9.3 Transformers, Generators & Hermetic HVAC Equipment

Transformers, Generators & Hermetic HVAC Equipment

Transformers and hermetic heating, ventilation, and air-conditioning (HVAC) equipment represent two highly specialized equipment classifications within the National Electrical Code. While general power circuits size conductors and overcurrent devices concurrently, transformers (NEC Article 450) and hermetic refrigerant motor-compressors (NEC Article 440) employ tailored sizing frameworks reflecting their distinct electromagnetic and thermodynamic properties.

Mastering three-phase transformer current math, Table 450.3(B) overcurrent protection rules, and the relationship between Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protective Device (MOCP) ratings is essential for journey level certification.

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1. Transformer Electrical Theory & Full-Load Current Formulas

A transformer is a static electromagnetic device that transfers electrical energy between circuits through inductive coupling without a change in frequency. Calculating full-load current is the mandatory prerequisite before applying any overcurrent protection rules.

Single-Phase Transformer Current Formula

For a single-phase transformer, the volt-ampere rating ($VA$) divided by the terminal line voltage ($V$) yields the full-load current ($I$):

$I = \frac{VA}{V} = \frac{\text{kVA} \times 1000}{V}$

Three-Phase Transformer Current Formula

In a balanced three-phase AC system, power is distributed across three line conductors separated by 120 electrical degrees. Calculating line current requires the square root of 3 ($\sqrt{3} \approx 1.732$):

$I = \frac{VA}{\sqrt{3} \times V_{\text{L-L}}} = \frac{\text{kVA} \times 1000}{1.732 \times V_{\text{L-L}}}$

Where $V_{\text{L-L}}$ is the line-to-line phase voltage (e.g., 480V or 208V).

Exam Math Tip: The constant product of $(1.732 \times 480\text{ V}) = 831.36$. The constant product of $(1.732 \times 208\text{ V}) = 360.26$. Committing these two denominators to memory saves significant time during the timed 3-hour NEC calculation portion of the exam.

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2. Transformer Overcurrent Protection: Systems 1,000V or Less (NEC Table 450.3(B))

NEC Article 450 protects the transformer windings from destructive thermal overload and internal faults. NEC Table 450.3(B) establishes two primary protection methods for transformers operating at 1,000 volts or less:

Summary of NEC Table 450.3(B) Protection Rules

Primary-Only Protection Rules

Under Primary-Only protection, a single OCPD on the supply side protects the transformer.

• When rated primary current is 9 amperes or greater, the maximum protective device rating is 125%.
• Table 450.3(B) Note 1: Where 125% of rated primary current does not correspond to a standard ampere rating of a fuse or nonadjustable circuit breaker under NEC 240.6(A), the next higher standard rating shall be permitted.

Primary and Secondary Protection Rules

When a facility designer desires a larger primary OCPD to prevent inrush nuisance tripping, secondary overcurrent protection must be added:

• The primary OCPD can be sized up to 250% of rated primary current. Note 1 does NOT apply to this primary rating; the device cannot exceed 250%.
• The secondary OCPD must be sized at not more than 125% of rated secondary current (for currents $\ge 9$A). Here, Note 1 does apply, permitting a round-up to the next higher standard rating.

Secondary Conductor Protection (NEC 240.4(F) & 240.21(C))

An essential distinction exists between protecting transformer windings under Article 450 and protecting the secondary conductors under Article 240:

• Under NEC 240.4(F), secondary conductors of single-phase transformers with other than a 2-wire secondary, and of three-phase transformers other than delta-delta 3-wire, are not considered protected by the primary overcurrent device (Section 5.5).
• Secondary conductors must be protected by an overcurrent device located at their termination or comply with the 10-foot or 25-foot transformer tap rules in NEC 240.21(C).

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Transformer Installation Rules (NEC 450)

3. Hermetic Refrigerant Motor-Compressors (NEC Article 440)

Air conditioning and commercial refrigeration equipment utilize hermetic refrigerant motor-compressors, where the motor windings and compressor mechanism are sealed inside a welded steel shell exposed directly to the circulating refrigerant gas and lubricating oil.

Because cold suction refrigerant gas continuously cools the motor windings, a hermetic motor can operate safely at higher current densities than a standard open-air motor of identical physical dimensions. However, if the refrigeration cycle loses refrigerant or airflow across the evaporator collapses, the motor will rapidly overheat. For this reason, NEC Article 440 replaces the standard Article 430 rules with dedicated sizing parameters.

Key Article 440 Nameplate Terminology

+--------------------------------------------------------------+
|              COMMERCIAL PACKAGED ROOFTOP UNIT                |
|                                                              |
| VOLTS: 460  |  PHASE: 3  |  HZ: 60                           |
| COMPRESSOR RLC: 24.0 A   |  LOCKED ROTOR AMPS (LRA): 140 A   |
| CONDENSER FAN MOTOR: 2.2 A (1/2 HP)                          |
|                                                              |
| MINIMUM CIRCUIT AMPACITY (MCA): 32.2 A                       |
| MAXIMUM OVERCURRENT PROTECTIVE DEVICE (MOCP): 50 A           |
| TYPE: HACR CIRCUIT BREAKER OR TIME-DELAY FUSE                |
+--------------------------------------------------------------+

