1 Scope
NOTE This specification covers dry-type and liquid-immersed distribution transformers used to step utility or premises medium-voltage service down to a building's utilization voltage, and to step the secondary 480V distribution down to 208Y/120V for receptacle and lighting branch circuits. (1.1)
NOTE Dry-type transformers under this standard are general-purpose, ventilated or sealed, single- or three-phase units complying with UL 1561 and IEEE C57.12.01. (1.2)
NOTE Liquid-immersed transformers under this standard are pad-mounted, compartmental, self-cooled, three-phase distribution transformers complying with IEEE C57.12.34 and IEEE C57.12.00. (1.3)
NOTE A transformer ties together the upstream utility or service entrance, the downstream distribution gear, the room or pad it sits on, and the grounding system. (1.4)
1.4.2 Pad-mounted units shall be coordinated with the concrete pad scope under Concrete Pads. 1.4.3 The transformer secondary is a separately derived system; the system bonding jumper, the grounding electrode conductor, and the secondary grounding electrode shall be installed in accordance with NEC 250.30 and the project's grounding standard.
NOTE This separately derived system requirement applies to every dry-type transformer that converts 480V three-phase to a 208Y/120V wye, which is the most common transformer application in this scope, and is the single most frequently missed installation requirement in the field. (1.4.4)
2 Referenced Standards
2.1 Equipment and installation shall comply with the latest adopted edition of the following standards.
2.1.1 Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
2.2 Standards Table
| Standard |
Title |
| NFPA 70 |
National Electrical Code (Article 450 — Transformers and Transformer Vaults) |
| IEEE C57.12.00 |
Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers |
| IEEE C57.12.01 |
Standard for General Requirements for Dry-Type Distribution and Power Transformers |
| IEEE C57.12.10 |
Standard Requirements for Liquid-Immersed Power Transformers |
| IEEE C57.12.34 |
Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers, 10 MVA and Smaller |
| IEEE C57.12.36 |
Standard Requirements for Liquid-Immersed Distribution Substation Transformers |
| IEEE C57.12.51 |
Standard for Ventilated Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase |
| IEEE C57.12.55 |
Standard for Dry-Type Transformers Used in Unit Installations, Including Unit Substations |
| IEEE C57.12.91 |
Standard Test Code for Dry-Type Distribution and Power Transformers |
| IEEE C57.110 |
Recommended Practice for Establishing Liquid-Immersed and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents |
| IEEE C57.116 |
Guide for Transformers Directly Connected to Generators |
| UL 1561 |
Standard for Dry-Type General Purpose and Power Transformers |
| UL 1562 |
Standard for Transformers, Distribution, Dry-Type, Over 600 Volts |
| 10 CFR Part 431 Subpart K |
DOE Energy Conservation Standards for Distribution Transformers |
| NEMA TP-2 |
Standard Test Method for Measuring the Energy Consumption of Distribution Transformers |
| ASTM D3487 |
Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus |
| ASTM D6871 |
Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus |
| IEC 61039 |
Classification of Insulating Liquids (K-class less-flammable designation) |
| NETA ATS |
Acceptance Testing Specifications for Electrical Power Equipment and Systems |
| ASCE 7 |
Minimum Design Loads and Associated Criteria for Buildings (seismic) |
3 Submittals
3.1 Action Submittals
3.1.1 Contractor shall submit the following for the Engineer's review prior to fabrication:
- Shop drawings showing overall dimensions, mounting and lifting provisions, primary and secondary termination locations, and required clearances
- Nameplate data including kVA rating, primary and secondary voltage, vector group, BIL, impedance, temperature class and rise, sound level, and weight
- Insulation class certification and verification of compliance with the DOE 10 CFR Part 431 efficiency table applicable to the unit category
- Performance data including no-load loss, load loss at rated load and at 50% of rated load, and per-unit impedance at the rated and nominal tap
- For liquid-immersed units: bushing, switch, and accessory arrangement drawings and the proposed insulating liquid type with safety data sheet
- Seismic certification documentation where required by the building code
☑ Shop drawings with dimensions and clearances
☐ Nameplate data sheet
☐ DOE efficiency compliance certificate
☐ No-load and load loss performance data
☐ Insulation class and temperature-rise certification
☐ Sound level certification
☐ Insulating liquid SDS (liquid-immersed only)
☐ Bushing and accessory arrangement (pad-mounted only)
☐ Seismic certification (where required)
