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SECTION 26 2200 - DRY TYPE TRANSFORMERS

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PART 1 - GENERAL
1.1 RELATED DOCUMENTS

A. Drawings and general provisions of the Contract, including General and Supplementary Conditions
and Division 01 Specification Sections, apply to this Section.

B. The Contractor's attention is specifically directed, but not limited, to the following documents for
additional requirements:
1. The current version of the Uniform General Conditions for Construction Contracts, State of
Texas, available on the web site of the Texas Facilities Commission.
2. The University of Houston’s Supplemental General Conditions and Special Conditions for
Construction.
1.2 DESCRIPTION OF WORK
A. Included: Provide transformer work as shown, scheduled, indicated and as specified.
B. Types: The types of low voltage transformers required for the project include, but are not limited to,
1. Dry- Type General Purpose Transformers.
2. K-Factor Rated Dry-Type Transformers.
3. Dry-Type Harmonic Mitigating Transformers.
1.3 STANDARDS
A. Products shall be designed, manufactured, tested and installed in compliance with the following
standards:
1. Institute of Electrical and Electronics Engineers (IEEE)
a. The following IEEE/ANSI standards when referenced in NEMA ST-20 or UL 1561:
1) C57.94 “Recommended Practice for Installation, Application, Operation, and
Maintenance of Dry-Type Distribution and Power Transformers”
2) C57.96, “IEEE Guide for Loading Dry-Type Distribution and Power Transformers”
3) C57.12.91, “IEEE Standard Test Code for Dry-Type Distribution and Power
Transformers”

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4) C57.12.01, “IEEE Standard General Requirements for Dry-Type Distribution and Power
Transformers Including Those with Solid Cast and/or Resin Encapsulated Windings”
5) C57.12.70, “Standard Terminal Markings and Connections for Distribution and Power
Transformers”
2. National Electrical manufacture Association (NEMA)
a. NEMA ST-20, “Dry Type Transformers for General Application”
3. National Fire Protection Association
a. NFPA 70, “National Electrical Code (NEC)”
b. NFPA 70E, Standard for Electrical Safety in the Work Place
c. NFPA 5000, “Building Construction and Safety Code”
4. Underwriters Laboratories, Inc. (UL):
a. UL 1561, Dry-Type General Purpose and Power Transformers
b. UL50, Enclosures for Electrical Equipment, Non-Environmental Considerations
c. UL50E, Enclosures for Electrical Equipment, Environmental Considerations (Units self-
certified to NEMA 250 shall not be accepted – must have Third Party Testing)
d. UL969, Marking and labelling systems

B. The latest published edition of a reference shall be applicable to this Project unless identified by a
specific edition date.
C. All reference amendments adopted prior to the effective date of this Contract shall be applicable to
this Project.
D. All materials, installation and workmanship shall comply with the applicable requirements and
standards addressed within the following references:
1.4 QUALITY ASSURANCE
A. Listing and Labeling: Provide transformers specified in this Section that are listed by Underwriters
Laboratories (UL). Transformers shall meet all relevant NEMA, UL, CSA, and NFPA standards.
B. Minimum Linear Load Efficiency: NEMA TP-1 98.23 percent for 30 kVA, 98.4 percent for 45kVA, 98.6
percent for 75 kVA, 98.74 percent for 112.5kVA, 98.83 percent for 150 kVA, 98.94 percent for 225
kVA, 99.02 percent for 300 kVA, 99.14 percent for 500 kVA.
C. Manufacturers: Provide products complying with these specifications and produced by one of the
following:
1. Dry-Type General Purpose and K-Factor Rated Dry-Type Transformers.
a. Eaton.
b. ABB.
c. Square D Company.
d. Siemens.
e. Hevi-Duty Electric Corporation.
f. Power Smith.
g. Sola HD.

