A pad-mounted transformer is a ground-level transformer housed in an enclosure and installed on a prepared foundation. Buyers also search for a pad mount transformer or padmount transformer. Select the equipment by its electrical specification and installation arrangement, not its exterior appearance alone.
Before requesting offers, identify capacity, primary and secondary voltage, phase, frequency and the required connection arrangement. Obtain the utility or project specification where one applies.
Key Takeaways
- Match the specification before the price. kVA, voltage ratio, connection, impedance, cooling, accessories and condition define whether two offers are comparable.
- 3,750 and 5,000 kVA carry no federal minimum efficiency today. DOE's amended standards extend coverage to 5,000 kVA on April 23, 2029, and the required efficiency at those two ratings is lower than at 2,500 kVA.
- Specifying a non-standard impedance can remove the unit from DOE scope entirely — a "special-impedance transformer" is excluded from the definition of distribution transformer.
- Impedance is a switchgear decision, not a transformer decision. Discovering a mismatch after the switchgear order is a schedule event, not a change order.
- Pad mount is the minority of installed units but the majority of installed capacity — roughly 65% of U.S. distribution transformer capacity per NREL, against about 76% of unit count being pole-mount.
What a pad mount transformer is and where it sits#
The utility-to-facility interface#
No fence, no vault, no elevated structure — which is exactly why utilities and data center developers use them. They can be sited close to the load, in a landscaped area, without the civil work a substation requires.
Primary side runs 4.16 kV to 34.5 kV. Secondary side is most often 480Y/277 V for data center building service, or 208Y/120 V for smaller loads. Federally, a distribution transformer is defined as 34.5 kV or less input, 600 V or less output, 60 Hz, and 10 to 5,000 kVA for liquid-immersed units — a definition that matters more than it looks, for reasons covered below.
Within the full data center transformer chain, the pad mount sits at the building service position, taking campus medium-voltage distribution down to the 480 V that feeds low-voltage switchgear, UPS systems, and mechanical plant. At 2,000 to 2,500 kVA per service position, a single-building 20 MW facility works out to roughly eight to twelve units.
Pad mount vs. unit substation vs. dry-type#
| Attribute | Pad mount | Unit substation | Dry-type |
|---|---|---|---|
| Cooling / insulation | Liquid (mineral or ester) | Usually liquid | Air and solid |
| Location | Outdoors, at grade | Indoors or outdoors | Indoors, no vault required |
| Typical rating | 75–5,000 kVA | 750–5,000 kVA | 30–2,500 kVA |
| Includes switchgear | No | Yes — integrated | No |
| Footprint per kVA | Low | Higher | Highest |
| Lead time, 2026 | 40–65 wk | Varies by section | 20–32 wk |
| Fire code constraint | Outdoors avoids NEC 450 vault rules | Depends on liquid type | Room construction per 450.21 |
The selection is usually settled by location. Outdoors at grade, near the building, without integrated switchgear points to pad mount. Indoors in occupied space points to dry-type. Transformer and switchgear procured together as one tested assembly points to a unit substation.
Dead-front vs. live-front#
Dead-front construction uses fully insulated, shielded separable connectors — 200 A load-break elbows or 600 A deadbreak connectors — so no energized metal is exposed when the compartment door is open. Live-front uses exposed bushings with clamp connections.
Anything accessible to non-utility personnel should be dead-front, and in practice essentially all data center pad mounts are. The safety case is obvious; the operational case is that dead-front construction is what allows a technician to work in the compartment under normal PPE.
Pad-mounted transformer vs. pad-mounted switchgear#
A transformer changes voltage. Switchgear performs switching and protection functions. Although both can be installed in outdoor enclosures, they are not interchangeable equipment categories. Some transformer designs integrate protective devices, so compare the actual functions and drawings rather than relying on the enclosure description.
See the switchgear catalogue for switching equipment and the transformer catalogue for transformer sourcing.
Configurations that change the specification#
Radial feed vs. loop feed#
This is the single most consequential configuration choice, and it is frequently made by default rather than by decision.
Radial feed takes one primary cable in. Simple, lowest first cost, single point of failure — lose the feeder and the transformer is dark.
Loop feed takes two primary cable entries with internal switching, so the transformer sits on a loop rather than at the end of a radial spur. Either direction can serve it, and a fault on one leg is isolated while the other continues to feed.
