8.4 Transformers & Generators

8.4 Transformers & Generators

Quick Reference: Transformers and generators constitute the core power-transformation and on-site generation equipment in modern electrical distribution. NEC Article 450 governs transformers, establishing precise overcurrent protection thresholds in NEC Table 450.3(B) (differentiating primary-only from primary-plus-secondary protection) and mandating ventilation and accessibility clearances. Crucially, transformer overcurrent protection under Article 450 protects only the transformer windings—it does NOT protect secondary conductors, which must be independently audited under NEC 240.21(C). NEC Article 445 governs generators, requiring phase conductors to be sized at not less than $115%$ of the generator nameplate rating under NEC 445.13 and enforcing lockable disconnecting and emergency shutdown means under NEC 445.18.

1. Transformer Overcurrent Protection $\le 1000\text{ Volts}$ (NEC 450.3(B))

Transformer overcurrent protection requirements for systems operating at $1000\text{ V}$ or less are specified in NEC Table 450.3(B). Sizing is governed by two fundamental protection schemes: Primary Only Protection and Primary and Secondary Protection.

+---------------------------------------------------------------------------------------------------+
|              NEC TABLE 450.3(B) TRANSFORMER OVERCURRENT PROTECTION (<= 1000V)                     |
+------------------------------------+-----------------------+--------------------------------------+
| Protection Scheme & Rated Current  | Primary Protection    | Secondary Protection                 |
+------------------------------------+-----------------------+--------------------------------------+
| PRIMARY ONLY PROTECTION:           |                       |                                      |
| • Primary Current >= 9 Amperes     | 125% (Note 1 applies) | None Required                        |
| • Primary Current 2A to < 9 Amps   | 167% (Max)            | None Required                        |
| • Primary Current < 2 Amperes      | 300% (Max)            | None Required                        |
+------------------------------------+-----------------------+--------------------------------------+
| PRIMARY AND SECONDARY PROTECTION:  |                       |                                      |
| • Secondary Current >= 9 Amperes   | 250% (Max)            | 125% (Note 1 applies)                |
| • Secondary Current < 9 Amperes    | 250% (Max)            | 167% (Max)                           |
+------------------------------------+-----------------------+--------------------------------------+

Understanding Table 450.3(B) Note 1 (The Round-Up Rule)

Under Note 1 of Table 450.3(B), where $125%$ of the rated current does not correspond to a standard ampere rating of fuse or non-adjustable circuit breaker listed in NEC Table 240.6(A), the next higher standard rating is permitted, provided the rated current is $9\text{ amperes}$ or more.

Transformer Full-Load Current Formulas

$\text{Three-Phase Transformer FLC: } I = \frac{\text{kVA} \times 1000}{\sqrt{3} \times V_{\text{Line-to-Line}}} = \frac{\text{kVA} \times 1000}{1.732 \times V}$

$\text{Single-Phase Transformer FLC: } I = \frac{\text{kVA} \times 1000}{V}$

+---------------------------------------------------------------------------------------------------+
|               STANDARD THREE-PHASE 480V TO 208Y/120V TRANSFORMER FLC & OCPD MATRIX                |
+------------+------------------+-------------------+-----------------------+-----------------------+
| Size (kVA) | 480V Primary FLC | 208V Sec. FLC     | Max Primary-Only OCPD | Max Dual Sec. OCPD    |
|            |                  |                   | (125% + Note 1)       | (125% + Note 1)       |
+------------+------------------+-------------------+-----------------------+-----------------------+
| 15 kVA     | 18.04 A          | 41.64 A           | 25 A Breaker          | 60 A Breaker          |
| 30 kVA     | 36.08 A          | 83.27 A           | 50 A Breaker          | 110 A Breaker         |
| 45 kVA     | 54.13 A          | 124.91 A          | 70 A Breaker          | 175 A Breaker         |
| 75 kVA     | 90.21 A          | 208.18 A          | 125 A Breaker         | 300 A Breaker         |
| 112.5 kVA  | 135.32 A         | 312.27 A          | 175 A Breaker         | 400 A Breaker         |
| 150 kVA    | 180.42 A         | 416.36 A          | 250 A Breaker         | 600 A Breaker         |
| 225 kVA    | 270.63 A         | 624.54 A          | 350 A Breaker         | 800 A Breaker         |
| 300 kVA    | 360.84 A         | 832.72 A          | 500 A Breaker         | 1200 A Breaker (Note*)|
+------------+------------------+-------------------+-----------------------+-----------------------+