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2. Harmonic Mitigating Transformers:

a. Powersmiths International Corp.
b. Power Quality International, Inc.
c. Eaton.
D. UL Label: All transformers shall be UL-labeled, and have transformers efficiencies verified by a third
party NRTL.
1.5 SUBMITTALS
A. Shop Drawing submittals shall include, but not be limited to, the following:
1. Include data on features, components, ratings and performance for each type of transformer
specified. Include dimensioned plans, sections, and elevation views. Show minimum clearances
and installed devices and features.
a. Include outline and support point dimensions of enclosures and accessories, unit weight,
voltage, kVA, and impedance ratings and characteristics.
b. No load core loss, full load winding conductor loss, full load losses, efficiency at 25 percent,
50 percent, 75 percent and 100 percent rated loads, percent regulation with 80 percent and
100 percent power factor loads, sound level, tap configurations, insulation system type and
rated temperature rise.
2. Cut sheets of the transformers with load ratings, sound ratings, and all associated accessories
clearly indicated.
3. Include outline and support point dimensions of enclosures and accessories; unit weight;
voltage; kVA; impedance ratings and characteristics; loss data; efficiency at 25, 50, 75 and 100%
rated load; sound level; tap configurations; insulation system type, and rated temperature rise.
4. Additional information as required in Section 26 0001 “Electrical General Provisions”.
5. Wiring Diagrams: Detail wiring and identify terminals for tap changing and connecting field-
installed wiring.
6. Product Certificates: Signed by manufacturer of transformers certifying that the products
furnished comply with requirements, including harmonic performance guarantee.
7. Factory Test Reports: Base data for electrical characteristics on actual laboratory tests of typical
transformers for harmonic performance and energy efficiency. Testing to be conducted using
three (3) single-phase 120V nonlinear load banks with personal computer harmonic profile (100
percent current THD) at 35 percent or more of transformer nameplate load level. Test results to
be submitted at time of quotation to include transformer efficiency, change in voltage THD
between transformer primary and secondary terminals, ratio of 3rd harmonic current in primary
delta current profile compared to that in secondary phase current.
8. Copies of manufacturer’s design and routine factory tests required by referenced standards,
including results of zero sequence impedance and reactance tests.
1.6 PRODUCT DELIVERY, STORAGE, AND HANDLING
A. Transport, handle, store and protect products.
B. Deliver transformers individually wrapped for protection and mounted on shipping skids.
C. Store in a clean, dry space. Maintain factory wrapping or provide an additional heavy canvas or
plastic cover to protect units from dirt, water, construction debris, and traffic.

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D. Handle transformers carefully to avoid damage to material components, enclosure and finish. Use
only lifting eyes and brackets provided for that purpose. Damaged transformers shall be rejected and
not be installed on project.
E. Store transformers in a clean and dry space and protect from weather. Do not stack transformers.
F. Apply temporary heat according to manufacturer’s written instructions within the enclosure of each
ventilated-type unit throughout periods during which equipment is not energized and is not in a
space that is continuously under normal control of temperature and humidity.
G. Transformers shall not be used as work tables, scaffolds or ladders.
PART 2 - PRODUCTS
2.1 MATERIALS AND COMPONENTS
A. General: Except as otherwise indicated, provide transformer manufacturer's standard materials and
components as indicated by the published product information, designed and constructed as
recommended by the manufacturer, and as required for a complete installation.
2.2 DRY TYPE GENERAL PURPOSE, K-RATED AND ISOLATION TRANSFORMERS
A. General
1. Indoor transformers shall be dry type, 2-winding transformers, voltage, phase and kVA rated as
shown, noted or scheduled. Transformers shall be designed for 60-hertz operation, self-cooled
per NEMA Class AA and shall have manufacturer's standard impedance.
2. Primary winding of 3-phase standard transformers shall be delta-connected. Secondary
windings of 3-phase transformers shall be wye or as required to cancel zero sequence current
flux for phase cancellation type, with the common neutral brought out. Transformer primary
and secondary voltages shall be as shown on the Drawings.
3. Suppression transformers shall have the primary and secondary coils physically separated.
B. Construction
1. Transformer core shall be of a common core construction using cold rolled, oriented, high
permeability silicon steel. Windings shall be copper individual windings terminated with silver-
plated copper bars or wire electrically welded to the ends of the windings. Foil windings shall
not be acceptable.
2. Transformer coils shall be vacuum pressure impregnated (VPI) with non-hydroscopic,
thermosetting varnish and shall have a final wrap of electrical insulating material designed to
prevent injury to the magnet wire. Transformers having coils with magnet wire visible will not
be acceptable. The core and coil shall be completely isolated from the enclosure by means of
vibration absorbing mounts.
3. All ventilating openings shall be of the baffled type. Ventilated dry-type transformers installed
in sprinklered space shall have rain shields on all openings.
4. The base of the transformer shall be constructed of heavy gauge steel. The transformer
enclosure shall be degreased, cleaned, phosphatized, primed, and finished with baked enamel.
C. Taps: Transformers shall have taps as follows:
KVA Rating Taps
0.25 to 2 kVA, single phase No taps

