1 Scope

NOTE This specification covers three-phase distribution transformers with a primary above 600V, used to step utility or premises medium-voltage service down to a building's utilization voltage. (1.1)

NOTE Liquid-immersed transformers under this standard are pad-mounted, compartmental, self-cooled units complying with IEEE C57.12.34 and IEEE C57.12.00, installed on a customer-owned concrete pad. (1.2)

NOTE Medium-voltage dry-type transformers under this standard — ventilated, vacuum-pressure-impregnated (VPI), or cast-resin — comply with UL 1562 and IEEE C57.12.01 and are applied indoors or in dedicated enclosures where a liquid-filled unit is undesirable. (1.3)

NOTE A transformer ties together the upstream utility or service entrance, the downstream distribution gear, the pad or room it sits on, and the grounding system. (1.4)

1.4.1 The Contractor shall coordinate this scope with the upstream medium-voltage switchgear or service equipment (Medium Voltage SwitchgearMedium Voltage SwitchgearResolves to the current edition.sync/medium-voltage-switchgear, Electrical Service EntranceElectrical Service EntranceResolves to the current edition.sync/electrical-service-entrance), the downstream switchboards and panelboards (Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current edition.sync/low-voltage-switchboards, PanelboardsPanelboardsResolves to the current edition.sync/panelboards), the raceway and feeder cables serving the transformer (Raceways And ConduitRaceways and ConduitResolves to the current edition.sync/raceways-and-conduit, Conductors And CablesConductors and CablesResolves to the current edition.sync/conductors-and-cables), and the grounding system into which the transformer secondary will be bonded (Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current edition.sync/grounding-and-bonding).

1.4.2 Pad-mounted units shall be coordinated with the concrete pad scope under Concrete PadsConcrete Equipment PadsResolves to the current edition.sync/concrete-pads.

1.4.3 The transformer secondary is a separately derived system unless it constitutes the service; the system bonding jumper, the grounding electrode conductor, and the secondary grounding electrode connection shall be installed in accordance with NEC 250.30 (separately derived systems) or NEC 250.24 (services), as applicable, and Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current edition.sync/grounding-and-bonding.

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 C57.12.34	Standard Requirements for Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers, 10 MVA and Smaller
IEEE C57.12.90	Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEEE C57.12.91	Standard Test Code for Dry-Type Distribution and Power Transformers
IEEE C57.91	Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators
IEEE C57.104	Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers
IEEE 386	Standard for Separable Insulated Connector Systems for Power Distribution Systems Rated 2.5 kV through 35 kV
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
NEMA TR 1	Transformers, Step Voltage Regulators, and Reactors (audible sound levels)
NEMA ST 20	Dry-Type Transformers for General Applications
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, weight, mounting and lifting provisions, primary and secondary termination locations, compartment arrangements, and required clearances
Nameplate data including kVA rating, primary and secondary voltage, vector group, BIL, impedance, temperature class and rise, sound level, and weight
Verification of compliance with the DOE 10 CFR Part 431 efficiency table applicable to the unit category (medium-voltage dry-type or liquid-immersed), with the manufacturer's written efficiency certification
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, fuse, and accessory arrangement drawings and the proposed insulating liquid type with safety data sheet
Pad drawing with conduit window dimensions and grounding connection points (pad-mounted units)
Seismic certification documentation where required by the building code

Action Submittal Documentationcheckbox

☑ Shop drawings with dimensions, weight, 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, switch, and accessory arrangement (pad-mounted only)

☐ Pad drawing with conduit window and grounding points (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 turns ratio (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

Required Closeout Submittalscheckbox

☑ 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 Medium-voltage dry-type transformers shall be listed and labeled to UL 1562 by a Nationally Recognized Testing Laboratory.

4.2.2 Liquid-immersed pad-mounted units shall comply with IEEE C57.12.34 and IEEE C57.12.00 and shall bear the manufacturer's certification of compliance.

4.3 Efficiency Compliance

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 (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.

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

5.3.1 The maximum ambient temperature at the installation shall be as indicated in the datasheet.

Ambient Temperature (Maximum)select

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. (5.3.2)

5.4 Installation Altitude

5.4.1 The installation altitude shall be as indicated in the datasheet.

Installation Altitudeselect

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. (5.4.2)

5.4.3 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.

