Dry Type Distribution and Power Transformers Design Notes

This article is about the design, construction, two winding transformer and testing of Dry Type Cast Resin Distribution Transformers ranging from 15 to 500 kVA of both the general purpose type and K-factor rated for non-linear loads. Main keywords for this article are Dry Type Distribution and Power Transformers Design Notes. Transformer Basics. K-Factor Rated Transformers. How to Read Transformer Ratings. Transformer Testing List.

Dry Type Distribution and Power Transformers References

American National Standards Association / Institute for Electrical and Electronics Engineers (ANSI/IEEE)
C57.12.01 General Requirements for Dry-Type Distribution and Power Transformers,  Including those with Solid-Cast And/or Resin Encapsulated Windings
C57.12.50 Requirements for Ventilated Dry-Type Distribution Transformers, 1 to 500 kVA, Single Phase and 15 to 500kVA Three Phase
C57.12.70 Terminal Marking and Connections for Distribution and Power Transformers
C57.12.91 Test Code for Dry-Type Distribution and Power Transformers
C57.110 Recommended Practice for establishing Liquid-Filled and Dry-Type Power Transformer Capability When Supplying Non-sinusoidal Load Currents
American Society for Testing and Materials (ASTM)
D 1535 Method of Specifying Color by the Munsell System
National Electrical Manufacturer’s Association (NEMA)
ST20-1982 Dry Type Transformers For General Applications

Transformer Basics

Basically transformer is a device which changes voltage either ups or down. And it will not change the frequency or not current. For example you can not change frequency 50 Hz to 60 Hz.
One thing keep in mine transformer never work on DC Voltage it works only with AC voltages.

Dry Type Distribution and Power Transformers Design Notes as per NEC

  • Transformers shall conform to ANSI C57.12.50 and rated from 15 kVA to 500 kVA.
  • Transformers for indoor use shall be ventilated dry type with a 220 °C insulation system. The insulation levels and dielectric capabilities shall be in accordance with NEMA ST20.
  • Transformers for outdoor use in both classified and unclassified areas shall be nonventilated up to the limits available from the manufacturer. Above this limit they shall be ventilated and protected from the environment with weather shield kits.
  • Units shall be designed to operate with a rated maximum temperature rise of 150 °C.
  • Unless otherwise specified, electrical conductor shall be made of electrolytic copper,  with conductivity 98% IACS and arranged to brace coil layers and provide maximum ventilation. Aluminum windings shall be provided with conductivity 61% IACS.
  • Core laminations shall be tightly assembled. The complete core and coil assembly shall be vacuum impregnated with a non-hygroscopic thermo-setting varnish.
  • Core and coil assemblies shall be constructed to provide thermal, mechanical and magnetic short circuit withstand capabilities.
  • The core and coil assembly shall be installed on vibration dampening pads and securely bolted to the enclosure base.
  • Grounding shall be in accordance with ANSI C57.12.50 section 5.7.
  • On neutral grounded wye system, the neutral shall be grounded at the supply transformer neutral bushing.
  • The enclosure shall be of heavy gauge steel construction, with removable front and  rear covers to provide access to the terminal compartments.
  • Unless otherwise specified, finish coat shall have a nonmetallic pigment surface. Color of finish coat shall be light gray number 61 in accordance with ASTM D 1535 (Munsell Notation 8.3 G6 10/0.54) and covered with an epoxy coat for protection against environmental conditions.
  • Terminals shall be sized to carry 125 percent of the transformers full load current.
  • Transformers above 600 V shall have two separate terminal compartments for high voltage and low voltage terminals.
  • Transformers shall be approved for use in the area classification they are to be installed in. The majority of these transformers will probably be located in Class I, Division 2 areas. Wherever possible they shall be installed in unclassified areas.
  • Units installed outdoors shall have a NEMA 4X outdoor enclosure.
  • The enclosure shall be mechanically isolated from the transformer.  Corrosion resistant wall mounting brackets and fasteners shall be installed where specified.

