Transformers

Summary

Transformers change voltage between systems: utility pad-mounted liquid-filled units step medium voltage down to building service voltage, and indoor dry-type units step 480 V distribution down to 208Y/120 V for general loads. They are heavy, produce heat and hum, and need ventilation, clearances, and sound isolation, so their rooms, pads, and structural support must be located early.

Cols

Image

Dry-type: air-cooled, indoor general-purpose transformer, 600 V class
Liquid-filled / pad-mounted: oil- or ester-filled outdoor unit, often utility-owned
kVA: transformer capacity rating
Primary / secondary: input and output windings
Delta-wye: common configuration producing a grounded 208Y/120 V or 480Y/277 V secondary
K-factor: rating for harmonic-rich loads such as electronics
Separately derived system: secondary requires its own grounding and bonding
Impedance: affects available fault current downstream

Standards

Ratings

Indoor dry-type units commonly up to several hundred kVA; one university standard requires approval for indoor units above 75 kVA and prohibits wall mounting
Efficiency set by DOE federal efficiency standards for distribution transformers
Ventilation and clearances per manufacturer and NEC

Test standards

UL 1561: dry-type general purpose and power transformers
NEMA ST 20: dry-type transformers (referenced in specifications)
DOE efficiency standards: distribution transformer efficiency
NFPA 70 (NEC): transformer installation and vaults

Components

Core and coils: steel core with copper or aluminum windings
Enclosure: ventilated steel enclosure, or tank for liquid-filled units
Taps: for voltage adjustment
Vibration isolators: pads under core and coil assembly
Pad or vault: concrete pad outdoors; vault for some liquid-filled indoor units

Detailing

Locate dry-type units in electrical rooms with ventilation for heat loss
Isolate from structure and keep away from quiet rooms to limit hum
Provide working space and clearance from walls per the manufacturer
Coordinate utility pad-mounted transformer locations, screening, and bollards
Size floor or slab for transformer weight

Uses

Specific uses

480 V to 208Y/120 V step-down in commercial buildings
Utility service transformers for buildings and campuses
Isolation transformers for sensitive equipment
K-rated units for data and IT loads

Pros and cons

Pros

High-voltage distribution reduces feeder size and losses
Local step-down at point of use
Long service life with little maintenance

Cons

Heat, noise, and weight
Standby losses even when lightly loaded
Space and clearance in electrical rooms

Manufacturing and cost

Construction

Factory-built; set on pads or floor, anchored, connected with primary and secondary feeders, grounded, and tested

Main manufacturers and suppliers

Trade associations: NFPA and NECA.

Sources

University of Houston: 26 22 00 low-voltage transformers specification, https://www.uh.edu/facilities-services/departments/fpc/master-specs/26-22-00-low-voltage-transformers.pdf
Washington State University: 26 22 00 low-voltage transformers (May 2025), https://wpcdn.web.wsu.edu/wp-fais/uploads/sites/2870/2025/05/26-22-00-Low-Voltage-Transformers_20250501.pdf
Wisconsin DFD: 26 22 00 master specification, https://doa.wi.gov/DFDM_Documents/MasterSpecs/Electrical/262200.docx
Wikimedia Commons: Planar core assembly exploded.png, by Cyril BUTTAY (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Planar_core_assembly_exploded.png