(Note: For secondary currents over 800A, refer to NEC 240.4(C) conductor matching rules).*

2. Transformer Secondary Conductor Tap Rules (NEC 240.21(C))

A critical, frequently misunderstood principle in electrical plan review is the distinction between transformer protection (Article 450) and secondary conductor protection (Article 240).

The Secondary Protection Non-Reflection Rule (NEC 240.21(C)(1))

Under NEC 240.21(C)(1), primary overcurrent protective devices are considered to protect secondary conductors ONLY for 2-wire single-phase systems or delta-delta 3-wire 3-phase systems.

• For common 3-wire single-phase ($120/240\text{V}$) or 4-wire three-phase ($208\text{Y}/120\text{V}$ or $480\text{Y}/277\text{V}$) transformers, line-to-neutral unbalanced currents and single line-to-ground faults on the secondary do NOT reflect equally into the primary phase conductors.
• Therefore, primary overcurrent devices CANNOT protect secondary conductors! Every 4-wire secondary conductor run must be protected in accordance with the secondary tap rules of NEC 240.21(C).

Key Transformer Secondary Tap Rules (NEC 240.21(C))

1. Transformer Secondary 10-Foot Tap Rule (NEC 240.21(C)(2)): Conductor length does not exceed $10\text{ feet}$ ($3.0\text{ m}$), conductors are enclosed in raceway, terminate in a single OCPD, and have an ampacity:
   • Not less than the combined calculated load,
   • Not less than the rating of the terminating OCPD / panelboard main, AND
   • Not less than $\frac{1}{10}\text{th}$ of the primary OCPD rating multiplied by the primary-to-secondary voltage ratio ($\text{Ampacity} \ge \frac{1}{10} \times \text{OCPD}{\text{pri}} \times \frac{V{\text{pri}}}{V_{\text{sec}}}$).
2. Transformer Secondary 25-Foot Tap Rule (NEC 240.21(C)(6)): Conductor length does not exceed $25\text{ feet}$ ($7.5\text{ m}$), conductors are protected from physical damage, terminate in a single OCPD sized to the tap ampacity, and have an ampacity:
   • Not less than the primary OCPD rating multiplied by the primary-to-secondary voltage ratio divided by $3$ ($\text{Ampacity} \ge \frac{1}{3} \times \text{OCPD}{\text{pri}} \times \frac{V{\text{pri}}}{V_{\text{sec}}}$).

3. Transformer Ventilation & Installation Clearances (NEC 450.9 & 450.13)

Ventilation Requirements (NEC 450.9)

• Ventilation openings on transformer enclosures must not be obstructed by walls, panels, structural columns, or storage materials.
• Transformers must be installed with the manufacturer's marked clearance from walls or other obstructions (typically $6\text{ inches}$ to $12\text{ inches}$ for dry-type units).

Accessibility (NEC 450.13)

• General Rule (NEC 450.13): Transformers must be readily accessible to qualified personnel for inspection and maintenance.
• Exceptions for Dry-Type Transformers:
  • Open Installations (450.13(A)): Dry-type transformers $\le 1000\text{V}$ and $\le 50\text{ kVA}$ are permitted to be installed in open areas (e.g., mounted on elevated platforms or structural building columns) without being readily accessible.
  • Suspended Ceilings (450.13(B)): Dry-type transformers $\le 1000\text{V}$ and $\le 50\text{ kVA}$ are permitted to be installed above suspended ceilings, provided the space is fire-rated or non-combustible, has adequate ventilation, and is accessible via ceiling access panels.

