A transformer changes electricity from one voltage to another. In a commercial building, the utility's transformer brings the supply down to the building's service voltage, and dry-type transformers inside the building step 600 V down to the 120/208 V that receptacles and small equipment need. Transformers are simple, reliable and long-lived, but they need the right size, ventilation and grounding to stay that way.
Most building owners never think about their transformers until one hums loudly, runs hot or fails. Because they sit quietly in electrical rooms for decades, they are easy to forget during renovations, when new loads get added to a transformer that was sized for the original tenant.
What a transformer does
A transformer has two sets of windings on a shared iron core. Alternating current in the primary winding creates a changing magnetic field in the core, which induces a voltage in the secondary winding. The ratio of turns between the two windings sets the ratio of the voltages. There is no wire connecting the primary to the secondary; power passes through the magnetic field.
That last point matters for safety and design. Because the secondary side is not electrically tied to the primary, it is treated as a new system. It needs its own neutral, its own grounding and bonding, and its own overcurrent protection on the secondary conductors.
The common delta-wye transformer
The most common building transformer takes 600 V three-phase in on three wires, connected in a triangle called a delta, and puts out 120/208 V on a star-shaped connection called a wye, which has a centre point that becomes the neutral. That neutral is what lets one transformer serve both 208 V equipment, connected between phases, and 120 V receptacles, connected between a phase and the neutral.
Getting this grounding right is one of the details most often checked during inspections. The neutral is bonded to the transformer enclosure and the grounding system at one point only, usually at the transformer or the first disconnect after it. Bonding the neutral to ground again further downstream, a common mistake in renovations, lets normal neutral current flow on metal conduit and equipment frames, where it can cause shocks, interference with sensitive equipment and nuisance tripping of ground fault protection.
Types you will find in buildings
| Type | Where it is used | Things to know |
|---|---|---|
| Ventilated dry-type | Electrical rooms inside buildings, the most common type | Air-cooled through vents; needs clearance and a cool, clean room |
| Cast resin dry-type | Indoor sites with dust, moisture or higher reliability needs | Windings sealed in resin; more costly, more tolerant of harsh rooms |
| Liquid-filled | Outdoor pad-mounts, utility and large services | Oil or other fluid for cooling and insulation; needs containment and fluid testing |
| Buck-boost and small control transformers | Correcting voltage for one machine, control circuits | Small, but still need correct protection and labelling |
Sizing and placement
Transformers are rated in kVA, the apparent power they can deliver continuously without overheating. A designer sizes one from the calculated demand of the panels it feeds, plus room for growth. Too small and it runs hot and ages quickly; much too large and it costs more to buy, takes more space and wastes energy, because every transformer has core losses whenever it is energized, even with no load.
Placement matters as much as size. Dry-type transformers give off heat, so the room needs ventilation or cooling to carry it away. They hum at a low pitch, which travels through floors and walls, so designers keep them away from offices and bedrooms and mount them on vibration isolators. They need working clearance in front and air space around the vents, which is often lost when electrical rooms become storage rooms.
A transformer's impedance, printed on the nameplate as a percentage, also affects the rest of the system. It limits how much fault current can flow on the secondary side, which in turn affects the ratings needed for downstream panels and the arc flash results.
Efficiency
Federal energy efficiency regulations set minimum efficiency levels for many dry-type transformers sold in Canada. Older units installed before current rules can have noticeably higher losses. When an old transformer is lightly loaded, those losses are a steady cost day and night, which is one reason to consider replacement during major renovations.
Harmonics
Computers, LED drivers and variable frequency drives draw current in short pulses rather than smooth waves. These harmonic currents heat transformers more than the same amount of ordinary load. Where harmonic loads are heavy, designers may choose transformers rated for them (often called K-rated) or size up.
Keeping transformers healthy
- Keep vents clear and the room free of storage, dust and moisture.
- Check that room temperature stays within the manufacturer's limits, especially in summer.
- Include transformers in infrared thermography scans of electrical equipment under load.
- Have connections, grounding and enclosure condition checked during planned maintenance shutdowns.
- For liquid-filled units, follow the manufacturer's schedule for fluid sampling and testing.
- Listen for changes in noise, and note any burning smell or discolouration.
Maintenance on transformers is done de-energized by qualified electrical workers. Opening energized transformer enclosures exposes people to serious shock and arc flash hazards.
A typical problem
A tenant in an older office building complains that the electrical closet on their floor is hot and smells of warm dust. The closet holds a dry-type transformer and two panels. Over several renovations, the floor has gained a kitchen, a server rack and more equipment, all fed from the same transformer. Storage boxes are stacked against its vents. A thermographic scan shows the transformer running much hotter than it should. The fix is in three parts: clear the vents and stop the room being used for storage, add ventilation to the closet, and rebalance or move some of the load. A load calculation shows whether the existing transformer can carry the remaining load or whether a larger one is needed.
How ehZee can help
We size and place transformers as part of new distribution designs and assess existing ones when loads change. See our power distribution design service, and read about grounding and bonding and power quality and harmonics.
Common questions
How long do dry-type transformers last?
Commonly several decades when they are kept within their temperature limits. Heat is the main thing that shortens insulation life, so overloading and poor ventilation are the usual causes of early failure.
Why does our transformer hum?
All transformers hum because the core vibrates slightly with the alternating magnetic field. Louder humming than usual can mean loose mounting, missing isolators or overloading, and is worth having checked.
What information is on the nameplate?
Usually the kVA rating, primary and secondary voltages and connections, impedance, temperature rise, insulation class and the manufacturer's details. Record it during surveys; it is needed for load, short-circuit and arc flash calculations.
Can we add load to an existing transformer?
Only after checking its rating against the measured and calculated demand of everything it feeds. Panels downstream may also need checking. A load calculation answers the question before the work is designed.