6 Type and Configuration

6.1 Construction Type

6.1.1 The transformer construction type shall be as indicated in the datasheet.

Transformer Constructionradio

○ Liquid-immersed, pad-mounted (outdoor)

○ Dry-type, ventilated / VPI (indoor electrical room)

○ Dry-type, cast resin (indoor or outdoor, severe environment)

NOTE Liquid-immersed pad-mounted transformers are the standard form for a medium-voltage utility-style service to a customer building; the sealed tank tolerates outdoor exposure and requires only a concrete pad. (6.1.2)

NOTE Medium-voltage dry-type units avoid insulating liquid entirely — no containment, no fire-point clearances — at the cost of a larger footprint, a conditioned or ventilated room, and higher first cost per kVA. (6.1.3)

NOTE Cast-resin construction provides the highest tolerance for contaminated, humid, or corrosive environments and the highest short-circuit withstand among dry types; it is commonly specified for industrial, marine, and mission-critical service. (6.1.4)

6.2 kVA Rating

6.2.1 The kVA rating shall be as indicated in the datasheet, sized for the present load with allowance for future growth.

kVA Ratingrange

kVA

75112.51502253005007501000150020002500

Per drawings — one-line diagram (deferred by default)

NOTE The ratings above follow the preferred rating series of IEEE C57.12.34; manufacturers may offer additional intermediate ratings. (6.2.2)

6.3 Voltage

6.3.1 Primary and secondary voltage ratings shall be as indicated in the datasheet.

Primary Voltageselect

4.16 kV three-phase

12.47 kV three-phase, delta or grounded-wye

13.2 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 — one-line diagram (deferred by default)

Secondary Voltageselect

480Y/277V three-phase, four-wire

208Y/120V three-phase, four-wire

480V three-phase, three-wire (delta)

240/120V three-phase, four-wire (delta)

Per drawings — one-line diagram (deferred by default)

6.3.2 The primary voltage and winding configuration shall match the serving utility's distribution system; the Contractor shall confirm the utility's nominal voltage, grounding method, and available fault duty before ordering.

6.4 Vector Group

6.4.1 The vector group shall be as indicated in the datasheet and shall be compatible with the primary system grounding method and the secondary voltage configuration selected.

Vector Groupradio

○ Dyn1 (delta primary, wye secondary with neutral, 30° lag)

○ Dyn11 (delta primary, wye secondary with neutral, 30° lead)

○ Yyn0 (grounded wye - wye with neutral, no phase shift)

○ Dd0 (delta-delta, no phase shift)

NOTE A delta primary with grounded-wye secondary (Dyn) is the most common arrangement for a customer substation; it isolates the secondary neutral from the primary system and blocks triplen harmonics. A grounded wye-wye is common on utility loop-feed systems because it avoids ferroresonance during single-phase switching of cable-fed transformers. (6.4.2)

6.4.3 The vector group selection shall match any parallel transformer banks; mismatched vector groups cannot be paralleled.

6.5 Basic Insulation Level (BIL)

6.5.1 The BIL of each winding shall be as indicated in the datasheet, selected per IEEE C57.12.00 Table 4 (liquid-immersed) or IEEE C57.12.01 (dry-type) for the system voltage class and grounding method, and coordinated with the protective margin of the surge arresters provided.

Primary BILselect

30 kV BIL (5 kV class)

60 kV BIL (8.7 kV class; 15 kV class dry-type standard)

95 kV BIL (15 kV class liquid-immersed standard; 15 kV class dry-type optional high)

125 kV BIL (25 kV class)

150 kV BIL (34.5 kV class)

NOTE Dry-type transformers carry a lower standard BIL than liquid-immersed units of the same voltage class; specifying the optional higher BIL on a 15 kV class dry-type unit (95 kV in place of 60 kV) is common practice where the unit is exposed to switching transients or where arrester protective margins are thin. (6.5.2)

6.6 Tap Changer

6.6.1 Transformers shall be furnished with full-capacity primary taps to adjust the turns ratio for variations in supply voltage, with the tap arrangement as indicated in the datasheet.