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3-15 kVA, single phase Two 2.5% taps below normal

voltage
25-100 kVA, single phase Six 2-1/2% taps, two above and four
below normal voltage
167-250kVA, single phase Four 2-1/2% taps, two above and
two below normal voltage
9-15 kVA, three phase Two 2.5% taps below normal
30-300 kVA, three phase Six 2-1/2% taps, two above and four
below normal voltage
500 kVA, three phase Four 2-1/2% taps, two above and
two below normal voltage
D. Temperature Rating: Transformers shall utilize an insulation system that has been properly
temperature classified and approved by UL and shall have temperature ratings as follows.
KVA Rating Temperature Rating
0.25 to 2 kVA Maximum winding temperature rise
of 80°C with an insulation system
temperature classification of 155°C
3-9 kVA Maximum winding temperature rise
of 115°C with an insulation system
temperature classification of 185°C
15 kVA and above Maximum winding temperature rise
of [150°C] with an insulation system
temperature classification of 220°C.
E. K-Factor Non-Linear Load Ratings: Transformers, where shown on the drawings, shall be K Factor
rated to serve non-linear loads. Transformer primary and secondary temperature shall not exceed
220°C at any point in the coils while carrying the transformer full rated capacity of non-linear loads at
the load K Factor shown on the drawings.
F. K-Factor Rated Transformer Neutrals: Neutral terminals for phase cancellation K Factor rated
transformers shall be designed for 200 percent of the secondary phase conductor ampacity
G. Load Rating: Transformers shall be capable of operating at 100% of nameplate rating continuously
while in an ambient temperature not exceeding 40°C.
H. Vibration Isolation: Each transformer core and coil shall be mounted in the transformer enclosure on
rubber vibration isolators. Vibration isolators shall limit the transmission of sound from the 120 Hz
harmonic to 10% of its un-isolated level, and shall be certified as such on the Shop Drawings.
I. Grounding: The core and coils shall be visibly grounded to the frame of the transformer cubicle by
means of a flexible grounding strap of adequate size.
J. Sound Rating: The transformer shall have sound levels equal to or lower than those ratings
established in the latest revision of NEMA ST-20 and as shown in the following table. Sound ratings
shall be measured per ANSI C89.91.
Transformer Rating
KVA Maximum Sound Level
(600 Volt Class) Decibels Per NEMA ST-20

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0-09 40
10-50 45
51-150 50
151-300 55
301-500 60
K. Testing:
1. The manufacturer shall have thoroughly tested each transformer for proper operation before
shipment.
2. The manufacturer shall have performed the following additional tests on units identical to the
design type being supplied. Furnish proof-of-performance of these tests in the form of test data
sheets upon request.
a. Sound levels.
b. Temperature rise tests.
c. Full-load core and winding losses.
d. Percent regulation with 80 and 100% power factor load.
e. Percent impedance.
f. Exciting current.
g. Insulation resistance.
h. Efficiency at 1/4, 1/2, 3/4, and full load.
i. Noise attenuation on suppression transformers.
L. Shield: Provide an electrostatic shield between the transformer primary and secondary to attenuate
source side line interference for transformers indicated to be shielded and for all transformers with a
K-factor rating greater than 1.0.
M. Transformer shall have minimum efficiency per DOE10 CFR431.
2.3 DRY TYPE HARMONIC MITIGATING TRANSFORMERS
A. General: Transformers shall be dry type, 2-winding transformers, rated as shown, shall have
manufacturer's standard impedance and shall include windings to mitigate harmonics in secondary
windings.
1. Primary winding of 3-phase transformers shall be delta-connected. Secondary windings of
3-phase transformers shall be wye connected, with the common neutral brought out.
Transformer primary and secondary voltages shall be as shown on the Drawings.
2. Transformers shall be designed to treat all major harmonics produced by phase-neutral, phase-
phase, and 3-phase six-pulse electronic equipment: 3rd, 5th, 7th, and 9th in addition to other
zero sequence currents.
3. 3rd harmonic and other zero sequence currents shall be treated in the secondary windings the
transformer through flux cancellation, and shall not be coupled into the primary winding.
Trapping these currents in the primary delta winding, as is the case for the delta-wye
transformer configuration, is not acceptable.
4. [5th and 7th harmonics for single output transformers are treated by introducing the
appropriate primary-to-secondary phase-shift in the transformer such that these currents

