HVAC Sizing: Conductor Ampacity & Breaker for 28.5A MCA / 45A MOP Unit
Determine minimum branch circuit conductor gauge and maximum breaker size for an air conditioning unit with 28.5A MCA and 45A MOP under NEC 440.
Calculated strictly in accordance with NEC Article 440.4(B), 440.22, & 440.32.
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NEC 430.6(A)(1) mandates that conductor ampacity and breaker sizing MUST be based on the NEC table values (Table 430.250), not the motor nameplate current.
Step-by-Step Solution
Formal engineering derivation citing applicable National Electrical Code articles.
Per NEC 440.32, branch circuit conductors must have an ampacity not less than the nameplate Minimum Circuit Ampacity (MCA): 28.5 Amperes.
Select conductor from Table 310.16 at 75°C: 10 AWG copper is rated for 35 Amperes, satisfying the 28.5A MCA requirement.
Per NEC 440.22, the protective device must not exceed the Maximum Overcurrent Protection (MOP) rating: 45 Amperes.
Result: 10 AWG copper conductors protected by a 40A or 45A circuit breaker.