11.4 Air Conditioning & Refrigeration Equipment (NEC Article 440: MCA, MOCP & Disconnects)
Key Takeaways
Air-conditioning and refrigeration equipment containing hermetic refrigerant motor-compressors are governed specifically by NEC Article 440, superseding general Article 430 rules due to suction gas cooling.
Under NEC 440.4(C), if the equipment nameplate lists a Branch-Circuit Selection Current (BCSC) greater than marked rated-load current, BCSC must be used for conductor, disconnect, and OCPD sizing.
Minimum Circuit Ampacity (MCA; NEC 440.32, 440.35) represents 125% of compressor rated current plus 100% of concurrent auxiliary loads; field branch conductors must have allowable ampacity ≥ MCA.
Maximum Overcurrent Protective Device (MOCP; NEC 440.22) is sized at not more than 175% of compressor rated current (max 225% exception); field circuit breakers or fuses must not exceed nameplate MOCP.
NEC 440.14 mandates that the equipment disconnecting means must be located 'within sight from' (visible and ≤50 ft), readily accessible, and NOT mounted on panels designed to be removed for servicing.
Air Conditioning & Refrigeration Equipment (NEC Article 440: MCA, MOCP & Disconnects)
Air conditioning, heat pump, and commercial refrigeration systems operate with hermetic refrigerant motor-compressors—specialized assemblies where the electric motor and compressor pump are completely sealed within a welded steel or cast-iron shell exposed directly to refrigerant and lubricating oil. Because the motor windings are continuously cooled by cold suction gas returning from the evaporator, a hermetic motor can carry significantly higher electrical loads without overheating than a conventional open-air motor of the same physical dimensions. Consequently, the National Electrical Code dedicates Article 440 specifically to hermetic equipment, superseding standard Article 430 requirements.
1. Article 440 Scope & Nameplate Terminology (NEC 440.4)
On the Oklahoma Journeyman Electrician examination, electricians are not required to calculate internal compressor physics from scratch. Instead, manufacturers perform listed laboratory calculations under UL 1995 / UL 60335-2-40 and stamp the exact field requirements onto the equipment nameplate. Understanding how the NEC interprets these nameplate values is essential:
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| ANATOMY OF AN HVAC EQUIPMENT NAMEPLATE |
| |
| MODEL: AC-4800-X VOLTS: 208/230V 1-PHASE 60 HZ |
| COMPRESSOR RLC: 24.2 A LOCKED ROTOR (LRA): 144.0 A |
| OUTDOOR FAN MOTOR FLA: 2.1 A 1/4 HP 1100 RPM |
| ----------------------------------------------------------------------- |
| MINIMUM CIRCUIT AMPACITY (MCA): 32.4 AMPERES |
| MAX OVERCURRENT PROTECTIVE DEVICE: 50 AMPERES |
| MAX FUSE OR HACR CIRCUIT BREAKER |
+-----------------------------------------------------------------------------+
Key Terms and Definitions:
- Rated-Load Current (RLC): The current drawn by the compressor under maximum rated operating conditions (temperature, head pressure, suction pressure).
- Branch-Circuit Selection Current (BCSC; NEC 440.4(C)): A current value marked on the nameplate that is greater than the rated-load current, used where the compressor is designed for sustained peak loading. Where marked, BCSC must be used in place of RLC for all calculations.
- Locked-Rotor Amperes (LRA): The instantaneous current drawn when the compressor rotor is mechanically stalled at startup (typically 5 to 7 times running current). Used to determine disconnect interrupting capacity and horsepower ratings.
2. Minimum Circuit Ampacity (MCA) & Conductor Sizing (NEC 440.32–440.35)
The Minimum Circuit Ampacity (MCA) stamped on the equipment nameplate dictates the smallest allowable conductor ampacity that can be installed in the field:
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| HOW THE MANUFACTURER CALCULATES MCA |
| |
| MCA = (1.25 x Compressor RLC/BCSC) + Sum(All Concurrent Fan/Pump Motors) |
| |
| FROM NAMEPLATE EXAMPLE: |
| MCA = (1.25 x 24.2 A) + 2.1 A = 30.25 A + 2.1 A = 32.35 A -> 32.4 Amps |
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Field Conductor Sizing Rules:
- Select from NEC Table 310.16: Match the conductor's 75°C ampacity rating directly to the MCA value. For an MCA of 32.4A, select #8 AWG Copper (rated 50A at 75°C) or #8 AWG Aluminum (rated 40A at 75°C). (Note: #10 AWG Cu has a 75°C rating of 35A, which also satisfies 32.4A where termination temperatures permit).
- Small Conductor Rule Exemption (NEC 240.4(G)): Article 440 exempts HVAC equipment from the general 15A, 20A, and 30A caps of NEC 240.4(D). If an MCA is 18.0A and the MOCP is 30A, #12 AWG Copper (rated 20A/25A) is 100% code-compliant on a 30A breaker!