Dry-Type Transformers Installed Indoors (NEC 450.21)

• Transformers rated $\le 112.5\text{ kVA}$ must maintain at least $12\text{ inches}$ ($305\text{ mm}$) separation from combustible materials unless separated by a fire-resistant thermal insulating barrier.
• Transformers rated $> 112.5\text{ kVA}$ must be installed in a dedicated transformer room of 1-hour fire-resistant construction, EXCEPT where the transformer has Class 155 or higher insulation systems ($80^\circ\text{C}$ rise or higher) and is separated from combustible materials by at least $6\text{ feet}$ horizontally and $12\text{ feet}$ vertically.

4. Generator Sizing, Conductors & Disconnecting Means (NEC Article 445)

Stationary and portable engine-driven generators supply backup, legally required, and emergency power systems.

Generator Conductor Sizing (NEC 445.13(A))

Under NEC 445.13(A), phase conductors from the generator output terminals to the first distribution device containing overcurrent protection must have an allowable ampacity not less than $115%$ of the generator nameplate full-load current rating. $\text{Minimum Generator Conductor Ampacity} = 1.15 \times \text{Generator Nameplate Full-Load Amperes}$ (Exception: Where the design and operating conditions of the generator prevent the generator from operating at an overload, conductors are permitted to be sized at $100%$).

Generator Neutral Conductor (NEC 445.13(B))

The neutral conductor must be sized to carry the maximum calculated unbalanced load in accordance with NEC 220.61, but cannot be smaller than the minimum size equipment grounding conductor required under NEC Table 250.122 based on the generator rating.

Generator Disconnecting Means & Emergency Shutdown (NEC 445.18 & 445.19)

1. Disconnecting Means (NEC 445.18(A)): Generators must be equipped with a disconnecting means that disconnects all ungrounded conductors simultaneously. The disconnect must be lockable in the open position in accordance with NEC 110.25.
2. Emergency Shutdown (NEC 445.18(C) & 445.19):
   • One- and Two-Family Dwellings (NEC 445.18(C)): For stationary dwelling unit generators, an outdoor emergency shutdown device must be provided at a readily accessible outdoor location outside the building.
   • Commercial & Industrial Installations (NEC 445.19): Generators with output ratings $> 15\text{ kW}$ must be equipped with an emergency shutdown switch located outside the generator room enclosure or remote from the generator.

