TryBuildCalcMCA & MOCP from Motor RLA/FLA Calculator (NEC 440.32 / 440.22 — From Component Ratings)
Calculate MCA/MOCP from motor ratings.
🕒 Last updated: September 1, 2026
Inputs
ℹ️The rated-load amps (RLA) of the largest single motor on the circuit — almost always the compressor. If there are two compressors, enter the larger one here and the smaller one under Other Motor Loads below.
ℹ️Sum of the full-load amps (FLA) of every other motor sharing this circuit — condenser fan, indoor blower, a second compressor, etc. Enter 0 if there are none.
ℹ️Sum of any non-motor loads sharing this circuit — electric heat strips, controls. Enter 0 if there are none. Continuous electric-heat loads can have their own additional sizing considerations beyond this simplified combination — see Limitations.
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
35 A
MCA — Minimum Circuit Ampacity
MCA used: 24.5 A
Wire ampacity (base): 35 A
Calculated: 125% of largest motor + other loads
MOCP — Max Overcurrent Protection
MOCP used: 35 A
175% base: 33.5 A, 225% cap: 42.5 A
Rounded up to the next standard size
Interpretation note: two documented MOCP rounding conventions exist
This result (35 A) rounds UP to the next standard breaker size, matching the majority of published NEC sources — but some sources instead round DOWN to the largest standard size at or below the 175% base by default, reserving a move toward the 225% ceiling strictly for a demonstrated motor-starting problem. That conservative reading gives 30 A (breaker: 30 A) for the same inputs. Confirm the preferred interpretation with a licensed electrician or your local AHJ, especially for larger equipment where the gap matters more.
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.
MCA & MOCP from motor RLA/FLA ratings
For field-assembled or custom systems without a single combined nameplate MCA/MOCP — such as a mismatched split system with separate indoor and outdoor unit listings — MCA and MOCP can be calculated directly from the compressor's rated-load current plus other loads.
This page is pre-set to a common single-compressor scenario with a small fan-motor load — edit the inputs above to match your actual equipment's individual component ratings.
- Enter the largest motor's (almost always the compressor's) RLA or FLA from its own individual nameplate.
- Sum any other motors (condenser fan, indoor blower) into Other Motor Loads.
- This from-scratch mode is a simplified single-largest-motor model — always prefer a combined nameplate value when the equipment has one.
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 | From component ratings | 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 | 24.5 A |
| Wire size | 10 AWG (copper) |
Step 2 — MOCP
| Calculation | Result |
|---|---|
| MOCP | 35 A (175% base 33.5 A, 225% cap 42.5 A) |
| Breaker/fuse size | 35 A |
Therefore, for this equipment you need approximately 10 AWG copper wire on a 35 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.