BS 7671 (IET Wiring Regulations) sizes motor circuits under its own separate framework, without a direct MCA/MOCP equivalent structured the same way |
| General guidance | HVAC/motor circuit sizing rules are code-specific and vary by edition — always confirm against the exact table and edition your local authority has adopted before finalizing a real installation. |
Final Verdict
Read the nameplate first — MCA sizes the wire, MOCP sizes the breaker, and both already have their safety margins built in. Recalculating from scratch is only for equipment that genuinely doesn't have a combined listing.
- Use the nameplate MCA and MOCP directly whenever the equipment has them — they reflect actual UL listing testing.
- MCA sizes the wire (conductor ampacity ≥ MCA); MOCP sizes the breaker/fuse (rating ≤ MOCP) — never swap the two.
- Only the single largest motor gets the 125%/175%/225% multiplier — other loads are added at 100%.
- MOCP rounds up to the next standard breaker size, but never past the 225% ceiling — round down instead when it would.
- Apply the lower of the wire's insulation rating and the terminal/equipment rating per NEC 110.14(C).
- Size the equipment grounding conductor separately, from NEC Table 250.122 based on the breaker's OCPD rating.
Related calculators
Use these calculators when you need to turn this reference information into project quantities:
- MCA & MOCP Calculator
Find the required wire and breaker size for AC/HVAC equipment per NEC Article 440.
- Wire Size Calculator
Find the correct AWG wire size from load current, run length, and derating conditions.
- Circuit Breaker Size Calculator
Size the breaker that pairs with a given wire and load.
- Equipment Grounding Conductor Size Calculator
Find the minimum EGC size per NEC Table 250.122, with the 250.122(B) upsize check applied automatically.
Related resources
- Wire Gauge Selection Guide
Complete guide to selecting the correct wire gauge (AWG) — base ampacity by insulation rating, the derating factors that reduce it (conductor count, ambient temperature), the small-conductor breaker cap, and a worked example.
- Equipment Grounding Conductor Sizing Guide
Complete guide to sizing an equipment grounding conductor (EGC) — how NEC Table 250.122 works, the 250.122(B) proportional-increase rule, copper vs. aluminum, and a worked example.