Why K-Factor Rated Transformers are Required

  • When transformers are to serve non-linear loads they shall be K-Factor rated.
  • The K-Factor is the ratio between the additional losses due to harmonics and the eddy current losses at 60 Hz.
  • Non-linear rated transformers shall be suitable for nonsinusoidal loads and harmonic  distortion as required in ANSI/IEEE C57.110.
  • Non-linear rated transformers shall be UL listed and shall bear the UL marking on its nameplate along with the specific K-Factor rating or equal.
  • The transformers shall be designed with and labeled for one of the following K-Factor rating designations: K4 for 50 percent non-linear load K13 for 100 percent non-linear load  K20 for 150 percent non-linear load K30 for 200 percent non-linear load
  • The core of K-Factor rated transformers shall be designed to withstand voltage distortion and high frequency harmonic currents.
  • The core shall be magnetic flux density.
  • Primary and secondary windings shall be designed to minimize stray losses, skin effect losses and excessive heating from harmonic currents.
  • Coils shall not exceed the specified winding temperature rise, the corresponding hot spot temperature rating or the 220 °C insulation rating while carrying the specified nonlinear load.
  • The neutral bus for single phase, 3 wire or 3 phase, 4 wire shall be sized for 200 percent of rated current. 
  • An electrostatic shield shall be provided between the primary and secondary windings and grounded to a common point within the transformer enclosure.

How to Read Transformer Ratings


  • All transformers shall be 3 phase delta primary and wye secondary dry type cast resin and suitable for indoor and outdoor operation.
  • Unless otherwise specified on data sheets standard voltage ratings shall be supplied. The primary voltage shall not exceed 34.5 kV, with secondary voltage to be one of the following:
    a. 120 V, 240 V single phase 2 wire
    b. 208 Y / 120 V three phase 4 wire
    c. 480 Y / 277 V three phase 4 wire
  • Unless otherwise specified, four full-capacity, off-load taps, two 21/2 percent above nominal and two 21/2 percent below nominal shall be provided on the high voltage winding.

Sound Levels

  • Unless otherwise specified, transformer sound levels shall conform to ANSI/NEMA as follows:
    10 to 50 kVA                      45 dB
    51 to 150 kVA                    50 dB
    151 to 300 kVA                 55 dB
    301 to 500 kVA                 60 dB

Primary Basic Impulse Insulation Level (BIL)

Transformer windings and bushings insulation level shall not be less than the following

Terminal Marking

  • Terminal designations shall be as defined by ANSI C57.12.70 with each terminal clearly marked with oil resistant yellow paint. The identification of terminal connections shall be shown on the instruction nameplate.

Transformer Testing List Required

  • Transformers shall be tested in accordance with ANSI C57.12.91 and applicable industry standards. Unless otherwise specified, the following tests shall be required.
    (i) Ratio: Rated voltage and all taps
    (ii) Excitation loss at rated voltage and nameplate connection
    (iii) Resistance: On all windings and at all tap positions
    (iv) Polarity and phase relations
    (v) Excitation current at rated voltage and at nameplate connection
    (vi) Load loss and impedance corrected to nameplate connected rated voltage and rated temperature rise, at 50 °C ambient and rated kVA
    (vii) Sound level testing
    (viii) Temperature rise of each winding
    (ix) Dielectric: Applied potential, at power frequency, for each winding to all other windings, the core and the enclosure or frame
    (x) Dielectric: Induced potential
    (xi) Dielectric: Partial discharge, coils less than 1.2 kV
    (xii) Dielectric: Impulse
    (xiii) Insulation Resistance: Each winding to ground and from winding to winding using a megger type tester, or as required
    (xiv) Short circuit
    (xv) Full load and impartial


Transformer Accessories

  • Each transformer shall be equipped with all vendor’s normally furnished standard accessories.
  • If surge arresters are specified on data sheet, they shall be mounted to allow for ready disconnection (without removal) to permit independent testing of transformer and arresters.
  • Unless otherwise specified on data sheet, 3 phase hot spot temperature indicators shall be provided on transformers 300 kVA and 500 kVA.
  • Nameplates shall be made of stainless steel securely fastened at eye level height to the front of the transformer enclosure with stainless steel screws and shall conform to ANSI C57.12.01, Table 5. As a minimum the nameplate shall include the following:
    a. Complete electrical data
    b. Wiring Diagram
    c. Tap Connections                                                                                                                                                                                                                                                                      d. Manufacturer’s Name
    e. Catalog Number
    f. Mfg. Serial No.
    g. Date of Manufacture
    h. Impedance
    i. Weight
    j. Ambient Temperature
    k. Vector Group

    Dry Type Distribution and Power Transformers Design Notes. Transformer Basics. K-Factor Rated Transformers. How to Read Transformer Ratings.


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1 Response

  1. March 24, 2020

    […] Outdoor transformers shall be dry-type (unless specified otherwise in request for quotes and purchasing documents), and shall conform to applicable sections of Dry Type Distribution and Power Transformers Design Notes.   […]

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