Loop feed adds cost at the transformer and requires additional primary cable and switching. On a data center campus where the medium-voltage distribution is the shared layer beneath everything, it is close to always worth it. Radial feed on a critical building service position undoes redundancy that has been paid for everywhere else in the design.
Specify the primary connection arrangement in the enquiry. Eaton's pad-mounted transformer guide describes radial- and loop-feed options as well as livefront and deadfront constructions. Confirm the exact equipment arrangement against project drawings before purchase.
Winding configurations#
Delta-wye is the data center standard, for three reasons: the wye secondary provides a neutral for 277 V lighting and single-phase loads, the delta primary traps triplen harmonics rather than passing them upstream, and the grounded wye secondary gives a defined ground-fault current path for protection.
Wye-wye appears occasionally where the utility requires a primary neutral. It passes harmonics more readily and needs careful attention to grounding.
Tap changers#
De-energized tap changers — typically four 2.5% taps, two above and two below nominal — allow output voltage adjustment to compensate for line drop. De-energized is literal: the transformer must be off before operating the tap.
Set taps during commissioning based on measured voltage, not on assumption. It is a free adjustment during startup and an outage afterward. Note also that a unit with a tap range of 20% or more is excluded from the federal definition of a distribution transformer, and therefore from the efficiency standards.
Fusing and protection#
Bay-O-Net fuses are the common arrangement — oil-immersed, load-break, replaceable from outside the tank without pulling the unit. Usually backed by an internal partial-range current-limiting fuse for high-magnitude faults.
Internal weak-link fuses carry a lower first cost and are non-replaceable; a fuse operation means the transformer is scrap. Acceptable on small utility distribution units, not on a data center service transformer. Specify Bay-O-Net with a current-limiting backup on anything critical.
Cooling class and fluid type#
Most pad mounts in this range are ONAN — oil natural, air natural — with no fans or pumps.
Fluid choice matters more than it used to. NEC 450.23 draws its less-flammable line at a 300 °C fire point, which conventional mineral oil does not meet and natural-ester fluids do. Qualifying as less-flammable can reduce clearance and barrier requirements near buildings, and ester fluids biodegrade, which simplifies spill containment. The tradeoffs are higher fluid cost, higher viscosity at low temperature, and a moisture-tolerance profile that differs from mineral oil.
On a constrained site the reduced separation distance is frequently what decides it.
Sizing a pad mount transformer#
kVA selection method#
- Establish connected load on the transformer's secondary.
- Apply a demand factor — carefully. Conventional commercial demand factors assume diversity a data center does not have; IT load runs at a near-constant fraction of design capacity around the clock.
- Apply harmonic derating per IEEE C57.110, or specify a K-rated unit. Switch-mode power supplies produce harmonic currents that cause eddy-current heating scaling with the square of harmonic order.
- Add growth headroom matched to the buildout plan.
- Select the next standard kVA rating above the calculated value.
- Verify impedance against the downstream fault-current study.
Standard kVA ratings#
Three-phase pad mount transformers are built to standard ratings — 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 3750 and 5000 kVA. Specifying between them adds cost and lead time for no benefit. Data center building service clusters at 1000, 1500, 2000, 2500 and 3000 kVA.
Impedance, fault current, and the regulatory trap#
Standard impedance for this class is set by regulation as much as by practice: DOE's normal impedance range for three-phase liquid-immersed units from 750 to 5,000 kVA is 5.0 to 7.5%. Lower impedance improves voltage regulation and raises secondary fault current; higher impedance does the reverse.
Going outside that band has a consequence most specifiers do not expect. A transformer built to operate outside the normal impedance range for its rating is a special-impedance transformer, and special-impedance transformers are excluded from the federal definition of a distribution transformer — which removes the unit from the efficiency standards altogether. That may be exactly what a project needs, but it should be a decision rather than a side effect, and it changes what "DOE compliant" means on the submittal.
The other constraint is downstream. Available fault current at the transformer secondary determines the interrupting rating the low-voltage switchgear must carry. Because switchgear now runs past a year on many lineups, discovering an impedance mismatch after the switchgear order is a schedule event, not a change order. Fix the impedance value in the fault study before either order goes out.
Temperature rise and loading#
Liquid-immersed transformers in this class are standard at 65 °C average winding rise over a 30 °C average ambient. Real overload capability exists and is modeled in IEEE C57.91, but it depends on the load having a duty cycle that allows cooling. A data center load does not. Plan on continuous rating, not on nameplate-plus-overload.