3.1.2 Procurement shall not proceed until submittals are reviewed and returned.
3.2 Closeout Submittals
3.2.1 Contractor shall provide the following at substantial completion:
- Certified factory test reports for each transformer
- Field test reports for installed transformers including insulation resistance, winding resistance, and TTR readings
- Operation and maintenance manuals including recommended inspection intervals and torque values for accessible bus and cable connections
- For liquid-immersed units: initial dissolved gas analysis sample report and the post-energization liquid sample if specified
- Warranty documentation
☑ Certified factory test reports for each transformer
☐ Field test reports (insulation resistance, winding resistance, TTR)
☑ Operation and maintenance manuals with inspection intervals and torque values
☐ Initial dissolved gas analysis sample report (liquid-immersed only)
☐ Post-energization liquid sample report (liquid-immersed, where specified)
☑ Warranty documentation
4 Quality Assurance
4.1 Manufacturer Qualifications
4.1.1 Transformers shall be manufactured by a company regularly engaged in the production of distribution transformers complying with the referenced IEEE and UL standards, with a minimum of five years of documented production experience for the transformer type specified.
4.1.2 The manufacturer shall maintain an ISO 9001 certified quality management system.
4.2 Listing and Labeling
4.2.1 Every transformer shall be listed and labeled by a Nationally Recognized Testing Laboratory to the applicable UL standard for the construction type (UL 1561 for dry-type general purpose and power transformers 600V and below, UL 1562 for dry-type distribution transformers over 600V) or to the manufacturer's standard scope for liquid-immersed pad-mounted units complying with IEEE C57.12.34.
4.3 Efficiency Compliance
● DOE 10 CFR Part 431 minimum (current effective date)
○ NEMA Premium Efficiency (TP-1 level, where higher than DOE minimum)
Per drawings
4.3.1 Every transformer manufactured for installation in the United States shall comply with the applicable energy conservation standard of 10 CFR Part 431 Subpart K based on its category (low-voltage dry-type, medium-voltage dry-type, or liquid-immersed).
4.3.2 Efficiency shall be established at 50% of nameplate-rated load using the test method of NEMA TP-2.
4.3.3 The manufacturer shall provide a written certification of efficiency compliance with each unit.
NOTE NEMA Premium efficiency transformers exceed the DOE minimum at the cost of larger physical size and higher first cost; the energy savings over the life of the transformer typically justify the premium for units operating at moderate to high load factor. (4.3.4)
4.3.5 The Engineer should select NEMA Premium for transformers with load factors above 35%.
5 Environmental and Service Conditions
5.1 Transformers shall be suitable for continuous operation under usual service conditions as defined in IEEE C57.12.00 (liquid-immersed) and IEEE C57.12.01 (dry-type).
5.2 Where the installation involves unusual service conditions — elevated altitude, ambient extremes, contaminated or corrosive atmospheres, harmonic-rich loads, or unusual loading cycles — the manufacturer shall be notified at the time of order and the unit shall be derated or modified as appropriate.
5.3 Ambient Temperature
30°C average / 40°C maximum (standard)
40°C average / 50°C maximum
Outdoor pad-mounted, full sun exposure
NOTE The standard rating reference of IEEE C57.12.00 and C57.12.01 is a 24-hour average ambient of 30°C with a maximum of 40°C. (5.3.1)
5.3.2 Units installed in unventilated rooms with no mechanical cooling shall be derated for the actual room ambient.
5.3.3 The Contractor shall confirm the room temperature rise from heat-producing equipment with the mechanical contractor and report any condition expected to exceed the standard ambient.
5.4 Installation Altitude
Below 3,300 ft (1,000 m) - no derating
3,300 - 6,600 ft (1,000 - 2,000 m)
Above 6,600 ft (2,000 m) - consult manufacturer
NOTE Insulation dielectric strength and convective cooling both decrease with altitude; per IEEE C57.12.00 and C57.12.01, derating begins at 1,000 m elevation. (5.4.1)
5.4.2 Units at altitudes above 1,000 m shall be derated for both BIL and kVA capacity, with the specific derating factors taken from the applicable IEEE table.
5.5 Indoor / Outdoor Installation
● Indoor, conditioned electrical room
○ Indoor, unconditioned space
○ Outdoor
5.5.1 Indoor dry-type transformers shall be located in a dedicated electrical room or in a space meeting the clearance and ventilation requirements of NEC Article 450 Part II.
