Transformers

12.1.0 Introduction12.1.1 Typical Transformer Weights (lb) by kVA12.1.2 Transformer Full-Load Current, Three-Phase, Self-Cooled Ratings12.1.3 Typical Impedances, Three-Phase, Liquid-Filled Transformers12.1.4 Approximate Transformer Loss and Impedance Data12.1.5 Transformer Primary (480-V, Three-Phase, Delta) and Secondary

(208-Y/120-V, Three-Phase, Four-Wire) Overcurrent Protection,Conductors and Grounding

12.1.6 NEC Table 450.3 (A), Maximum Rating or Setting of OvercurrentProtection for Transformers Over 600 V (as a Percentage of TransformerRated Current)

12.1.7 NEC Table 450.3 (B), Maximum Rating or Setting of Overcurrent Protectionfor Transformers 600 V and Less (as a Percentage of Transformer RatedCurrent)

12.2.1 Electrical Connection Diagrams12.3.1 Auto Zigzag Grounding Transformers for Deriving a Neutral, Schematic

and Wiring Diagram12.3.2 Auto Zigzag Transformer Ratings12.4.1 Buck-Boost Transformer Three-Phase Connection Summary12.4.2 Wiring Diagrams for Low-Voltage Single-Phase Buck-Boost Transformers12.4.3 Connection Diagrams for Buck-Boost Transformers in Autotransformer

Arrangement for Single-Phase System12.4.4 Connection Diagrams for Buck-Boost Transformers in Autotransformer

Arrangement for Three-Phase System12.5.1 Maximum Average Sound Levels for Transformers12.5.2 Typical Building Ambient Sound Levels12.6.1 Transformer Insulation System Temperature Ratings12.7.1 k-Rated Transformers

12.1.0 Introduction

Transformers are a critical part of electrical distribution systems because they areused most often to change voltage levels. This affects voltage, current (both loadand fault current levels), and system capacity. They also can be used to isolate,suppress harmonics, derive neutrals through a zigzag grounding arrangement,and reregulate voltage. The information that follows provides useful design andinstallation data.

Section

12.1

12

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12.1.1 Typical Transformer Weights (lb) by kVA

12.2 Section Twelve

TABLE 12.1.1

TABLE 12.1.2

12.1.2 Transformer Full-Load Current,Three-Phase, Self-Cooled Ratings

12.1.3 Typical Impedances, Three-Phase,Liquid-Filled Transformers (see page 12.3)

12.1.4 Approximate Transformer Loss andImpedance Data (see page 12.3)

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Transformers 12.3

TABLE 12.1.3 Typical Impedances—Three-Phase Transformers

TABLE 12.1.4 Approximate Loss and Impedance Data

Note: Values are typical. Refer to transformer manufacturer for exact values.

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12.1.6 Maximum Rating or Setting of OvercurrentProtection for Transformers Over 600 V (as a Percentage of Transformer Rated Current) [NEC Table 450.3(A)] (see page 12.5)

12.1.5 Transformer Primary (480-V, Three-Phase,Delta) and Secondary (208-Y/120-V, Three-Phase,Four-Wire) Overcurrent Protection, Conductors andGrounding

12.4 Section Twelve

TABLE 12.1.5

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Transformers 12.5

TABLE 12.1.6

TABLE 12.1.7

12.1.7 Maximum Rating or Setting of OvercurrentProtection for Transformers 600 V and Less (as aPercentage of Transformer Rated Current)

(© 2001, NFPA)

(© 2001, NFPA)

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12.2.1 Electrical Connection Diagrams

12.6 Section Twelve

12.2.1 Connection diagrams for transformers.

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Transformers 12.7

12.3.1 Auto zig-zag transformers, for developing a neutralfrom a three-phase, 3-wire supply.

12.3.1 Auto Zigzag Grounding Transformers forDeriving a Neutral, Schematic and Wiring Diagram

12.3.2 Auto Zigzag Transformer Ratings

TABLE 12.3.2

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12.4.1 Buck-Boost Transformer Three-PhaseConnection Summary

12.8 Section Twelve

TABLE 12.4.1 Three-Phase Connections

12.4.2 Wiring Diagrams for Low-VoltageSingle-Phase Buck-Boost Transformers

12.4.2 Low-voltage buck-boost drawings.

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12.4.3 Connection Diagrams for Buck-BoostTransformers in Autotransformer Arrangementfor Single-Phase System

Transformers 12.9

12.4.3 Connection diagrams, single-phase.

12.4.4 Connection Diagrams for Buck-BoostTransformers in Autotransformer Arrangementfor Three-Phase System (see page 12.10)

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12.5.1 Maximum Average Sound Levels forTransformers

12.10 Section Twelve

12.4.4 Connection diagrams, three-phase.

TABLE 12.5.1 Maximum Average Sound Levels (dB)

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12.5.2 Typical Building Ambient Sound Levels

Transformers 12.11

12.6.1 Total winding temperature, °C.

TABLE 12.5.2 Typical Sound Levels (dB)

12.6.1 Transformer Insulation System TemperatureRatings

12.7.1 k-Rated Transformers

Transformers used for supplying the nonsinusoidal high-harmonic-content (�5 per-cent) loads that are increasingly prevalent must be designed and listed for theseloads. ANSI C57.110-1986, “Recommended Practice for Establishing TransformerCapability When Supplying Non-Sinusoidal Load Currents,” provides a method forcalculating the heating effect in a transformer when high harmonic currents are pre-sent. This method generates a number called the k-factor, which is a multiplier thatrelates eddy-current losses in the transformer core due to harmonics to increasedtransformer heating. Transformer manufacturers use this information to designtransformer core/coil and insulation systems that are more tolerant of the higherinternal heating load than a standard design. Simply put, a k-rated transformer can

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tolerate approximately k times more internal heat than a similar standard designtransformer (e.g., a k-4 transformer can handle approximately four times the internalheating load of a similar ANSI standard non-harmonic-rated transformer with no lifeexpectancy reduction).

The k rating of a transformer addresses only increased internal heating. It doesnot address mitigation of the harmonic content of the transformer load.

12.12 Section Twelve

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