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subtract at the common bus with 5th and 7th harmonic currents produced by other similar
harmonic current sources fed from the same bus. See Drawings for phase shift of each
transformer. Fundamental current imbalance shall be reduced on the primary side of the
transformer compared to the secondary side.]
5. Harmonic cancellation performance guarantee shall be provided by the manufacturer.
B. Harmonic Cancellation: Zero sequence impedance/reactance shall be less than 0.95%/0.3%.
Transformer secondary shall be connected to cancel zero sequence current flux (3rd, 9th, 15th etc.).
These currents shall not be coupled into the primary windings of the transformer. [Provide windings
with 0 or 30 degrees phase shift with winding arrangements as required to cancel zero sequence
current flux (3rd, 5th, 7th 9th, 15th etc.) as scheduled on the drawings.]
C. Construction:
1. Transformer core shall be a common core construction using cold rolled, grain oriented, high
permeability silicon steel, formed as a coil. Windings shall be copper individual windings
terminated with silver-plated copper bars or wire electrically welded to the ends of the
windings. Neutral terminals shall be designed for 200 percent of the secondary phase conductor
ampacity. Foil windings shall not be acceptable.
2. Transformer coils shall be vacuum pressure impregnated (VPI) with non-hydroscopic,
thermosetting varnish and shall have a final wrap of electrical insulating material designed to
prevent injury to the magnet wire. Transformers having coils with magnet wire visible will not
be acceptable. The core and coil shall be completely isolated from the enclosure by means of
vibration absorbing mounts.
3. All ventilating openings shall be of the baffled type. Ventilated dry-type transformers installed
in sprinklered space shall have rain shields on all openings.
4. The base of the transformer shall be constructed of heavy gauge steel. The transformer
enclosure shall be degreased, cleaned, phosphatized, primed, and finished with baked enamel.
D. Taps: Three phase transformers shall have a minimum of four 2-1/2% taps, two above and two below
normal.
E. Temperature Rating: Transformers shall utilize an insulation system that has been properly
temperature classified and approved by UL. Transformers shall have a maximum winding
temperature rise of 115°C with an insulation system temperature classification of 220°C.
F. Non-Linear Load Ratings: Transformers, where shown on the drawings, shall be K-20 rated to serve
non-linear loads. Transformer primary and secondary temperature shall not exceed 220°C at any
point in the coils while carrying the transformer full rated capacity of non-linear loads at the load K
Factor shown on the drawings.
G. Load Rating:
1. Transformers shall be capable of operating at 100% of nameplate rating continuously while in an
ambient temperature not exceeding 40°C.
2. Transformers shall be capable of meeting the daily overload requirements of ANSI Standard
C57.12.
H. Energy Efficiency Ratings: Transformers shall have minimum efficiency per DOE10 CFR431.
I. Vibration Isolation: Each transformer core and coil shall be mounted in the transformer enclosure on
rubber vibration isolators. Vibration isolators shall limit the transmission of sound from the 120 Hz
harmonic to 10% of its un-isolated level, and shall certified as such on the Shop Drawings.