Primary Tapsselect

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

NOTE The ±2 × 2.5% arrangement (giving a total tap range of −5% to +5% in 2.5% steps) is the established default for distribution transformers; wider tap ranges are appropriate where the supply voltage is known to vary substantially. (6.6.2)

6.6.3 Taps shall be de-energized only; load tap changers are not within the scope of this standard.

6.6.4 Tap changers shall be operable only with the transformer de-energized; an interlock or warning label shall be provided to enforce this.

6.7 Impedance

6.7.1 Where a specific impedance is required to limit the available secondary short-circuit current or to parallel with another unit, the impedance voltage in percent shall be as indicated in the datasheet.

Impedance Voltage at Rated Tap (%Z)range

%

23455.75677.5

Manufacturer's standard (by default)

6.7.2 Where no impedance is indicated, the manufacturer's standard impedance for the kVA rating shall apply, and the manufacturer shall report the design impedance in the action submittals for use in the project's short-circuit study.

6.7.3 Any impedance indicated shall be coordinated with the project's short-circuit study and shall limit the available secondary short-circuit current to a value compatible with the downstream secondary distribution equipment's interrupting rating.

NOTE IEEE C57.12.34 assigns a standard impedance to each kVA rating; departing from the standard value is a special order and should be reserved for cases where the short-circuit study requires it. (6.7.4)

6.7.5 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.

6.8 Sound Level

6.8.1 The maximum audible sound level shall not exceed the NEMA TR 1 value (liquid-immersed) or the NEMA ST 20 value (dry-type) for the transformer's kVA rating and cooling class, reduced by the amount indicated in the datasheet.

Sound Level Reduction Below NEMA Standard Valueselect

0 dB (NEMA standard sound level)

3 dB below NEMA standard value

5 dB below NEMA standard value

6.8.2 Sound level shall be measured per IEEE C57.12.90 (liquid-immersed) or IEEE C57.12.91 (dry-type).

NOTE NEMA TR 1 and NEMA ST 20 tabulate the maximum average sound level as a function of the transformer's kVA rating, so an absolute decibel limit is meaningful only relative to those tables; the specification therefore states the required reduction below the NEMA value rather than an absolute level. (6.8.3)

6.8.4 Where the transformer is installed adjacent to occupied space or a property line with a noise ordinance, the Engineer should specify a sound level 3 to 5 dB below the NEMA value.

7 Construction — Liquid-Immersed Pad-Mounted

7.1 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.

7.2 Liquid-immersed units shall be rated for a 65°C average winding temperature rise at rated load in accordance with IEEE C57.12.00.

7.3 Tank Construction

7.3.1 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.

7.3.2 Tank wall and cover thicknesses shall be sufficient to withstand the full vacuum required for processing of the dielectric liquid without permanent deformation.

7.4 Enclosure and Cabinet

7.4.1 The cabinet finish color shall be as indicated in the datasheet.

Cabinet Finishradio

● Munsell green 7GY3.29/1.5 (IEEE C57.12.34 pad-mounted standard)

○ ANSI 61 gray (light gray)

○ ANSI 70 gray

7.4.2 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.

7.4.3 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.

7.4.4 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. (7.4.5)

7.5 Bushings and Terminations

7.5.1 The high-voltage bushing arrangement shall be as indicated in the datasheet.

High-Voltage Bushing Arrangementradio

○ Live-front (porcelain bushings, exposed terminals)

● Dead-front (universal bushing wells with 200A elbow connectors)

○ Dead-front (600A bushing wells with apparatus connectors)

7.5.2 Separable insulated connectors (elbows) shall comply with IEEE 386.

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. (7.5.3)

NOTE Live-front construction is used where the primary cable is a permanent overhead-to-underground transition or where the local utility requires it. (7.5.4)

NOTE 600A apparatus connections are appropriate for primary loop currents above the 200A elbow rating and for units at the top of the kVA range. (7.5.5)

7.6 Primary Switching

7.6.1 The primary switching arrangement shall be as indicated in the datasheet and shall match the primary feed configuration (radial or loop) shown on the one-line diagram.