Worked example — 2 MW IT load at 480 V#
| Step | Input | Result |
|---|---|---|
| IT load | 2,000 kW | — |
| Mechanical and losses at 1.25 PUE | × 1.25 | 2,500 kW total facility |
| Power factor correction | ÷ 0.95 | ≈ 2,630 kVA |
| Harmonic derating, illustrative 8% | ÷ 0.92 | ≈ 2,860 kVA |
| Growth headroom | × 1.10 | ≈ 3,150 kVA |
| Next standard rating | — | 3,750 kVA |
The derating figure is illustrative; compute it from a measured spectrum per C57.110 rather than assuming a percentage. The jump from 3,000 to 3,750 kVA looks like over-sizing. It is the standard rating, and specifying 3,200 kVA to "right-size" it produces a custom unit at higher cost and longer lead time — and, at a non-standard rating, a longer conversation about which efficiency row applies.
Efficiency standards and the April 23, 2029 compliance date#
DOE's April 2024 rule requires compliance with amended standards for equipment manufactured or imported on and after April 23, 2029. The final rule softened the core-material requirement so that roughly 75% of the market can comply using grain-oriented electrical steel rather than shifting to amorphous alloy, and extended the compliance window from three years to five.
3,750 kVA and 5,000 kVA — two standard pad mount ratings that carry no federal minimum efficiency until April 23, 2029, when coverage extends to 5,000 kVA.
Read the test point before comparing efficiency numbers. Liquid-immersed efficiency is certified at 50% per-unit load, with reference temperatures of 20 °C for no-load loss and 55 °C for load loss. Low-voltage dry-type is certified at 35% load — so a dry-type and a liquid-filled efficiency figure are not directly comparable, and neither describes a unit held near nameplate.
Three-phase liquid-immersed minimum efficiency, before and after April 23, 2029#
| kVA | Manufactured before 4/23/2029 | Manufactured on or after 4/23/2029 |
|---|---|---|
| 500 | 99.35% | 99.38% |
| 750 | 99.40% | 99.43% |
| 1,000 | 99.43% | 99.46% |
| 1,500 | 99.48% | 99.51% |
| 2,000 | 99.51% | 99.53% |
| 2,500 | 99.53% | 99.55% |
| 3,750 | not covered | 99.54% |
| 5,000 | not covered | 99.53% |
All values at 50% per-unit load per 10 CFR 431.196. Note the shape at the top of the table: the required efficiency falls above 2,500 kVA, so a 5,000 kVA unit is held to a looser standard than a 2,500 kVA unit. That is a deliberate feature of how DOE set levels by equipment class, and it means "meets DOE 2029" carries different weight at different ratings.
What the rule restricts, and what it does not#
The rule governs what may be manufactured or imported after the compliance date. It does not require replacement of installed units, and the manufacturing cutoff is fixed and knowable, which prices the pre-compliance option. Whether a specific pre-compliance unit sitting in a distributor's channel can still be sold after that date is a question for counsel on the individual transaction rather than a general rule.
Site and installation requirements#
The pad. Reinforced concrete, sized to the unit's footprint with margin, with cable entry provisions. Level, and rated for the unit's filled weight — pull that number from the manufacturer's drawing rather than estimating, because it drives both the pad design and the rigging plan.
Clearances. Utility and manufacturer requirements typically call for substantial clear space at the door side for cabinet swing and operation, with lesser clearance on the remaining sides. Verify against the serving utility's standard — they vary, and the utility's requirement governs at the service position.
Grounding. Ground grid connected to the tank, the enclosure, and the secondary neutral per NEC Article 250 and IEEE 142.
Oil containment. Required where a spill would reach a waterway or where local code demands it. 40 CFR Part 112 SPCC rules apply above threshold volumes. Less-flammable ester fluids can reduce but do not always eliminate the requirement.
Pad mount transformer cost and quotation scope#
A useful price comparison starts with matching specifications: kVA, voltage ratio, connection, impedance, cooling, accessories and condition. Request an itemized quotation covering the transformer and any testing, freight, unloading, installation or commissioning included. Record the date and location of the offer.