5.5.2 Outdoor dry-type units shall have a sealed (NEMA 3R) enclosure.
5.6 Harmonic Loading
Linear loads only (lighting, motors)
Moderate nonlinear (≤ 30% of load, mixed office)
Heavy nonlinear (> 30% of load, data center / VFD)
5.6.1 Where the connected load is rich in harmonic currents — switched-mode power supplies, variable frequency drives, LED drivers in quantity, and similar electronic loads — the transformer shall be specified with a K-factor rating appropriate to the load per IEEE C57.110, or with an oversized neutral and increased capacity to accommodate the same harmonic content.
NOTE A K-factor rated transformer carries an electrostatic shield, oversized neutral, transposed conductors or other construction features that limit harmonic-induced heating; using a standard transformer on a heavily nonlinear load consumes the transformer's nameplate capacity in winding and stray losses without delivering the same kVA to the load. (5.6.2)
6 Type and Configuration
● Dry-type, ventilated (indoor, general purpose)
○ Dry-type, sealed / non-ventilated (indoor or outdoor)
○ Dry-type, cast resin (indoor or outdoor, severe environment)
○ Liquid-immersed, pad-mounted (outdoor)
NOTE Ventilated dry-type transformers are the default for indoor commercial and institutional applications below 1000 kVA; sealed dry-type units are appropriate for outdoor or dirty environments where opening the core and coil to the ambient atmosphere is unacceptable. (6.1)
NOTE Cast resin transformers provide the highest tolerance for contaminated, humid, or corrosive environments and the highest short-circuit withstand at additional first cost; they are commonly specified for industrial, mining, and offshore service. (6.2)
NOTE Liquid-immersed pad-mounted transformers are the standard form for medium-voltage utility-style service entrances to a customer building. (6.3)
6.4 Phase
● Three-phase
○ Single-phase
6.5 kVA Rating
15 kVA
30 kVA
45 kVA
75 kVA
112.5 kVA
150 kVA
225 kVA
300 kVA
500 kVA
750 kVA
1000 kVA
1500 kVA
2000 kVA
2500 kVA
Per drawings
NOTE Standard ratings above follow the NEMA preferred rating series; manufacturers may offer additional intermediate ratings. (6.5.2)
6.5.3 The selected rating shall account for present load with allowance for future growth and the connected load's harmonic content.
6.6 Voltage and Vector Group
480V three-phase, three-wire
480V three-phase, four-wire
208V three-phase
12.47 kV three-phase, delta or grounded-wye
13.8 kV three-phase, delta or grounded-wye
23 kV three-phase, grounded-wye
34.5 kV three-phase, grounded-wye
Per drawings
208Y/120V three-phase, four-wire
480Y/277V three-phase, four-wire
240/120V three-phase, four-wire (delta)
240/120V single-phase, three-wire
Per drawings
● Dyn1 (delta primary, wye secondary with neutral, 30° lag)
○ Dyn11 (delta primary, wye secondary with neutral, 30° lead)
○ Yyn0 (wye-wye with neutral, no phase shift)
○ Dd0 (delta-delta, no phase shift)
NOTE For 480V to 208Y/120V step-down transformers, Dyn1 is the established US-market default; the delta primary provides a path for triplen harmonic circulating currents and isolates the secondary neutral from the primary system, while the wye secondary with neutral provides a four-wire system for branch circuit loads. (6.6.1)
6.6.2 The vector group selection shall match any parallel transformer banks; mismatched vector groups cannot be paralleled.
6.7 Basic Insulation Level (BIL)
10 kV BIL (600V class)
30 kV BIL (5 kV class)
60 kV BIL (8.7 kV class)
95 kV BIL (15 kV class)
125 kV BIL (25 kV class)
150 kV BIL (34.5 kV class)
6.7.1 The BIL of each winding shall be coordinated with the applicable system voltage class and the upstream protective device.
6.7.2 For medium-voltage primary windings, the BIL shall be selected per IEEE C57.12.00 Table 4 for the system voltage and the grounding method, and shall be coordinated with the protective margin of any surge arresters provided.
6.8 Tap Changer
Two 2.5% above and two 2.5% below nominal (±2 × 2.5%)
Four 2.5% above and four 2.5% below nominal (±4 × 2.5%)
Two 5% below nominal only
No taps
6.8.1 Transformers shall be furnished with full-capacity taps to adjust the turns ratio for variations in supply voltage.
