MCA & MOCP Calculator (NEC Article 440 — HVAC Circuit Sizing)

Find your AC/HVAC wire and breaker size instantly.

Use this MCA & MOCP calculator to find the required wire and breaker size for air-conditioning and refrigeration equipment, per NEC Article 440 — either from the equipment's nameplate MCA/MOCP, or calculated from scratch from motor RLA/FLA ratings.

🕒 Last updated: September 1, 2026

Inputs

Input Method

Nameplate MCA

ℹ️Minimum Circuit Ampacity, printed directly on the equipment's nameplate — this sets the minimum required wire ampacity.

Nameplate MOCP

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.

Wire Material

▾

ℹ️Copper and aluminum have different ampacity tables — aluminum needs a larger wire size for the same MCA.

Insulation Rating

▾

ℹ️The wire's own temperature rating (e.g. THHN is 90°C).

Terminal Rating

▾

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

Apply Ambient Temperature / Conductor Count Derating?

Cost

Enable Cost Estimation?

Wire Size (Copper)

10 AWG

Breaker / Fuse Size

40 A

MCA — Minimum Circuit Ampacity

MCA used: 27 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 →

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What Is an MCA & MOCP Calculator?

An MCA & MOCP calculator finds the minimum wire size and maximum breaker/fuse size for air-conditioning and refrigeration equipment, per NEC Article 440. MCA (Minimum Circuit Ampacity) sets the smallest allowable conductor ampacity; MOCP (Maximum Overcurrent Protection) sets the largest allowable breaker or fuse rating. Both are usually printed directly on the equipment's nameplate — this calculator converts them into an actual wire size and breaker size, or calculates them from scratch from motor RLA/FLA ratings when no combined nameplate values exist.

MCA and MOCP are deliberately different from a simple load-current calculation — MCA already includes a 125% safety margin for the largest motor, and MOCP is allowed to run as high as 225% of that same motor's rated-load current specifically to survive its startup inrush current without nuisance tripping.

Why nameplate values matter:

Manufacturers calculate MCA/MOCP as part of UL listing testing, which can reflect equipment-specific factors a generic formula can't fully capture
Using the nameplate values is standard practice and takes precedence over recalculating from individual component ratings
The from-scratch calculation path exists for field-assembled or custom systems that don't have a single combined nameplate rating
Confusing MCA (for the wire) with MOCP (for the breaker) — or using either as a simple load current — is a common, code-significant mistake

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	27 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.

Standard OCPD Sizes Reference (NEC 240.6(A))

Both the MOCP calculation and the final breaker/fuse selection round to one of these standard ampere ratings — never an arbitrary in-between value.

Standard Sizes (Amps)
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000

MCA/MOCP Percentage Reference

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)

When should you use this MCA & MOCP calculator?

Sizing the branch-circuit wire and breaker for a new AC condenser, heat pump, mini-split, or refrigeration unit.
Converting a nameplate's MCA/MOCP values into an actual wire gauge and breaker/fuse size.
Calculating MCA/MOCP from scratch for a field-assembled system without a single combined nameplate rating.
Cross-checking an HVAC installer's proposed wire/breaker size against an independent estimate.
Planning material and cost for a dedicated HVAC circuit run.

Quick MCA/MOCP Sizing Tips

Always use the nameplate MCA/MOCP when the equipment has one — recalculating from component ratings is only for equipment without a combined listing.
MCA sizes the wire; MOCP sizes the breaker/fuse — they are never the same number and are never interchangeable.
The breaker/fuse size should generally equal the MOCP value directly, not something smaller — going smaller risks nuisance tripping on compressor startup.
Check the terminal rating on the disconnect/breaker, not just the wire's own insulation rating — the lower of the two governs per NEC 110.14(C).
Apply ambient temperature and conductor-count derating for wire routed through hot attics or shared conduits with several other circuits.
For dual-compressor or multi-motor equipment, double-check which motor is genuinely the largest before assigning the 125%/175%/225% factors.

Common Mistakes

Using MCA to size the breaker (or MOCP to size the wire) — they answer different questions and are never interchangeable.
Recalculating MCA/MOCP from component ratings when the nameplate already provides combined values — the nameplate should always take precedence.
Forgetting the 225% hard ceiling when rounding the MOCP base up to the next standard breaker size.
Treating MCA as a simple load current instead of a 125%-inflated design value — it already includes the safety margin, so it shouldn't be inflated again.
Ignoring the equipment/terminal temperature rating and sizing the wire off its insulation rating alone.
Assuming a smaller-than-MOCP breaker is always "safer" — undersizing risks nuisance tripping on compressor startup inrush current.