Confirm delivery milestones for the actual equipment. Unused surplus still requires evidence of storage, preservation, completeness and warranty status; used or refurbished equipment needs condition and test documentation. A catalogue entry does not prove that a matching unit is ready to ship.
Buying a used pad-mounted transformer#
Start with nameplate photographs and the used pad-mount transformer compatibility checklist. Identify differences from the project specification before agreeing commercial terms. Request transformer equipment with your required rating, voltages, connection and delivery location.
Nameplate verification#
The nameplate is the primary document. Confirm kVA rating, primary and secondary voltage, winding configuration, impedance, temperature rise, cooling class, fluid type, year of manufacture, and serial number. Photograph it and verify the serial against any documentation the seller provides. Year of manufacture drives the PCB question below; impedance drives whether the unit was ever in DOE scope.
Fluid testing before purchase#
| Test | Detects | Standard |
|---|---|---|
| Dissolved gas analysis | Internal arcing, partial discharge, overheating | IEEE C57.104 |
| Moisture content | Insulation wetting | ASTM D1533 |
| Dielectric breakdown | Insulating strength | ASTM D877 / D1816 |
| Acidity and interfacial tension | Oil degradation | IEEE C57.106 |
| PCB screening | Regulated contamination | 40 CFR Part 761 |
PCB screening on any pre-1979 unit is mandatory, not advisory. Taking title to a contaminated transformer means taking on a regulated-waste disposal liability that can exceed the equipment's value.
Electrical testing#
- Turns ratio (TTR) — winding and tap changer integrity
- Winding resistance — connection and conductor condition
- Insulation resistance and polarization index — insulation system condition
- Insulation power factor — moisture and contamination in the insulation
- Excitation current — core and winding condition
Testing costs a fraction of a percent of purchase price and catches nearly every failure mode that matters. Refusing a seller's offer to witness testing is where used-equipment purchases go wrong.
Refurbishment scope#
A proper refurbishment includes a full electrical test, oil processing or replacement, gasket replacement, bushing inspection and replacement as needed, tank cleaning and repainting, tap changer service, and a documented final test report. "Reconditioned" with no defined scope means whatever the seller chose to do.
Get the scope in writing before purchase, and the final test report before payment. Size the unit against your density profile and redundancy configuration in SecondWatt's power system configurator, then check current pad mount transformer inventory by rating, condition tier, and test documentation status.
FAQ: pad mount transformer specification and procurement#
What is a pad mount transformer?#
A liquid-immersed distribution transformer in a locked, tamper-resistant steel enclosure mounted on a concrete pad at grade. It steps medium voltage — typically 4.16 kV to 34.5 kV — down to utilization voltage such as 480Y/277 V, without requiring a vault, fence, or elevated structure.
How do you size a pad mount transformer?#
Establish connected load, apply a demand factor appropriate to a near-constant data center load profile, derate for harmonics per IEEE C57.110 or specify a K-rated unit, add growth headroom, then select the next standard kVA rating above the result. Verify impedance against the downstream fault-current study before ordering, because the fault current sets the switchgear interrupting rating.
What is the difference between loop feed and radial feed?#
Radial feed has one primary cable entry, which is a single point of failure. Loop feed has two entries with internal switching, so the transformer can be served from either direction and a fault on one leg is isolated. Loop feed costs more at the transformer and in primary cable, and is standard on critical data center service positions.
How much does a pad mount transformer cost?#
Compare offers only when kVA, voltage ratio, connection, impedance, cooling, accessories and condition match. Request an itemized quotation that separates the transformer from testing, freight, installation and commissioning, and record the offer date and location.
What is the lead time for a pad mount transformer?#
Delivery depends on the specific equipment and supplier. Confirm dispatch and delivery milestones in the offer; a catalogue entry or a surplus listing does not by itself establish that a matching unit is ready to ship.
Do DOE efficiency standards apply to every pad mount transformer?#
No. The federal definition covers liquid-immersed units from 10 to 5,000 kVA at 34.5 kV or less input and 600 V or less output, and excludes several categories outright — including special-impedance transformers and units with a tap range of 20% or more. Coverage above 2,500 kVA begins with equipment manufactured on or after April 23, 2029.
Can you buy a used pad mount transformer?#
Yes. Start with nameplate photographs and the compatibility checklist, then require dissolved gas analysis, oil quality testing, turns ratio, insulation power factor, and — on any pre-1979 unit — PCB screening before taking title.