6.8.2 Taps shall be de-energized only; load tap changers are not within the scope of this standard.
NOTE The ±2 × 2.5% arrangement (giving a total tap range of −5% to +5% in 2.5% steps) is the established default for general-purpose distribution transformers; wider tap ranges are appropriate where the upstream supply voltage is known to vary substantially or where the transformer feeds long secondary feeders. (6.8.3)
6.8.4 Tap changers shall be operable only with the transformer de-energized; an interlock or warning label shall be provided to enforce this.
6.9 Impedance
27.5
23455.7567
Default: 5.75 %
Per drawings
6.9.1 The impedance voltage in percent shall be selected to limit available secondary short-circuit current to a value compatible with the downstream secondary distribution equipment's interrupting rating, while not being so high as to produce excessive secondary voltage drop or regulation.
NOTE A 5.75% impedance is a typical default for 300–1000 kVA dry-type units stepping 480V down to 208Y/120V. (6.9.2)
6.9.3 Where the transformer must be paralleled with another transformer on a common bus, the impedance of paralleled units shall be within ±7.5% of each other and the vector groups shall match.
7.1 Core and Coil
7.1.1 Core laminations shall be of grain-oriented, low-loss electrical steel.
7.1.2 Cores shall be clamped, mitered, and step-lap stacked to minimize no-load (core) losses and audible noise.
7.1.3 Coils shall be wound from copper or aluminum conductor, vacuum impregnated or encapsulated as appropriate for the insulation class.
NOTE Copper windings have higher conductivity per cross-section, produce a smaller and lighter transformer of equivalent rating, and provide better long-term joint reliability at terminations; aluminum windings carry a lower first cost but require larger conductor cross-section, larger overall size, and careful joint design. (7.1.4)
NOTE Copper is the default for installations where the transformer will be loaded continuously near nameplate rating or where size is constrained. (7.1.5)
7.2 Insulation Class and Temperature Rise
105°C (Class A) — discontinued for distribution sizes
150°C (Class B)
180°C (Class F)
220°C (Class H / R)
80°C rise
115°C rise
150°C rise
NOTE Insulation class refers to the maximum temperature the insulation system can withstand continuously without unacceptable thermal degradation; temperature rise is the temperature increase of the winding above the ambient under rated load. (7.2.1)
NOTE A 220°C insulation system with a 115°C rise leaves approximately 65°C of thermal headroom for overload, hot-spot allowance, and end-of-life margin, which is the established commercial standard for ventilated dry-type transformers. (7.2.2)
7.2.3 A 220°C system with an 80°C rise is recommended where the transformer may be subject to harmonic loading or to short-term overloads; the lower rise extends life and provides margin for overload duty.
7.3 K-Factor Rating
K-1 (standard, linear load)
K-4 (light nonlinear load)
K-13 (heavy nonlinear load)
K-20 (very heavy nonlinear load, data center)
7.3.1 K-factor selection shall be coordinated with the connected load harmonic spectrum per IEEE C57.110.
NOTE K-4 is generally adequate for mixed commercial loads with a moderate proportion of electronic equipment; K-13 is appropriate for floors heavily loaded with computers and electronic equipment; K-20 is appropriate for data centers and other installations where nonlinear loads dominate. (7.3.2)
NOTE The K-factor transformer is internally constructed to handle the resulting eddy-current and stray losses at full nameplate kVA without derating; a standard (K-1) transformer applied to a nonlinear load must be derated, defeating the purpose of nameplate selection. (7.3.3)
7.4 Sound Level
4075
45505560646770
Default: 60 dB
7.4.1 Sound level shall be measured per IEEE C57.12.91.
NOTE The default values reflect NEMA standard sound levels for the kVA range. (7.4.2)
7.4.3 Where the transformer is installed near occupied space — adjacent to a classroom, an office, or a residential dwelling unit — the Engineer should specify a sound level 3 to 5 dB below the NEMA standard, which generally requires a larger, more lightly loaded core.
7.4.4 Acoustic isolation pads or vibration isolators shall be specified where structure-borne transmission is a concern.
7.5 Enclosure
NEMA 1 (indoor, general purpose, ventilated)
NEMA 2 (indoor, drip-proof)
NEMA 3R (outdoor, rain-tight, ventilated)
NEMA 4 (indoor/outdoor, watertight)
NEMA 4X (indoor/outdoor, watertight, corrosion-resistant stainless)
7.5.1 Enclosure ventilation openings shall be sized to support the transformer's natural-convection cooling and shall be screened against the entry of vermin and large debris.
