HVAC & Refrigeration MCA / MOCP Interpreter

HVAC and refrigeration equipment is one of the most routinely mis-wired load types in field practice — because contractors familiar with Article 430 motor calculations try to apply the same calculation-based logic to equipment governed by Article 440, where the rules are fundamentally different. Under Article 440, the manufacturer marks two values on the nameplate: MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection). These values are already calculated by the equipment manufacturer per NEC 440.33 and 440.22 — the installer's job is simply to select a conductor with ampacity at or above MCA and an OCPD with a rating at or below MOCP. This interpreter makes both checks explicit and immediate. Enter the nameplate MCA (1–800 A) and MOCP (1–1,200 A) from sliders or use one of six presets — RTU, Condensing Unit, Heat Pump, Walk-in Cooler, Wrong Breaker (intentional OCPD violation), and Undersized Wire (intentional conductor violation). Select equipment type, voltage (120–480 V), phase, conductor material (copper or aluminum), insulation type, temperature rating, and AWG or kcmil size. The Results tab shows Quick Glance Gauges for both conductor and OCPD compliance, the two Sizing Paths (MCA → Conductor and MOCP → OCPD) with your selected values annotated, and an Interpretation Notes panel. The Live Circuit Visualizer draws the branch circuit with values annotated and color-coded pass/fail. The Trace tab walks through every check with NEC section citations: 440.4(B) for nameplate marking, 440.32/440.33 for motor-compressor sizing, 440.22 for OCPD, 240.4(G) for the Article 240-to-440 cross-reference, and 110.14(C) for terminal temperature. The Learn tab explicitly contrasts the Article 440 nameplate-driven approach against the Article 430 calculation-driven approach — the single most important distinction for technicians making the transition. The Design Challenge tab presents scored nameplate scenarios. NEC 2017 and 2020 are supported with edition notes.

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A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

Example data — not a real field report

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• 01Explain the difference between Article 440 (nameplate-driven) and Article 430 (calculation-driven) sizing and identify which applies to hermetic refrigerant motor-compressor equipment
• 02Read an HVAC equipment nameplate and locate the marked MCA and MOCP values, then explain what each value represents and which NEC sections require the manufacturer to mark them
• 03Select the minimum conductor size whose ampacity meets or exceeds the marked MCA after applying temperature correction and bundling adjustment factors per NEC 310.15
• 04Select an OCPD whose rating does not exceed the marked MOCP per NEC 440.22 and apply next-standard-size logic from NEC 240.6(A) where appropriate
• 05Identify the two most common HVAC wiring violations — OCPD exceeding marked MOCP and conductor below marked MCA — and explain the compliance path for correcting each
• 06Distinguish the Article 440 hermetic motor-compressor scope from Article 422 appliances and explain when combination-load equipment nameplate marking under 440.4(B) applies

• —Not a design document, stamped calculation, or approval record.
• —Not a substitute for engineering analysis by a licensed professional.
• —Not a reproduction of NEC tables — values are computed and cited, and you must verify against the code edition enforced in your jurisdiction.
• —Not connected to live equipment and not a safety device.