8.3 Air Conditioning, Refrigeration & Heating

8.3 Air Conditioning, Refrigeration & Heating

Quick Reference: Air conditioning and refrigeration equipment incorporate hermetic refrigerant motor-compressors, where the compressor and motor operate inside a sealed metallic housing exposed directly to refrigerant suction gas. Because refrigerant gas provides superior cooling compared to standard ambient air, hermetic compressors operate at significantly higher current densities per horsepower than standard motors. Consequently, NEC Article 430 motor tables do NOT apply to hermetic compressors. Instead, the plans examiner must enforce NEC Article 440, verifying branch-circuit conductors against the marked Minimum Circuit Ampacity (MCA) and overcurrent devices against the marked Maximum Overcurrent Protective Device (MOCPD) on the equipment nameplate. Fixed electric space heating systems are governed separately under NEC Article 424 as continuous electrical loads.

1. Hermetic Compressor Terminology & Nameplate Ratings (NEC Article 440)

Under NEC 440.2 and 440.4, manufacturers must provide specific technical markings on HVAC nameplates:

+---------------------------------------------------------------------------------------------------+
|                         NEC ARTICLE 440 HVAC TERMINOLOGY MATRIX                                   |
+---------------------------------------+-----------------------------------------------------------+
| Term / Abbreviation                   | Code Definition & Engineering Significance                |
+---------------------------------------+-----------------------------------------------------------+
| Rated-Load Current (RLC)              | The steady-state operating current of the compressor      |
| (NEC 440.2 / 440.4(A))                | motor under rated voltage, frequency, and refrigeration   |
|                                       | load conditions.                                          |
+---------------------------------------+-----------------------------------------------------------+
| Branch-Circuit Selection Current      | The rated current value marked on the nameplate when the  |
| (BCSC) (NEC 440.2 / 440.4(C))         | compressor design allows sustained running current higher |
|                                       | than RLC. Where marked, BCSC REPLACES RLC in all sizing.  |
+---------------------------------------+-----------------------------------------------------------+
| Minimum Circuit Ampacity (MCA)        | The minimum allowable ampacity required for field-installed|
| (NEC 440.4(B) / 440.32 / 440.33)      | branch-circuit conductors supplying the equipment.        |
+---------------------------------------+-----------------------------------------------------------+
| Maximum Overcurrent Protective Device | The maximum permissible rating of the branch-circuit fuse |
| (MOCPD / MOCP) (NEC 440.4(B) / 440.22)| or HACR-type circuit breaker protecting the circuit.      |
+---------------------------------------+-----------------------------------------------------------+

How MCA is Calculated by Manufacturers (UL 1995 / UL 60335-2-40)

For multimotor equipment (e.g., packaged rooftop units with compressors, condenser fans, and blower motors): $\text{MCA} = (1.25 \times \text{Largest Motor/Compressor Current}) + \sum \text{All Other Simultaneous Motor and Auxiliary Currents}$

[!IMPORTANT] Plan Review Principle — No Double-Derating for MCA: Because the $125%$ continuous multiplier for the largest compressor is already engineered into the manufacturer's marked MCA, the electrical plans examiner must simply verify that the field conductor allowable ampacity (from NEC Table 310.16 at $75^\circ\text{C}$) is greater than or equal to MCA. Applying a second $1.25$ factor to MCA is a common engineering mistake that unnecessarily inflates wire sizes.

2. Conductor Sizing & Overcurrent Protection Rules (NEC Article 440)

Branch-Circuit Conductor Sizing (NEC 440.32 & 440.33)

• Single Motor-Compressor (NEC 440.32): Conductor ampacity must be not less than $125%$ of the motor-compressor rated-load current (RLC) or branch-circuit selection current (BCSC).
• Multimotor Equipment (NEC 440.33 / 440.35): Field conductors supplying multimotor equipment must have an allowable ampacity equal to or greater than the marked MCA.

Overcurrent Protective Device Sizing & Nameplate Mandates (NEC 440.22 & 110.3(B))

Under NEC 440.22, branch-circuit overcurrent protection for HVAC equipment is sized to accommodate compressor starting inrush while providing short-circuit and ground-fault protection. The rating is limited by the MOCPD marked on the manufacturer's nameplate.