NOTE NEMA 1 ventilated enclosures are the indoor commercial default; NEMA 3R is required for outdoor installation. (7.5.2)
7.5.3 NEMA 4X stainless construction should be specified where the transformer is exposed to salt spray, washdown, or aggressive industrial atmospheres.
7.6 Mounting
Floor-mounted
Wall-mounted (≤ 75 kVA)
Trapeze / hung from structure (≤ 75 kVA)
NOTE Floor mounting is the default for all dry-type transformers above 75 kVA and is preferred for all sizes where floor space is available. (7.6.1)
7.6.2 Wall-mounting and hung-mounting shall be limited to smaller units (typically 75 kVA and below) and shall be supported by structural attachment evaluated for the transformer's weight plus an allowance for seismic and vibration loading.
7.6.3 The Contractor shall verify that the supporting wall or structure can carry the transformer load before mounting.
7.7 Connections and Terminations
○ Compression lugs (factory-installed)
● Mechanical lugs (UL listed for conductor material and size)
○ Bus connection to adjacent equipment
7.7.1 Primary and secondary connection compartments shall be separate from the core and coil compartment and accessible without exposing personnel to live core and coil parts.
7.7.2 Termination provisions shall accommodate the conductor materials and sizes shown on the contract drawings and shall be sized for the maximum overcurrent device rating protecting the transformer windings.
8.1 Liquid-immersed pad-mounted units shall be constructed per IEEE C57.12.34, providing a tamper-resistant, compartmental enclosure suitable for unsupervised public installation on a customer-owned concrete pad.
8.2 Tank Construction
8.2.1 The transformer tank shall be of welded steel construction, leak-tested at the factory at a positive internal pressure for a duration sufficient to detect leaks at all welds, gasketed joints, and bushing penetrations.
8.2.2 Tank wall and cover thicknesses shall be sufficient to withstand the full vacuum required for processing of the dielectric liquid without permanent deformation.
8.3 Enclosure and Cabinet
● Manufacturer's standard green (Munsell 7GY3.29/1.5)
○ Manufacturer's standard gray (ANSI 70)
Per drawings — architectural drawings
8.3.1 The transformer cabinet shall comply with IEEE C57.12.34 for tamper resistance, with a hood, sill, and removable doors providing access to the high- and low-voltage compartments.
8.3.2 The high-voltage compartment shall be accessible only after the low-voltage compartment has been opened — a "dead-front" arrangement that prevents accidental contact with energized primary terminations.
8.3.3 A pentahead bolt and padlocking provisions shall be furnished per IEEE C57.12.34.
NOTE The Munsell green specified in IEEE C57.12.34 is the utility-industry standard color and is the appropriate default for any installation visible from the public way. (8.3.4)
8.4 Bushings and Terminations
○ Live-front (porcelain bushings, exposed terminals)
● Dead-front (universal bushing wells with 200A elbow connectors)
○ Dead-front (600A bushing wells with apparatus connectors)
NOTE Dead-front construction is the standard for customer-owned pad-mounted transformers because it removes exposed primary potentials from the compartment when the elbow connectors are mated, eliminating the requirement for utility-grade clearances within the cabinet. (8.4.1)
NOTE Live-front construction is used where the primary cable is a permanent overhead-to-underground transition or where the local utility requires it. (8.4.2)
8.5 Primary Switching
No primary switch (separate primary disconnect upstream)
Two-position, loadbreak (radial feed)
Four-position, loadbreak (loop feed with sectionalizing)
NOTE A loadbreak primary switch within the transformer cabinet allows the transformer to be isolated from the primary feeder without an additional pad-mounted switching device. (8.5.1)
NOTE For loop-feed configurations where two primary cables enter the transformer and the load is fed from either source, a four-position switch provides the ability to feed from either source and to sectionalize the loop. (8.5.2)
8.6 Liquid Type
○ Mineral oil (ASTM D3487)
● Natural ester (FR3 / vegetable oil, ASTM D6871) — less-flammable K-class
○ Silicone fluid — less-flammable K-class
NOTE Mineral oil is the historical baseline and remains the lowest-cost insulating liquid; it has a fire point of approximately 165°C and is classified as flammable. (8.6.1)
NOTE Natural ester fluids per ASTM D6871 have a fire point of approximately 360°C, qualifying as K-class less-flammable per IEC 61039, and are biodegradable; they are increasingly the default for new pad-mounted installations because they reduce the NEC 450.27 outdoor clearance requirements and the spill-containment burden. (8.6.2)
NOTE Silicone fluids are K-class and chemically very stable but are non-biodegradable and substantially more expensive than esters. (8.6.3)
8.6.4 Less-flammable liquids shall be used where the transformer is installed within the clearance distances of combustible building walls per NEC 450.23.
