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Memorial Hermann Best Practices
July, 2013
SECTION 26 22 13 LOW VOLTAGE DISTRIBUTION TRANSFORMERS
PART 1 - GENERAL
1.01
WORK INCLUDED
A.
Comply with the provisions of Sections 26 05 00.
B.
Provide dry type transformers as shown on drawings.
1.02
RELATED SECTIONS
A.
Section 26 05 26: Grounding and Bonding for Electrical Systems
B.
Section 26 05 53: Identification for Electrical Systems.
1.03
REFERENCES
A.
ANSI/IEEE C57.96 - Distribution and Power Transformers, Guide for Loading Dry-Type (appendix
to ANSI C57.12 standards)
B.
ANSI/IEEE C89.2 - Dry Type Transformers for General Applications
C.
ANSI/NFPA 70 - National Electrical Code
D.
IEEE C57.12.01 - General Requirements for Dry-Type Distribution and Power Transformers
Including Those with Solid Cast and / or Resin-Encapsulated Windings
E.
IEEE C57.12.91 - Test Code for Dry-Type Distribution and Power Transformers
F.
NEMA ST 20 - Dry Type Transformers for General Applications
G.
UL 506 - Specialty Transformers
1.04
SUBMITTALS
A.
Submit products for review.
B.
Submit dimensioned equipment location layouts in accordance with section 26 05 00.
1.05
A.
WARRANTY
Manufacturer warrants equipment to be free from defects in materials and workmanship for
greater of 1 year from date of installation or 18 months from date of delivery.
PART 2 - PRODUCTS
2.01
ACCEPTABLE MANUFACTURERS
A.
Square D
B.
Eaton/Cutler-Hammer
C.
General Electric
D.
Siemens
LOW VOLTAGE DISTRIBUTION TRANSFORMERS
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Memorial Hermann Best Practices
July, 2013
2.02
EQUIPMENT REQUIREMENTS
A.
Provide general purpose, self cooled dry type transformers designed for 60 Hz operation as
shown on drawings. Transformers to be designed, manufactured and tested in accordance with
the latest ANSI, NEMA and IEEE Standards. Transformers to be listed and labeled in
accordance with UL 1561.
B.
Transformers to be compliant with NEMA TP-1 energy efficiency requirements per EPAct
2005/CFR 10 Part 431.
2.03
INSULATION SYSTEM
A.
Provide ventilated type transformers for 15 KVA and larger with a UL recognized 220 degree
insulation system. Base the KVA ratings on an allowable 80, 115 or 150 degree C winding
temperature rise above a 30 degree hot spot.
B.
Transformer design KVA Rating shall be suitable for a 30 degree C average and 40 degree C
maximum ambient temperature.
2.04
CORE AND COIL
A.
Core construction to be of non-aging electrical grade grain-oriented silicon steel to minimize
hysteresis and eddy current losses. Core lamination shall be tightly assembled.
B.
Provide windings wound of high quality copper.
C.
Arrange ventilated windings to brace coil layers and provide maximum ventilation. Construct core
and coil assemblies to provide short circuit withstand capability as defined by ANSI and NEMA
standards. Securely bolt complete assembly to the enclosure base using vibration dampening
pads to reduce noise.
D.
The complete core and coil shall be impregnated with non-hygroscopic, thermo-setting polyester
varnish to provide a high dielectric and flame retardant seal. The shield of varnish to the coils
shall effectively impregnate the entire core and coil assembly that results in a unit which is
virtually impermeable to moisture, dust, salt air and other contaminants. Cast encapsulated core
and windings in a resin compound to provide a moisture-proof, shock-resistant, high dielectric
seal.
E.
Provide full capacity taps in the high-voltage windings of transformers. For 3 phase transformers
rated below 15 KVA, provide two 5% full capacity below normal taps . For 3 phase transformers
rated 15 KVA and above, provide two 2.5% full capacity above normal taps and four 2.5% full
capacity below normal taps
F.