+---------------------------------------------------------------------------------------------------+
|                         NAMEPLATE OCPD WORDING VS. PERMITTED DEVICE                               |
+----------------------------------------------+----------------------------------------------------+
| Equipment Nameplate Marking                  | Permitted Overcurrent Protective Device            |
+----------------------------------------------+----------------------------------------------------+
| "MAX FUSE SIZE 40A"                          | Fuses ONLY (Max 40A). Circuit breakers PROHIBITED  |
|                                              | by NEC 110.3(B) and NEC 440.21!                    |
+----------------------------------------------+----------------------------------------------------+
| "MAX FUSE OR HACR CIRCUIT BREAKER 40A"       | Either Fuses OR HACR-rated circuit breakers        |
|                                              | rated up to 40A.                                   |
+----------------------------------------------+----------------------------------------------------+
| "MAX OVERCURRENT PROTECTIVE DEVICE 40A"      | Standard circuit breakers, HACR breakers, or fuses |
|                                              | rated up to 40A.                                   |
+----------------------------------------------+----------------------------------------------------+

[!CAUTION] Plan Review Pitfall — Breaker on "Max Fuse Only" Units: When an HVAC manufacturer lists equipment under UL standards specifying "MAX FUSE SIZE," installing an inverse-time circuit breaker (even one marked HACR) is a direct violation of NEC 110.3(B) (Installation and Use of Listed Equipment) and NEC 440.21. If the upstream panelboard has circuit breakers, a fused disconnect switch must be installed at the unit containing fuses not exceeding the marked maximum fuse size.

HVAC Disconnecting Means (NEC 440.14)

• Location: The disconnecting means must be located within sight from (visible and not more than $50\text{ feet}$ away) and readily accessible from the air-conditioning or refrigerating equipment.
• Mounting on Equipment Enclosure: The disconnect switch is permitted to be mounted directly on or within the HVAC equipment, provided it does not obstruct access panels, cover manufacturer nameplates, or obscure diagnostic/service access doors.
• Ampere & HP Rating (NEC 440.12): The disconnect must be rated for at least $115%$ of the sum of all nameplate currents (or marked RLC/BCSC).

Rooftop Servicing Receptacle (NEC 210.63(A))

Under NEC 210.63(A), a $125\text{V}$, single-phase, $15\text{A}$ or $20\text{A}$ rated receptacle outlet is mandatory for servicing HVAC and refrigeration equipment:

1. Location: Located on the same roof or mezzanine level within $25\text{ feet}$ ($7.5\text{ m}$) of the equipment.
2. GFCI Protection: Receptacle must be GFCI protected (NEC 210.8(B)(3)).
3. Power Supply Connection: The receptacle shall not be connected to the load side of the HVAC disconnecting means (to ensure power remains available during maintenance when the unit disconnect is opened).

3. Fixed Electric Space-Heating Equipment (NEC Article 424)

Fixed electric space heating includes electric baseboards, duct heaters, unit heaters, central electric furnaces, and radiant heating cables.

Continuous Load Classification (NEC 424.3(B))

Under NEC 424.3(B), all fixed electric space-heating equipment (including resistance heating elements and accompanying fan/blower motors) is legally classified as a continuous load. $\text{Minimum Branch-Circuit Conductor Ampacity} = 1.25 \times \text{Total Heating Load Current}$ $\text{Minimum Branch-Circuit OCPD Rating} = 1.25 \times \text{Total Heating Load Current}$

Resistance Heating Element Subdivision (NEC 424.22(B))

To limit thermal damage during an internal heater short circuit, electric space heating equipment with resistance elements rated more than $48\text{ Amperes}$ must be subdivided by the manufacturer into smaller heating element loads:

• Each subdivided load must not exceed $48\text{ Amperes}$.
• Each subdivided circuit must be protected by a supplementary or branch-circuit overcurrent device rated at not more than $60\text{ Amperes}$.

+---------------------------------------------------------------------------------------------------+
|              FIXED ELECTRIC SPACE HEATING SUBDIVISION RULES (NEC 424.22(B))                       |
+------------------------------------+----------------------------------+---------------------------+
| Total Heater Resistance Current    | Element Subdivision Required?    | Maximum OCPD Per Stage    |
+------------------------------------+----------------------------------+---------------------------+
| <= 48 Amperes                      | No (Single circuit permitted)    | Sized per 424.3(B)        |
| > 48 Amperes (e.g., 72A / 36 kW)   | YES (Subdivide into <= 48A loads)| Max 60A per stage         |
| Large Furnace (e.g., 144A / 72 kW) | YES (Subdivide into 3+ stages)   | Max 60A per stage         |
+------------------------------------+----------------------------------+---------------------------+

Disconnecting Means for Electric Heating (NEC 424.19)

• Fixed electric space-heating equipment must have a means to disconnect all ungrounded conductors.
• The disconnect must be located within sight from the heating equipment or must be lockable in the open position in accordance with NEC 110.25.
• Thermostatically Controlled Switches (NEC 424.20): Thermostats with a marked "OFF" position are permitted to serve as the disconnect only if they directly break all ungrounded conductors, have a manual mechanical disconnect position, and cannot reclose automatically when switched off.

