How it works & the NEC rules behind it
Air conditioning and heat pump systems are governed by NEC Article 440, which provides specialized rules distinct from standard Article 430 motor circuits. Equipment nameplates provide two critical engineering parameters: MCA and MOCP.
Core Sizing Rules (NEC Article 440):
- 1. Minimum Circuit Ampacity (MCA, §440.33): Defines the minimum conductor current carrying capacity:
MCA = (1.25 × Compressor RLA) + Fan Motor FLA + Auxiliary Heat Amps.
Conductors are sized from NEC Table 310.16 to carry at least the MCA. - 2. Maximum Overcurrent Protective Device (MOCP, §440.22): Protects against short circuits and ground faults without tripping on compressor startup:
MOCP = (1.75 × Compressor RLA) + Fan Motor FLA.
Where the calculated value does not correspond to a standard size, the next lower standard size is used, or up to 225% RLA if the 175% breaker trips on inrush (§440.22(A) Exception). - 3. Breaker Larger Than Wire (§440.6): Unlike general branch circuits (§240.4(D)), it is standard and code-compliant in HVAC circuits for the circuit breaker (MOCP) to be rated higher than the wire ampacity, because the compressor has internal thermal overload protection.
- 4. Disconnect Switch Rating (§440.12): Must be rated for not less than 115% of total nameplate current and located within sight of the equipment (§440.14).