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J. Grounding: The core and coils shall be visibly grounded to the frame of the transformer cubicle by
means of a flexible grounding strap of adequate size.
K. Sound Rating: The transformer shall have sound levels equal to or lower than those ratings
established in the latest revision of NEMA ST-20 and as shown in the following table. Sound ratings
shall be measured per ANSI C89.91.
Transformer Rating
KVA Maximum Sound Level
(600 Volt Class) Decibels Per NEMA ST-20
0-09 40
10-50 45
51-150 50
151-300 55
301-500 60
L. Testing:
1. The manufacturer shall have thoroughly tested each transformer for proper operation before
shipment.
2. The manufacturer shall have performed the following additional tests on units identical to the
design type being supplied. Furnish proof-of-performance of these tests in the form of test data
sheets upon request.
a. Sound levels.
b. Temperature rise tests.
c. Full-load core and winding losses.
d. Percent regulation with 80 and 100% power factor load.
e. Percent impedance.
f. Exciting current.
g. Insulation resistance.
h. Efficiency at 1/4, 1/2, 3/4, and full load.
i. Noise attenuation on suppression transformers.
M. Shield: Provide a full width copper electrostatic shield between the transformer primary and
secondary to attenuate source side line interference.
PART 3 - EXECUTION
3.1 INSTALLATION OF TRANSFORMERS
A. General: Install transformers as indicated in accordance with the applicable requirements of the NEC
and the National Electrical Contractors Association “Standard of Installation”. Installation shall meet
or exceed all applicable federal, state and local requirements, referenced standards and conform to
codes and ordinances of authorities having jurisdiction.
B. Manufacturer’s Recommendations: All installation shall be in accordance with manufacturer’s
published recommendations.
C. Conduit Connections: All conduit directly connected to transformer enclosures shall be flexible steel
conduit extending for a minimum of 24 inches from transformer enclosure as measured along the

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conduit center line. Run a bonding jumper, sized per NEC Table 250-95, on outside of flexible
conduit.
D. Cable Connections: Make transformer cable connections with compression-type lugs suitable for
termination of 75 degrees C rated conductors. Position lugs so that field connections and wiring will
not be exposed to temperature above 75 degrees C.
E. Housekeeping Pads: All floor standing transformers shall be provided with a nominal reinforced
concrete housekeeping pad. Refer to Section 26 0501 "Electrical Basic Materials and Methods", for
additional requirements.
F. Transformer Isolation: Install floor-mounted transformers on concrete housekeeping pads with
vibration isolating pads suitable for isolating the transformer noise from the building structure in
accordance with Section 26 0501 "Electrical Basic Materials and Methods”. Maintain a minimum of
six (6) inches free air space between enclosure and walls.
1. Install transformer plumb and level.
2. For floor and trapeze transformer installations, use one pad type Korfund Elasto-Grip, waffle at
each corner of the transformer, sized for load of 50 pounds per square inch.
3. For wall hung transformer installations (15kVA and less), use spring type Korfund Series P.
Provide sound pads at each corner of the transformer sized for ½ inch deflection. Securely
anchor wall-mounting brackets to wall to provide adequate support.
G. Suspended Transformers: Suspend transformers (45kVA and less) from structure by means of
trapeze hangers constructed of ½ inch galvanized all-thread rods and metal framing channels. Make
double-nut connections between rods and channels. Locate transformers to provide adequate
ventilation and accessibility.
H. Identification: Refer to Section 26 0553 “Identifications for Electrical Systems” for transformer
identification.
3.2 TESTING
A. Pre-Energization Check: Check for damage and tighten connections prior to energizing transformer.
Verify removal of coil shipping anchor bolts before transformer is energized.
B. Insulation Tests: Prior to energization, check transformer windings for continuity. Test the insulation
resistance from primary phase winding to winding, primary winding to secondary winding, secondary
phase winding to winding, and from each winding to ground. Tests shall be made with a Biddle
Megger or equivalent test instrument at a voltage of not less than 1000 volts DC with readings taken
after 30 and 60 seconds of operation at Megger slip speed. Transformers which do not meet or
exceed manufacturers winding insulation resistance specifications or NETA ATS requirements shall be
replaced and the new transformer shall be tested, until an acceptable resistance is obtained.
C. Winding Current: During initial no-load energization, check current in each primary winding.
D. Tap Settings: Measure and record load current and voltage of transformers while loaded to verify
proper transformer tap setting. Select the appropriate tap setting on transformer so that the actual
secondary voltage is + 1/2 of a tap span at full load.
E. Test Submittals: Contractor shall furnish all instruments and personnel required for tests. Submit
four copies of certified test results to [Engineer] [Architect] for review. Reports shall include
transformer tested, date and time of tests, serial number, tap setting, input and output voltages,
primary and secondary winding currents, insulation test results, manufacturers winding insulation
resistance specifications, relative humidity, temperature, and weather conditions.

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F. Notification: Notify [Engineer] [Architect] in writing of any deviation from manufacturer's pre-
shipment test data.
END OF SECTION 26 2200

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