8.7 Liquid Preservation System
○ Sealed tank (no gas space access, pressure cycling within design)
● Sealed tank with pressure-vacuum bleeder
○ Conservator with bladder or diaphragm
NOTE Pad-mounted distribution transformers are sealed-tank construction. (8.7.1)
NOTE The pressure-vacuum bleeder allows the gas space above the liquid to vent slowly to atmosphere when internal pressure exceeds or falls below the operating range, while maintaining a positive seal under normal conditions to keep oxygen and moisture out of the liquid. (8.7.2)
9 Accessories
☑ Nameplate (stainless or anodized aluminum, engraved)
☐ Lifting eyes / lifting lugs
☐ Jacking provisions
☐ Grounding pad (NEMA two-hole, on tank/enclosure)
☐ Drain valve with sampler (liquid-immersed)
☐ Filter / fill connection (liquid-immersed)
☐ Pressure relief device (liquid-immersed)
☐ Liquid level gauge (liquid-immersed)
☐ Liquid temperature gauge (liquid-immersed)
☐ Winding temperature monitor (dry-type ≥ 750 kVA recommended)
☐ Forced-air cooling fans (dry-type, future addable)
☐ Surge arresters (primary, MOV elbow or distribution class)
☐ Bayonet fuses (current-limiting, in series, for liquid-immersed)
☐ Dial-type thermometer with alarm contact
9.1 Standard accessories shall be furnished as required by IEEE C57.12.00 / C57.12.34 (liquid-immersed) and C57.12.01 (dry-type).
9.2 Additional accessories below shall be specified where indicated.
9.3 Surge Arresters
○ Not required (transformer downstream of MV switchgear with arresters)
○ Distribution-class metal-oxide arresters on each primary phase
● Elbow arresters integral to dead-front bushing wells (pad-mounted)
9.3.1 Pad-mounted transformers fed by underground primary cable from an overhead system shall be protected by primary surge arresters because the cable's surge impedance does not adequately attenuate switching and lightning transients arriving from the overhead.
NOTE Elbow-style arresters integrated into the dead-front bushing wells are the cleanest installation. (9.3.2)
NOTE For transformers fed entirely from underground gear with arresters at the source, a separate arrester at the transformer is not strictly required, but is good practice for medium-voltage systems exposed to switching transients. (9.3.3)
9.4 Fusing
Bayonet expulsion fuse only
Bayonet expulsion fuse in series with current-limiting fuse (full range)
Bayonet expulsion fuse in series with current-limiting fuse (back-up)
External primary protection only (no internal fuses)
NOTE Bayonet-style expulsion fuses are removable from the front of the transformer compartment using a hot-stick or shotgun stick, and provide overload protection. (9.4.1)
NOTE A current-limiting fuse in series provides interruption of internal faults at fault levels beyond the expulsion fuse's capability and limits energy let-through. (9.4.2)
NOTE The combination of an expulsion fuse for overload duty and a current-limiting fuse for high-magnitude fault duty is the industry-standard internal primary protection for pad-mounted units. (9.4.3)
10 Testing
10.1 Factory Production Tests
10.1.1 Every transformer shall receive the production tests required by the applicable IEEE standard (C57.12.91 for dry-type, C57.12.90 for liquid-immersed).
10.1.2 At minimum, factory production tests shall include:
- Ratio test on all taps
- Polarity and phase relation
- No-load loss and excitation current
- Load loss and impedance voltage at rated current
- Applied potential (hi-pot) test
- Induced potential test
- Insulation resistance / power factor
- Leak test (liquid-immersed)
- Resistance of windings
10.1.3 Certified test reports for each transformer shall be furnished as part of the closeout submittals, identifying the unit by serial number and including all measured values.
10.2 Factory Witnessed / Acceptance Tests
○ Witnessed by Owner's representative
● Unwitnessed, certified test report only
○ Not required beyond production tests
10.2.1 Witnessed factory testing should be specified for transformers above 1500 kVA, for any unit in a critical service application, or where the manufacturer is unfamiliar to the Engineer.