Limitations

NEC-based only (US) — other regions use different HVAC circuit-sizing rules (IEC/BS 7671 and similar) and this calculator does not model them.
The from-scratch calculation is a simplified single-largest-motor model — it does not fully model every combination-load nuance (e.g. how continuous electric-heat strips interact with 424/422), so nameplate values should always be used when available.
Does not model the exception permitting further MOCP increase (beyond the standard rounding) when a motor genuinely won't start reliably at the calculated setting.
The 175%→225% MOCP rounding step has a genuinely documented interpretation split across published NEC sources — this calculator shows the majority (round-up-by-default) reading as the primary result, and a conservative round-down alternative alongside it in component-ratings mode; it does not adjudicate which is correct for your jurisdiction.
Wire sizing tops out at 4/0 AWG (branch/feeder scope) — very large commercial refrigeration equipment beyond that range is out of scope. Breaker/fuse sizing extends to 1000A.
Does not size the equipment's disconnect switch or evaluate disconnect location/accessibility requirements (NEC 440.14).
Assumes a single, non-parallel conductor run per phase.

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.

FAQ

MCA is the minimum ampacity the branch-circuit conductors supplying an air-conditioning or refrigeration unit must have, per NEC 440.32/440.33. It's calculated as 125% of the largest motor's rated-load current (almost always the compressor) plus 100% of every other motor and non-motor load sharing the circuit — the 125% factor already builds in a safety margin, so it should never be inflated again.

MOCP is the largest breaker or fuse rating permitted to protect that same circuit, per NEC 440.22. It starts at 175% of the largest motor's rated-load current plus other loads (rounded up to the next standard size), and can go as high as 225% of that same base if needed for the motor to start reliably — but 225% is a hard ceiling that can never be exceeded.

Check the equipment's nameplate — most air-conditioning, heat pump, and refrigeration equipment has both values printed directly on it (MOCP is sometimes labeled MOP or MFS for maximum fuse size). Use those nameplate values directly rather than recalculating; they reflect the manufacturer's UL listing testing for that specific equipment.

MCA sizes the wire (conductor ampacity must be at least MCA). MOCP sizes the breaker or fuse (its rating must not exceed MOCP). They are calculated differently (125% vs. up to 225%) and are never interchangeable — using MOCP to size wire, or MCA to size a breaker, produces the wrong answer either way.

A motor-compressor draws a large inrush current for a brief moment at startup — often several times its running current. Sizing the breaker only to the running-load current would cause it to nuisance-trip every time the compressor starts. NEC 440.22 permits the breaker to run up to 225% of the motor's rated-load current specifically to ride through that startup surge, while the wire itself only needs to handle the smaller continuous MCA value.

Published NEC sources genuinely disagree on how to round the 175% base MOCP when it doesn't land exactly on a standard breaker size. The majority reading (shown as this calculator's primary result) rounds UP to the next standard size by default — matching the same rounding tolerance the code grants generally for motor overcurrent protection. A more conservative reading treats 'not exceeding 175 percent' literally, rounding DOWN to the largest standard size at or below that base, and reserves any move toward the 225% ceiling strictly for a demonstrated starting-current problem rather than applying it automatically. When calculating from component ratings, this calculator shows both figures so you can apply whichever your local AHJ or licensed electrician prefers — nameplate MOCP values sidestep this entirely, since the manufacturer has already made that call.

Switch this calculator to 'From Component Ratings' mode and enter the largest motor's RLA/FLA, plus the sum of any other motor and non-motor loads sharing the circuit. This is common for field-assembled split systems where the indoor and outdoor units aren't matched from a single listing. Note this is a simplified single-largest-motor model — always prefer a nameplate value when one exists.

Yes — this calculator supports both copper and aluminum. Aluminum has lower conductivity for the same wire gauge, so it will generally need to be sized one or more gauges larger than copper for the same MCA. Confirm the equipment's terminals are rated for aluminum conductors before using it.

No — the equipment grounding conductor for this circuit is a separate conductor sized from NEC Table 250.122 based on the breaker's OCPD rating, not from the MCA. Use the Equipment Grounding Conductor Size Calculator for that separate sizing step.

No — this calculator is built entirely on the US National Electrical Code (NEC) Article 440. Other regions size HVAC/motor circuits under different rules entirely — IEC/BS 7671 (UK/Europe and many other countries) uses a different framework for motor circuit protection and conductor sizing. If your jurisdiction follows IEC/BS 7671 or another non-NEC standard, this calculator's output should not be used.

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