3. Maximum Overcurrent Protective Device (MOCP) & HACR Breakers (NEC 440.22)
The Maximum Overcurrent Protective Device (MOCP) represents the largest fuse or circuit breaker permitted to protect the branch circuit:
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| HOW THE MANUFACTURER CALCULATES MOCP |
| |
| STANDARD SIZING (NEC 440.22(A)): |
| MOCP = (1.75 x Compressor RLC) + Sum(Other Motors) |
| Rounded DOWN to the next standard rating (or up under specific test rules)|
| |
| ABSOLUTE MAXIMUM EXCEPTION (NEC 440.22(A) Exception 1): |
| If 175% cannot start unit, maximum is increased to 225% of RLC. |
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Critical Nameplate Markings & HACR Mandates (NEC 110.3(B) & 440.4(B))
Electricians must inspect the exact phrasing of the overcurrent protective device marking:
- "MAX FUSE SIZE: 50A": The branch circuit MUST be protected by fuses (such as a fused disconnect or fuse panel). Installing an inverse-time circuit breaker is a direct violation of NEC 110.3(B) (listing requirements).
- "MAX FUSE OR HACR BREAKER: 50A" (or "MAX CKT BKR: 50A"): The branch circuit is permitted to be protected by either fuses or an inverse-time circuit breaker listed for Heating, Air Conditioning, and Refrigeration (HACR) duty. Today, virtually all standard molded-case circuit breakers manufactured by major brands are UL-listed as HACR rated.
4. Disconnecting Means Requirements (NEC 440.14 & 440.12)
NEC 440.14 establishes stringent physical location requirements for air conditioning and refrigeration disconnect switches:
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| NEC 440.14 HVAC DISCONNECT RULES |
| |
| 1. LOCATION: Must be located WITHIN SIGHT FROM the equipment. |
| - Visible AND not more than 50 feet (15 m) away. |
| |
| 2. ACCESSIBILITY: Must be READILY ACCESSIBLE (Article 100). |
| - No portable ladders, no crawling, no removing obstacles. |
| |
| 3. MOUNTING PROHIBITION: The disconnect SHALL NOT be mounted on or |
| within panels designed to be removed for servicing the equipment. |
| - Screwing a disconnect box into the compressor access door is a |
| DANGEROUS CODE VIOLATION! |
| |
| 4. WORKING CLEARANCE (NEC 110.26): |
| - Must maintain 3 feet minimum depth, 30 inches width, and 6.5 ft |
| headroom in front of the disconnect switch. |
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Disconnect Sizing (NEC 440.12)
- Ampere Rating (440.12(A)(1)): Disconnect ampere rating must be at least 115% of the nameplate rated-load current (RLC) or branch-circuit selection current (BCSC).
- Horsepower Rating (440.12(A)(2)): Horsepower rating is determined by taking the compressor locked-rotor amperes (LRA) and converting it to horsepower using NEC Table 430.251(A) (1-phase) or Table 430.251(B) (3-phase).
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| HVAC BRANCH CIRCUIT SIZING SUMMARY MATRIX |
| |
| COMPONENT CODE SECTION SELECTION CRITERIA |
| ----------------------------------------------------------------------- |
| Conductor Size NEC 440.32 / 310.16 Ampacity >= Nameplate MCA |
| Overcurrent Device NEC 440.22 / 240.6 Rating <= Nameplate MOCP |
| Disconnect Amp Rating NEC 440.12(A)(1) Rating >= 115% of RLC / BCSC |
| Disconnect Location NEC 440.14 Within Sight (<= 50 ft) |
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An electrician is wiring a rooftop commercial air conditioning condensing unit. The equipment nameplate states: 'VOLTS: 208/230V 3-PHASE, COMPRESSOR RLC: 22.0A, FAN MOTOR FLA: 2.5A, MINIMUM CIRCUIT AMPACITY: 30.0A, MAX FUSE OR HACR BREAKER: 50A'. What is the minimum conductor size (75°C THHN Copper) and maximum circuit breaker rating permitted for this installation?
#6 AWG THHN Cu with a 60-ampere circuit breaker
#12 AWG THHN Cu with a 30-ampere circuit breaker
#8 AWG THHN Cu with a 70-ampere circuit breaker
#10 AWG THHN Cu with a 50-ampere HACR circuit breaker
The nameplate on a commercial refrigeration package unit explicitly states: 'MINIMUM CIRCUIT AMPACITY: 24.5A, MAXIMUM FUSE SIZE: 40A'. An installer protects the branch circuit with a 40-ampere standard molded-case circuit breaker. How does the National Electrical Code classify this installation?
It is a violation of NEC 110.3(B) and 440.4(B) because the nameplate specifically mandates fuses as the required overcurrent protection.
It is compliant because modern circuit breakers are automatically rated for HACR applications.
It is compliant provided the circuit breaker has an instantaneous trip setting of 800%.
It is a violation only if the conductor size is smaller than #6 AWG copper.
Where an air-conditioning condensing unit is installed on a residential concrete pad outside a house, where must the disconnecting means be installed under NEC 440.14?
Inside the main service panelboard located in the basement, provided the panelboard is unlocked.
Anywhere on the property as long as it is within 100 feet of the condensing unit.
Within sight from the condensing unit, readily accessible, and not mounted on or within panels designed to be removed for servicing.
Directly screwed onto the removable sheet-metal access cover of the compressor compartment.
An HVAC nameplate lists a hermetic compressor rated-load current (RLC) of 18.0 amperes and a 'Branch-Circuit Selection Current' (BCSC) of 22.0 amperes. Under NEC 440.4(C), which current value must the electrician use when sizing the branch-circuit conductors and disconnecting means?
The 18.0-ampere rated-load current because it represents standard running conditions.
The 22.0-ampere Branch-Circuit Selection Current because BCSC is greater than the rated-load current.
The average of the two current values (20.0 amperes).
The locked-rotor current divided by 6.
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