10.2.2 Where witnessed testing is specified, the manufacturer shall provide a minimum of two weeks advance notice of test readiness and shall submit the test procedure for review prior to testing.
10.3 Field Acceptance Tests
☑ Visual and mechanical inspection
☐ Insulation resistance, winding-to-winding and winding-to-ground (Megger)
☐ Turns ratio test (TTR) on all taps
☐ Winding resistance (DC) on all taps
☐ Insulation power factor / dissipation factor
☐ Polarity and phase relation verification
☐ Excitation current at applied test voltage
☐ Dielectric liquid sample analysis (liquid-immersed only)
☐ Pressure / vacuum decay test (liquid-immersed only)
☐ Grounding verification (tank, neutral, secondary)
☐ Operational test of all alarms and trip contacts
10.3.1 Field acceptance testing shall be performed by a qualified independent testing firm in accordance with NETA ATS Section 7.2.
10.3.2 Field acceptance tests shall be performed after installation is complete and before the transformer is energized for service.
10.3.3 Insulation resistance shall be measured at the voltage specified in NETA ATS for the winding voltage class, and shall meet or exceed the acceptance criteria in NETA ATS.
10.3.4 Where polarization index is measured, the ratio of the 10-minute reading to the 1-minute reading shall be 2.0 or higher for healthy insulation.
10.3.5 Liquid samples drawn from liquid-immersed units shall be analyzed for moisture (ASTM D1533), dielectric breakdown (ASTM D877 or D1816), interfacial tension, acidity, color, and visual condition.
10.3.6 An initial dissolved gas analysis sample should be drawn for baseline establishment regardless of unit size; on units 1000 kVA and larger, DGA is required.
10.4 Initial In-Service Inspection
10.4.1 Within 30 days of energization, the Contractor shall perform an infrared thermographic scan of the transformer and its primary and secondary connections under a load of at least 40% of nameplate rating, and shall report and correct any connection exceeding a 10°C rise above ambient.
11 Installation
11.1 Working Clearance
Per NEC 450.27 (mineral oil) — typically 25 ft to vertical surface
Per manufacturer's listing for less-flammable liquid (typically 3 ft)
Wall is non-combustible (no clearance required by 450.27)
11.1.1 Working space around the transformer shall comply with NEC 110.26 for low-voltage equipment and NEC 110.34 for medium-voltage equipment.
11.1.2 The required working space dimensions shall be coordinated with the room layout before pad or anchor work begins.
11.1.3 Pad-mounted transformers shall also have clearance from combustible surfaces per NEC 450.23 unless the unit is less-flammable-liquid filled and the manufacturer's listing reduces the clearance.
NOTE NEC 450.27 governs outdoor liquid-insulated transformer installations and requires substantial separation from combustible building walls for mineral-oil-filled units. (11.1.4)
NOTE A less-flammable K-class liquid (natural ester or silicone) per NEC 450.23, with the unit listed for indoor or close-to-building installation, allows much shorter separations and is one of the principal reasons for selecting an ester fluid on a constrained urban site. (11.1.5)
11.2 Pad Mounting (Liquid-Immersed)
11.2.1 The transformer shall be installed on a concrete pad sized and reinforced for the transformer footprint, the dead weight of the transformer including its liquid, and the seismic shear and uplift loads applicable to the project.
11.2.2 The pad shall include a sealed cable opening or stub-up arrangement sized to accommodate the primary and secondary cables and grounding conductors, with a vermin-resistant seal at the cable entry.
11.2.3 The Contractor shall coordinate pad dimensions, conduit penetrations, and grounding electrode connection points with the manufacturer's shop drawings before placing concrete; see Concrete Pads for construction requirements. 11.3 Indoor Installation (Dry-Type)
● Direct on housekeeping pad (4 in. minimum)
○ On vibration isolators
○ On factory-furnished base channel
11.3.1 Indoor dry-type transformers above 75 kVA shall be set on a reinforced concrete housekeeping pad at least 4 in. above finished floor, extending a minimum of 3 in. beyond the transformer base on all sides.
11.3.2 Vibration isolators should be specified where the transformer is installed on a structural floor above occupied space and where structure-borne hum would otherwise be objectionable.
11.4 Ventilation
11.4.1 Ventilated dry-type transformers shall be installed with at least the manufacturer's recommended clear space at each ventilation opening — typically 3 in. at the bottom inlet and 6 in. at the top outlet.