Primary Switchingselect

No primary switch (separate primary disconnect upstream)

Two-position, loadbreak (radial feed)

Four-position, loadbreak (loop feed with sectionalizing)

Per drawings — one-line diagram (deferred by default)

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. (7.6.2)

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. (7.6.3)

7.7 Primary Fusing

7.7.1 The internal primary fusing arrangement shall be as indicated in the datasheet.

Primary Fusingselect

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. (7.7.2)

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. (7.7.3)

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. (7.7.4)

7.8 Insulating Liquid

7.8.1 The insulating liquid type shall be as indicated in the datasheet.

Insulating Liquidradio

○ Mineral oil (ASTM D3487)

○ Natural ester (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. (7.8.2)

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 reduce the NEC outdoor clearance requirements and the spill-containment burden, and are increasingly specified for new pad-mounted installations. (7.8.3)

NOTE Silicone fluids are K-class and chemically very stable but are non-biodegradable and substantially more expensive than esters. (7.8.4)

7.8.5 A less-flammable K-class liquid shall be used where the transformer is installed within the clearance distances of combustible building surfaces established by NEC 450.23.

7.9 Liquid Preservation System

7.9.1 The liquid preservation system shall be as indicated in the datasheet.

Liquid Preservationradio

○ Sealed tank (no gas space access, pressure cycling within design)

● Sealed tank with pressure-vacuum bleeder

NOTE Pad-mounted distribution transformers are sealed-tank construction. (7.9.2)

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. (7.9.3)

8 Construction — Medium-Voltage Dry-Type

8.1 Core and Coil

8.1.1 The winding conductor material shall be as indicated in the datasheet.

Winding Conductor (Dry-Type)radio

○ Copper

○ Aluminum

8.1.2 Core laminations shall be of grain-oriented, low-loss electrical steel, clamped and stacked to minimize core losses and audible noise.

8.1.3 Ventilated and VPI windings shall be vacuum-pressure impregnated with a thermosetting varnish appropriate to the insulation class.

8.1.4 Cast-resin windings shall be cast under vacuum in epoxy resin, providing a sealed, moisture-impervious winding suitable for humid and contaminated atmospheres.

NOTE Copper and aluminum windings are both routinely specified and procurable from all major manufacturers; copper produces a smaller unit with better joint reliability, aluminum a lower first cost. (8.1.5)

8.2 Insulation Class and Temperature Rise

8.2.1 The insulation system class and the average winding temperature rise at rated load shall be as indicated in the datasheet.

Insulation System Temperature Class (Dry-Type)select

180°C (typical for cast resin)

220°C (typical for ventilated / VPI)

Average Winding Temperature Rise at Rated Load (Dry-Type)range

°C

80115150

8.2.2 The temperature rise selected shall be compatible with the insulation system class; a 150°C rise is available only on a 220°C system.

NOTE For 220°C ventilated / VPI systems a 150°C average rise is the standard design basis; cast-resin systems carry lower-temperature insulation classes and correspondingly lower standard rises per the manufacturer's listing. (8.2.3)

NOTE North American practice per IEEE C57.12.01 and UL 1562 identifies dry-type transformer insulation systems by their temperature class in °C; IEC letter designations (Class B, F, H) carry different temperature limits and are not used for transformers in the US market. (8.2.4)

8.2.5 A lower temperature rise (80°C on a 220°C system) extends insulation life and provides overload margin, and should be specified for units subject to harmonic loading or short-term overloads.

8.3 Enclosure

8.3.1 The dry-type enclosure rating shall be as indicated in the datasheet and shall correspond to the installation location: NEMA 1 for a dedicated indoor electrical room and NEMA 3R minimum for outdoor installation.

Enclosure Rating (Dry-Type)select

NEMA 1 (indoor, general purpose, ventilated)

NEMA 3R (outdoor, rain-tight, ventilated)

8.3.2 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.

9 Accessories

9.1 Liquid-immersed units shall be furnished with the standard accessories required by IEEE C57.12.34, including:

Nameplate (stainless or anodized aluminum, engraved)
Lifting lugs and jacking provisions
NEMA two-hole grounding pads on the tank
Drain valve with sampling device
Fill connection
Pressure relief device
Liquid level gauge
Dial-type liquid temperature gauge

9.2 Dry-type units shall be furnished with the standard accessories required by IEEE C57.12.01, including nameplate, lifting provisions, and grounding pads.