5. Step-by-Step Worked Plan Review Calculations

Worked Example 1: 75 kVA 480V to 208Y/120V Transformer Engineering Audit

• Parameters: A $75\text{ kVA}$, 3-phase, $480\text{V}$ primary to $208\text{Y}/120\text{V}$ secondary dry-type transformer. The single-line diagram specifies: (1) Primary OCPD = $125\text{A}$ breaker, (2) Primary conductors = $1\text{ AWG Cu}$, (3) Secondary conductors = $250\text{ kcmil Cu}$ ($8\text{ ft}$ run in EMT), (4) Secondary main breaker = $250\text{A}$ in a $208\text{Y}/120\text{V}$ distribution panel.
• Step 1: Calculate Primary Full-Load Current (FLC) $I_{\text{pri}} = \frac{75,000\text{ VA}}{\sqrt{3} \times 480\text{ V}} = \frac{75,000}{831.38} = \mathbf{90.21\text{ A}}$
• Step 2: Calculate Secondary Full-Load Current (FLC) $I_{\text{sec}} = \frac{75,000\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{75,000}{360.26} = \mathbf{208.18\text{ A}}$
• Step 3: Audit Primary Overcurrent Protection (NEC Table 450.3(B))
  • Primary-Only Protection: Multiplier = $125%$ of Primary FLC ($90.21\text{ A}$): $\text{Max Primary OCPD} = 1.25 \times 90.21\text{ A} = 112.76\text{ A}$
  • Apply Note 1: Next standard size above $112.76\text{A}$ in Table 240.6(A) = $125\text{ A}$ Breaker.
  • Finding: Proposed $125\text{A}$ primary breaker is FULLY COMPLIANT under Table 450.3(B).
• Step 4: Audit Secondary 10-Foot Tap Conductors (NEC 240.21(C)(2))
  • Secondary run is $8\text{ ft}$ ($\le 10\text{ ft}$).
  • $250\text{ kcmil Cu}$ at 75°C = $255\text{ A}$ ampacity.
  • Check 1: Ampacity ($255\text{A}$) $\ge$ Secondary OCPD ($250\text{A}$). PASS.
  • Check 2: 1/10th Primary OCPD ratio check: $\text{Min Ampacity} = \frac{1}{10} \times 125\text{ A} \times \left( \frac{480\text{ V}}{208\text{ V}} \right) = 12.5 \times 2.308 = 28.85\text{ A}$
  • Conductor ampacity ($255\text{A} \ge 28.85\text{A}$). PASS.
  • Finding: Secondary tap is FULLY COMPLIANT.

Worked Example 2: 150 kW Commercial Standby Generator Conductor Sizing

• Parameters: A $150\text{ kW} / 187.5\text{ kVA}$, $480\text{Y}/277\text{V}$, 3-phase diesel standby generator ($0.8\text{ Power Factor}$). Single-line diagram shows $4/0\text{ AWG}$ THHN Copper conductors from generator terminals to the automatic transfer switch (ATS).
• Step 1: Calculate Generator Full-Load Rated Current $I_{\text{gen}} = \frac{187,500\text{ VA}}{\sqrt{3} \times 480\text{ V}} = \frac{187,500}{831.38} = \mathbf{225.53\text{ A}}$
• Step 2: Calculate Minimum Conductor Ampacity (NEC 445.13(A)) $\text{Minimum Ampacity} = 1.15 \times 225.53\text{ A} = \mathbf{259.36\text{ A}}$
• Step 3: Select Conductor Size (NEC Table 310.16 at 75°C)
  • $4/0\text{ AWG Cu}$ is rated for $230\text{ A}$ at 75°C ($230\text{A} < 259.36\text{A}$ $\rightarrow$ DEFICIENT!).
  • Required Conductor: $300\text{ kcmil THHN Copper}$ ($285\text{ A}$ at 75°C) or $250\text{ kcmil Cu}$ ($255\text{ A}$ is also under $259.36\text{A}$).
  • Conclusion: REJECT DRAWING. Conductor must be upsized to at least $300\text{ kcmil Cu}$.

6. Plans Examiner Verification Checklist: Transformers & Generators

• Table 450.3(B) OCPD Check: Verify transformer primary OCPD does not exceed 125% (with Note 1 round-up) for primary-only, or 250% primary / 125% secondary for dual protection.
• Secondary Conductor Protection (240.21(C)): Confirm 3-wire and 4-wire transformer secondaries have dedicated overcurrent protection and meet 10-ft or 25-ft tap rules.
• Transformer Ventilation Clearances (450.9): Confirm drawings indicate required manufacturer spacing (minimum 6–12 inches) from walls and unobstructed louvers.
• Transformer Accessibility & Vaults (450.13 / 450.21): Confirm units >112.5 kVA meet 1-hour fire separation or have Class 155+ insulation with required 6-ft horizontal clearance.
• 115% Generator Conductor Sizing (445.13): Verify generator phase conductors have an allowable ampacity $\ge 115%$ of nameplate full-load current.
• Generator Lockable Disconnect (445.18): Verify generator disconnecting means is lockable in the open position under NEC 110.25 and includes required emergency shutdown controls.