4. Worked Plan Review Calculations & Common Traps

Worked Example 1: Commercial Rooftop Unit (RTU) Audit

• Nameplate Data: 460V, 3-Phase RTU. Marked MCA = 58.4 A, Marked MOCPD = 90 A ("Max Fuse or HACR Breaker"). Compressor 1 RLC = $22.0\text{ A}$, Compressor 2 RLC = $22.0\text{ A}$, Condenser Fan = $2.4\text{ A}$, Supply Blower = $6.5\text{ A}$. Single-line diagram specifies: $6\text{ AWG}$ THHN Cu conductors, $100\text{A}$ standard circuit breaker, and disconnect located $60\text{ ft}$ away on the mechanical penthouse wall.
• Step 1: Audit Conductor Sizing (NEC 440.33)
  • Required conductor ampacity $\ge \text{MCA} = 58.4\text{ A}$.
  • From Table 310.16 at 75°C: $6\text{ AWG Copper}$ is rated at $65\text{ A}$.
  • Conductor ampacity ($65\text{A} \ge 58.4\text{A}$) is COMPLIANT.
• Step 2: Audit Overcurrent Protection (NEC 440.22)
  • Proposed OCPD is a $100\text{A}$ circuit breaker.
  • Nameplate explicitly states MOCPD = $90\text{ A}$.
  • Finding: CODE DEFICIENCY. The $100\text{A}$ breaker exceeds the marked MOCPD of $90\text{A}$. Must be replaced with a $90\text{A}$ or smaller HACR circuit breaker or fuse.
• Step 3: Audit Disconnect Location (NEC 440.14)
  • Proposed disconnect is $60\text{ ft}$ away.
  • Under Article 100, "within sight from" is capped at $50\text{ ft}$.
  • Finding: CODE DEFICIENCY. Disconnect must be relocated to within $50\text{ ft}$ and in direct line of sight from the RTU.

Worked Example 2: Commercial Electric Duct Heater Sizing

• Project Details: A $30\text{ kW}$, $480\text{V}$, 3-phase electric duct heater is installed in a commercial HVAC supply duct.
• Step 1: Calculate Full-Load Current $I = \frac{\text{Watts}}{\sqrt{3} \times V} = \frac{30,000\text{ W}}{1.732 \times 480\text{ V}} = \frac{30,000}{831.38} = \mathbf{36.08\text{ A}}$
• Step 2: Check Subdivision Requirement (NEC 424.22(B))
  • $36.08\text{ A} \le 48\text{ A} \rightarrow$ Subdivision into multiple stages is NOT required.
• Step 3: Calculate Minimum Conductor Ampacity & OCPD (NEC 424.3(B)) $\text{Minimum Ampacity} = 1.25 \times 36.08\text{ A} = \mathbf{45.10\text{ A}}$
  • Conductor (Table 310.16 at 75°C): $8\text{ AWG THHN Copper}$ (rated $50\text{ A}$).
  • Next standard OCPD rating (NEC 240.6): $50\text{ A}$ circuit breaker.

5. Plans Examiner Verification Checklist: HVAC & Electric Heating

• MCA Conductor Sizing (440.32/440.33): Verify field conductors have allowable ampacity equal to or greater than marked MCA; ensure 125% factor is NOT applied twice.
• MOCPD Compliance (440.22 / 110.3(B)): Verify branch OCPD rating does not exceed marked MOCPD; check if "MAX FUSE ONLY" is marked and prohibit circuit breakers accordingly.
• Disconnect Proximity & Access (440.14): Confirm disconnect is within sight from ($< 50\text{ ft}$) and readily accessible from the HVAC unit.
• Rooftop Servicing Receptacle (210.63): Confirm 125V GFCI receptacle within 25 ft on the same roof level, powered independently from the HVAC disconnect.
• Continuous Heating Load Sizing (424.3(B)): Apply 125% multiplier to all electric space heating loads for conductor and OCPD sizing.
• Heating Element Subdivision (424.22(B)): For resistance heaters $> 48\text{A}$, verify elements are subdivided into stages $\le 48\text{A}$ protected by OCPDs $\le 60\text{A}$.