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Transformer glossary

The terms that appear on a quote, a drawing or a nameplate, defined plainly. If a supplier uses one of these and assumes you know it, this is what they meant.

Term	Meaning
Apparent power kVA	Voltage times current, with no allowance for power factor. Transformers are rated in kVA because heating depends on total current whether or not that current does useful work.
Approval drawing	The manufacturer's drawing issued after order and signed by the buyer before production starts. Typically four to five weeks, and it sits in front of the quoted lead time rather than inside it.
Bay-O-Net fuse	A replaceable fuse in an oil immersed holder accessible from outside the tank, used to protect the transformer without opening it. Usually paired with a current limiting backup fuse.
BIL Basic impulse level	The peak voltage a transformer's insulation can withstand from a lightning or switching surge, in kilovolts. A 15 kV class pad mount is normally 95 kV BIL on the high voltage side.
Bushing	An insulated terminal that carries a conductor through the tank wall without touching it. On a dead front pad mount the high voltage bushings take loadbreak elbows.
Dead front	Construction where no energised part is exposed when the cabinet is open. Connections are made with insulated loadbreak elbows. Standard on anything the public can reach.
Delta	A winding connection where the three windings form a closed triangle with no neutral point. Tolerates unbalanced load well and traps triplen harmonics.
DGA Dissolved gas analysis	A test that identifies developing faults inside a sealed transformer from the gases dissolved in its oil. Acetylene indicates arcing, ethylene serious overheating, hydrogen partial discharge.
DOE 2016	The federal efficiency standard for distribution transformers under 10 CFR Part 431, measured at 50 percent of nameplate load. Mandatory until April 2029. Applies up to and including 2500 kVA three phase.
Five legged core	A three phase core with two extra return legs, standard on pad mounted distribution transformers because it handles unbalanced and single phase loading without saturating.
FR3 Natural ester	A vegetable oil based insulating fluid with a fire point above 300 C, classified as less flammable and biodegradable. Costs more than mineral oil and is specified where fire or environmental rules demand it.
Full capacity taps	Taps that carry rated kVA at every position. Reduced capacity taps do not, which matters if the unit will run on a tap for long periods.
Grounded wye GrdY, GY	A wye connection with the neutral point bonded to ground. 12470 GY/7200 means 12470 volts phase to phase and 7200 phase to neutral on a grounded system.
Impedance %Z	The primary voltage needed to drive full load current through a shorted secondary, as a percentage of nominal. Sets voltage regulation and available fault current, and decides whether two units can be paralleled.
Live front	Construction where bushings and connections are exposed behind the door. Older style, still specified on some utility and industrial work.
Load loss Copper loss	Power lost in the windings, rising with the square of the current. Dominates on a heavily and constantly loaded unit such as a data centre transformer.
Loadbreak elbow	An insulated, deadbreak or loadbreak connector used to land a medium voltage cable on a dead front bushing. 200 A elbows are standard on pad mounted distribution units.
Loop feed	Six high voltage bushings so the unit sits in a loop and can be fed from either direction, letting a section be isolated without dropping every customer on the run.
No load loss Core loss, iron loss	Power consumed whenever the unit is energised, regardless of load. It runs 24 hours a day for the life of the transformer, which is why it dominates the running cost of a lightly loaded unit.
ONAN	Oil Natural Air Natural. The cooling class of a standard pad mounted transformer, meaning oil circulates by convection and air moves across the tank without fans.
Pad mount	A distribution transformer in a locked, tamper resistant steel cabinet, set on a concrete pad at ground level, serving an underground distribution system.
Radial feed	Two high voltage bushings, fed from one direction only. Cheaper than loop feed, with no alternate supply path.
Separately derived system	A system with no direct electrical connection to the supply conductors, which a two winding transformer creates. It gets its own neutral to ground bond at the transformer.
TAA Trade Agreements Act	Determines whether a product can be supplied on US federal contracts, based on country of manufacture. Only 32 of the 172 UL listed pad mount manufacturers are TAA eligible.
Taps	Connection points on the high voltage winding that adjust the ratio, normally two above and two below nominal at 2.5 percent each, to correct for a supply that runs persistently high or low.
Temperature rise	Average winding temperature rise above ambient at full load, in degrees Celsius. 65 C rise is the modern standard for liquid filled distribution transformers.
UL XPLH	The UL category covering liquid filled distribution transformers rated over 600 volts. A listing under XPLH is what lets a unit be accepted by a US inspector.
Wye Star	A winding connection where one end of each winding meets at a common neutral point, giving both a phase to phase and a phase to neutral voltage. Written 480Y/277 or 208Y/120.

28 terms. Free to cite with a link to this page. More detail on specifications, guides and this page as it grows.