11.4.2 The transformer room ventilation shall remove the transformer's losses (heat load) so that the room ambient remains within the standard rating.
11.4.3 The Contractor shall coordinate with the mechanical contractor on the heat load contribution.
11.5 Grounding
11.5.1.1 The transformer tank or enclosure shall be bonded to the building equipment grounding system at the grounding pad provided by the manufacturer, using a conductor sized per NEC 250.122 for the upstream overcurrent device.
11.5.1.2 The grounding connection shall be made with a listed two-hole compression lug bolted to the manufacturer's grounding pad.
11.5.2 Secondary System Grounding
● At the transformer secondary
○ At the first downstream disconnecting means
● Building grounding electrode system (common GEC tap)
○ Nearest effectively grounded structural metal member
○ Nearest effectively grounded metal water pipe (within 5 ft of building entry)
11.5.2.2 The system bonding jumper sizing shall follow NEC Table 250.102(C)(1) based on the secondary phase conductors.
11.5.3 Neutral Treatment
11.5.3.1 The secondary neutral (X0) bushing of a wye-secondary transformer shall be brought out to a neutral terminal in the secondary compartment, from which the neutral conductor to the downstream distribution equipment originates.
11.5.3.2 The neutral shall be bonded to the equipment grounding system at exactly one point — the system bonding jumper — and shall remain isolated from the equipment grounding conductor downstream of that point.
11.5.3.3 Any re-bonding of the neutral at a downstream panelboard creates parallel neutral paths and shall not be done.
11.6 Labeling
☑ Equipment designation (matching one-line)
☐ kVA, primary voltage, secondary voltage, vector group
☐ Upstream feeder source identification
☐ Available fault current at terminals (NEC 110.24)
☐ Arc flash warning label (NFPA 70E / IEEE 1584)
☐ Lockout/tagout caution at primary disconnect
11.6.2 The available fault current at the secondary terminals shall be calculated from the upstream system and the transformer impedance, and shall be labeled on the equipment per NEC 110.24 where the transformer is service equipment or where the secondary distribution equipment requires the label.
11.7 Cleanup and Energization
11.7.1 Before energization, the Contractor shall remove all temporary shipping braces, blocking, and packaging materials from inside the transformer compartments.
11.7.2 Before energization, the Contractor shall verify that all tap connections are in the position indicated on the contract drawings.
11.7.3 Before energization, the Contractor shall verify all torque on accessible bus and cable connections per the manufacturer's specification using a calibrated torque tool.
11.7.4 Before energization, the Contractor shall confirm that the field acceptance tests have been completed and accepted.
12 Delivery, Storage, and Handling
12.1 Transformers shall be delivered to the site only after the installation location is ready to receive them.
12.2 Where temporary storage is unavoidable, units shall be stored indoors in a clean, dry location, supported on the manufacturer's shipping skids or equivalent, and protected from accidental damage.
12.3 Dry-type transformers shall not be stored outdoors uncovered.
12.4 Liquid-immersed units may be stored outdoors on their pad provided the cabinet doors are sealed and the tank is at the proper liquid level.
12.5 Where a dry-type transformer has been stored or exposed to a damp environment, the Contractor shall measure insulation resistance before energization and shall arrange a dry-out cycle (heating with the windings de-energized) if the insulation resistance is below the acceptance criterion.
12.6 Lifting and rigging shall use only the lifting provisions identified by the manufacturer.
12.7 A liquid-immersed transformer shall not be lifted by the cabinet, the bushings, or any accessory mounting; lifting forces shall be applied only at the lifting lugs on the tank.
13 Warranty
1 year from substantial completion
2 years from substantial completion
5 years from substantial completion
13.1 The manufacturer shall warrant each transformer against defects in materials and workmanship for the warranty period selected.
13.2 Warranty shall cover replacement or repair of the transformer including labor for removal and reinstallation at the project site.
13.3 The Contractor shall warrant the installation, including all connections, for the project warranty period.
14 Spare Parts
☑ One set of primary fuses for each fuse type and rating (liquid-immersed)
☐ One spare gasket set per pad-mounted unit
☐ One spare load-break elbow connector per voltage class
☐ One set of arrester replacements per voltage class
14.1 Spare parts shall be delivered to the Owner at substantial completion, identified by the equipment they serve, and stored at a location designated by the Owner.