9.3 Optional accessories shall be furnished as indicated in the datasheet.

Optional Accessories Furnishedcheckbox

☐ Liquid temperature gauge with alarm contacts

☐ Pressure-vacuum gauge

☐ Winding temperature monitor with alarm contacts

☐ Sudden-pressure relay

☐ Dissolved-gas monitor (single-gas or multi-gas)

☐ Forced-air cooling fans (dry-type, future addable)

☐ Alarm contact wiring to terminal block for remote monitoring

NOTE Alarm and monitoring contacts are recommended for any transformer serving a critical load or monitored by a building automation or SCADA system. (9.4)

10 Surge Arresters

10.1 Surge arrester provisions shall be as indicated in the datasheet, coordinated with the primary feed exposure and the arrester protective margin against the winding BIL.

Surge Arrestersradio

○ 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)

10.2 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 line.

NOTE Elbow-style arresters integrated into the dead-front bushing wells are the cleanest installation for dead-front pad-mounted units. (10.3)

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 systems exposed to switching transients. (10.4)

11 Testing

11.1 Factory Production Tests

11.1.1 Every transformer shall receive the production tests required by the applicable IEEE test code (C57.12.90 for liquid-immersed, C57.12.91 for dry-type).

11.1.2 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
Leak test (liquid-immersed)
Resistance of windings

11.1.3 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.

11.2 Factory Witnessed / Acceptance Tests

11.2.1 Factory acceptance test witnessing shall be as indicated in the datasheet.

Factory Acceptance Test Witnessingradio

○ Witnessed by Owner's representative

● Unwitnessed, certified test report only

○ Not required beyond production tests

11.2.2 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.

11.2.3 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.

11.3 Field Acceptance Tests

11.3.1 Field acceptance tests shall be performed as indicated in the datasheet.

Field Acceptance Tests Requiredcheckbox

☑ Visual and mechanical inspection

☑ Insulation resistance, winding-to-winding and winding-to-ground (Megger)

☑ Turns ratio test (TTR) on all taps

☑ Winding resistance (DC)

☐ Insulation power factor / dissipation factor

☐ Polarity and phase relation verification

☐ Excitation current at applied test voltage

☑ Dielectric liquid sample analysis (liquid-immersed only)

☑ Grounding verification (tank, neutral, secondary)

☐ Operational test of all alarms and trip contacts

11.3.2 Field acceptance testing shall be performed by a qualified independent testing firm in accordance with NETA ATS Section 7.2.

11.3.3 Field acceptance tests shall be performed after installation is complete and before the transformer is energized for service.

11.3.4 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.

11.3.5 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.

11.3.6 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.

11.3.7 An initial dissolved gas analysis sample should be drawn for baseline establishment regardless of unit size; on units 1000 kVA and larger, a baseline DGA per IEEE C57.104 is required.

11.4 Initial In-Service Inspection

11.4.1 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.

12 Installation

12.1 Clearances

12.1.1 Working space around the transformer shall comply with NEC 110.26 for low-voltage compartments and NEC 110.34 for medium-voltage compartments.

12.1.2 The required working space dimensions shall be coordinated with the pad or room layout before pad or anchor work begins.

12.1.3 Outdoor liquid-insulated transformers shall be separated from combustible building surfaces in accordance with NEC 450.27; for mineral-oil units this requires substantial separation from combustible walls and openings, while a unit filled with a listed less-flammable liquid may be installed at the reduced separation permitted by NEC 450.23 and the liquid's listing.

NOTE The reduced clearance available with a K-class liquid is one of the principal reasons for selecting an ester fluid on a constrained urban site. (12.1.4)

12.1.5 Indoor dry-type units shall be located in a dedicated electrical room meeting the clearance and ventilation requirements of NEC Article 450 Part II.

12.2 Pad Mounting (Liquid-Immersed)

12.2.1 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.

12.2.2 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.

12.2.3 The Contractor shall coordinate pad dimensions, conduit penetrations, and grounding electrode connection points with the manufacturer's shop drawings before placing concrete; see Concrete PadsConcrete Equipment PadsResolves to the current edition.sync/concrete-pads for construction requirements.