1. Rated-Load Current (RLC): The operating current drawn by the motor-compressor under rated refrigeration load, voltage, and frequency.
2. Branch-Circuit Selection Current (BCSC): A marked value provided by the manufacturer when the compressor design requires conductors or disconnects larger than those determined by RLC. When marked, BCSC must be used in place of RLC for all calculations (NEC 440.4(C)).
3. Minimum Circuit Ampacity (MCA): The absolute minimum allowable ampacity for the branch circuit conductors supplying the unit (NEC 440.35). Derived per NEC 440.32: $\text{MCA} = (125\% \times \text{Compressor RLC or BCSC}) + \sum (\text{All Concurrent Auxiliary Motor Loads})$ In the nameplate above: $(1.25 \times 24.0\text{ A}) + 2.2\text{ A} = 30.0\text{ A} + 2.2\text{ A} = 32.2\text{ A}$.
4. Maximum Overcurrent Protective Device (MOCP): The maximum rating of the branch-circuit short-circuit and ground-fault protective device permitted to protect the circuit (NEC 440.22). Derived under 440.22 as not more than 175% of RLC (or up to 225% if starting inrush demands it).

The Conductor Ampacity vs. Breaker Sizing Relationship

In standard residential and commercial branch circuits, the overcurrent device rating must not exceed the conductor ampacity (e.g., #10 AWG copper rated 30A protected by a 30A breaker).

In hermetic HVAC circuits governed by Article 440, conductors are sized to the MCA, while the protective device is sized up to the MOCP:

• Conductors: Must have an allowable ampacity $\ge \text{MCA}$ from Table 310.16 (typically evaluated at 75°C).
• Circuit Breaker / Fuse: Must have a standard rating $\le \text{MOCP}$.

Exam Trap: Candidates often believe installing #10 AWG copper (rated 35A at 75°C) on a 50A circuit breaker violates NEC 240.4. It does not! Under NEC 240.4(G), specific equipment circuits—including Article 440 air conditioning—are exempt from the small conductor rule. The internal thermal overload protector inside the hermetic compressor protects the #10 AWG conductors from continuous overload, while the 50A breaker provides short-circuit protection.

HVAC Equipment Disconnecting Means (NEC 440.14)

Under NEC 440.14, the disconnecting means for air-conditioning and refrigerating equipment must be:

1. Located within sight from the equipment (visible and not more than 50 feet distant).
2. Readily accessible (accessible without portable ladders, removing obstacles, or climbing over ductwork).
3. Sized with an ampere rating of not less than 115% of the nameplate RLC or BCSC, whichever is greater (NEC 440.12(A)(1)).

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4. Generators (NEC Article 445)

Example: a 60 kW, 240 V single-phase generator has a nameplate current of 60,000 / 240 = 250 A. The conductors to the first overcurrent device need at least 250 x 1.15 = 287.5 A of ampacity.

5. Worked Calculation Examples

Example 1: Three-Phase Transformer Sizing & Overcurrent Protection

Scenario: A 45 kVA, dry-type transformer has a 480V, 3-phase delta primary and a 208Y/120V, 3-phase, 4-wire wye secondary. The transformer will be installed with primary-only overcurrent protection using an inverse time circuit breaker. Calculate full-load currents and select the maximum standard primary circuit breaker under NEC Table 450.3(B).

Step 1: Calculate Rated Primary Full-Load Current $I_{\text{primary}} = \frac{45,000\text{ VA}}{\sqrt{3} \times 480\text{ V}} = \frac{45,000}{831.36} = 54.13\text{ amperes}$

Step 2: Calculate Rated Secondary Full-Load Current $I_{\text{secondary}} = \frac{45,000\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{45,000}{360.26} = 124.91\text{ amperes}$

Step 3: Size the Primary OCPD under Table 450.3(B)

• Primary current ($54.13\text{A}$) is $\ge 9$ amperes, so the maximum protection multiplier is 125%. $\text{Maximum Setting} = 54.13\text{ A} \times 1.25 = 67.66\text{ amperes}$
• Check NEC 240.6(A) standard ratings: 50A, 60A, 70A, 80A...
• 67.66A is not a standard size.
• Under Table 450.3(B) Note 1, round UP to the next higher standard rating: 70 amperes.
• Maximum Standard Primary Breaker: 70 amperes.

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Example 2: Sizing Conductors and Protection for a Packaged HVAC Unit

Scenario: A commercial rooftop air handler has a nameplate listing:

• Rated Voltage: 460V, 3-phase
• Minimum Circuit Ampacity (MCA): 38.0 amperes
• Maximum Overcurrent Protective Device (MOCP): 60.0 amperes (HACR type)
• Terminals are rated for 75°C.

Select the minimum size copper THHN branch-circuit conductors and determine the maximum standard circuit breaker rating.

Step 1: Size Branch Circuit Conductors (NEC 440.35)

• The conductor ampacity must be equal to or greater than the marked MCA (38.0A).
• Enter NEC Table 310.16 (75°C Copper Column):
  • #10 AWG Cu = 35 amperes (insufficient; 35A < 38A)
  • #8 AWG Cu = 50 amperes (complies; 50A $\ge$ 38A)
• Selected Conductor: #8 AWG THHN Copper.

Step 2: Size the Overcurrent Protective Device (NEC 440.22)

• The protective device must not exceed the marked MOCP (60.0A).
• A 60-ampere HACR circuit breaker is a standard rating in NEC 240.6(A) and exactly matches the MOCP ceiling.
• Final Installation: #8 AWG THHN Copper conductors protected by a 60A HACR circuit breaker.