Where called for on drawings, provide shielded isolation transformers with full width electrostatic
shields with a maximum primary to secondary coupling capacitance of 33 picofarads. Provide
shielded isolation transformers with attenuation that equals or exceeds:
1.
Common mode: 0-1.5kHz: 120dB
2.
Common mode: 1.5k-10kHz: 90dB
3.
Common mode: 10k-100kHz: 65dB
4.
Common mode: 100k-1MHz: 40dB
5.
Transverse mode: 1.5k-10kHz: 52dB
6.
Transverse mode: 10k-100kHz: 30dB
7.
Transverse mode: 100k-1MHz: 30dB
LOW VOLTAGE DISTRIBUTION TRANSFORMERS
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Memorial Hermann Best Practices
July, 2013
G.
K-rated, non-linear load transformers constructed of insulating materials exceeding NEMA ST20
standards and shall comply with 220 degree C., UL component recognized insulation systems
requirements.
H.
K-rated transformers shall be capable of carrying full rated, non-sinusoidal load without either
primary or secondary coils exceeding temperatures of 200 degrees C at any spot.
I.
K-rated transformers shall be listed for 155 degree C, 80 degree C temperature rise.
J.
K-rated transformers shall have an impedance of 3 to 5 percent and a minimum reactance of 2
percent to reduce third harmonic current effects on the neutral.
K.
K-rated transformers shall have six, 5% full capacity primary taps and two, 5% full capacity
secondary taps.
L.
K-rated transformers shall be supplied with electrostatic shields with maximum primary to
secondary coupling capacitance of 33 picofarads and attenuation ratings equal to or exceeding
the ratings shown for shielded transformers.
M.
Provide transformers with a minimum K-factor rating of 4, unless noted otherwise.
N.
Transformer impedance for transformers rated below 112.5 kVA shall limit available secondary
fault current to less than 10,000 amperes.
2.05
ENCLOSURES
A.
Provide NEMA 1 ventilated enclosures of heavy gauge steel construction for indoor transformers.
Provide weather shields for ventilated transformers installed outdoors conforming to the
requirements of NEMA 250, Type 3R. Finish enclosures in ANSI 61 gray paint.
B.
Unless otherwise specified, sound levels shall be in accordance with values allowed by NEMA
ST-20.
PART 3 - EXECUTION
3.01
INSTALLATION REQUIREMENTS
A.
Provide flexible conduit to and from transformer.
B.
Install transformers in accordance with manufacturer's recommendations.
C.
Provide vibration isolation devices whether floor mounted or hung from structure as follows:
1.
Install floor mounted transformers on two (2) layer, one (1) inch thick, rubber neoprene
pads with galvanized shims (Mason Super "W" or equal), mounted on a 4" concrete
housekeeping pad.
2.
For suspended transformers provide spring hangers (Mason 30N or equal) with double
neoprene elements at the top of the device with spring seated neoprene cups at the base
of the device for sway bracing purposes.
D.
Provide working clearances around transformers in accordance with Article 110 of the National
Electrical Code.
E.
Provide separation and clearance around transformers in accordance with manufacturer's
recommendation to allow adequate cooling.
F.
Ground transformers in accordance with Articles 250 and 450 of the National Electrical Code.
LOW VOLTAGE DISTRIBUTION TRANSFORMERS
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Memorial Hermann Best Practices
July, 2013
G.
3.02
Mount floor mounted transformers on one inch (1") thick rubber pad.
FIELD TESTING
A.
Torque all connections to manufacturer's recommendation.
B.
Check the insulation resistance of primary and secondary windings with a 1000 vdc "megger".
Minimum acceptable value is 3 megohms.
C.
Clean all construction debris, trash, and dust from enclosure, windings, and area around
transformer. Use industrial type vacuum cleaner to remove dust from windings.
D.
Clean or replace all filters associated with cooling air intake.
END OF SECTION
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