12.3 Ventilation (Dry-Type)

12.3.1 Ventilated dry-type transformers shall be installed with at least the manufacturer's recommended clear space at each ventilation opening.

12.3.2 The transformer room ventilation shall remove the transformer's losses (heat load) so that the room ambient remains within the standard rating; the Contractor shall coordinate the heat load contribution with the mechanical contractor.

12.4 Grounding

12.4.1 Tank / Enclosure Grounding

12.4.1.1 The transformer tank or enclosure shall be bonded to the grounding electrode system at the grounding pads provided by the manufacturer, using conductors sized per NEC 250.122 for the upstream overcurrent device, with a listed two-hole compression lug at each pad.

12.4.1.2 Pad-mounted transformer grounding shall include a grounding electrode loop or ground rods at the pad as required by the serving utility and Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current edition.sync/grounding-and-bonding.

12.4.2 Secondary System Grounding

12.4.2.1 Where the transformer secondary is a separately derived system, the Contractor shall install the system bonding jumper and a grounding electrode conductor at the source or at the first disconnecting means (but not both) per NEC 250.30, connected to the grounding electrode system per Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current edition.sync/grounding-and-bonding.

12.4.2.2 Where the transformer secondary constitutes the service, the grounded conductor and grounding electrode connections shall comply with NEC 250.24 and the service entrance requirements of Electrical Service EntranceElectrical Service EntranceResolves to the current edition.sync/electrical-service-entrance.

12.4.2.3 The secondary neutral shall be bonded to the grounding system at exactly one point and shall remain isolated from the equipment grounding conductor downstream of that point.

12.5 Labeling

12.5.1 Field-installed labels shall be provided as indicated in the datasheet and shall comply with Equipment LabelingElectrical Equipment LabelingResolves to the current edition.sync/equipment-labeling.

Transformer Labelingcheckbox

☑ Equipment designation (matching one-line)

☑ kVA, primary voltage, secondary voltage, vector group

☑ Upstream feeder source identification

☐ Available fault current at terminals (NEC 110.24, where applicable)

☐ Arc flash warning label (NFPA 70E / IEEE 1584)

☐ Lockout/tagout caution at primary disconnect

☑ Danger — high voltage signage per utility and AHJ requirements

12.5.2 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 where required by NEC 110.24.

12.6 Cleanup and Energization

12.6.1 Before energization, the Contractor shall remove all temporary shipping braces, blocking, and packaging materials from inside the transformer compartments.

12.6.2 Before energization, the Contractor shall verify that all tap connections are in the position indicated on the contract drawings.

12.6.3 Before energization, the Contractor shall verify all torque on accessible bus and cable connections per the manufacturer's specification using a calibrated torque tool.

12.6.4 Before energization, the Contractor shall confirm that the field acceptance tests have been completed and accepted.

13 Delivery, Storage, and Handling

13.1 Transformers shall be delivered to the site only after the installation location is ready to receive them.

13.2 Where temporary storage is unavoidable, dry-type units shall be stored indoors in a clean, dry location, supported on the manufacturer's shipping skids or equivalent, and protected from accidental damage.

13.3 Dry-type transformers shall not be stored outdoors uncovered.

13.4 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.

13.5 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.

13.6 Lifting and rigging shall use only the lifting provisions identified by the manufacturer.

13.7 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.

14 Warranty

14.1 The manufacturer shall warrant each transformer against defects in materials and workmanship for the warranty period indicated in the datasheet.

Warranty Periodselect

1 year from substantial completion

2 years from substantial completion

5 years from substantial completion

14.2 Warranty shall cover replacement or repair of the transformer including labor for removal and reinstallation at the project site.

14.3 The Contractor shall warrant the installation, including all connections, for the project warranty period.

15 Spare Parts

15.1 Spare parts shall be furnished as indicated in the datasheet.

Spare Parts Furnishedcheckbox

☐ One set of primary fuses for each fuse type and rating

☐ 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

NOTE Spare primary fuses should be furnished for every fused pad-mounted transformer; a blown bayonet fuse is otherwise a multi-day outage waiting on a replacement. (15.2)

15.3 Spare parts, where furnished, shall be delivered to the Owner at substantial completion, identified by the equipment they serve, and stored at a location designated by the Owner.