Common transformer installation issues

Improper Secondary Ground
If the secondary of the transformer is not grounded properly, the output voltage will look ok between the phases but it will float and not be referenced to earth ground.

Back-Feeding Delta Primary/Wye Secondary Transformers
While a base wye secondary transformer can be field modified to backfed, the field modifications may violate U.L., NEC or local code and the transformer’s warranty. Don’t back-feed delta/wye transformers.

Back-Feeding Transformers above 1 kVA
Back feeding larger transformers can result in high inrush currents upon transformer energization and nuisance tripping of circuit breakers and fuses. Due to a number of factors which affect inrush, this issue is difficult to predict and costly to fix. The best way to handle this is to purchase transformers wound as step-up. If this isn’t feasible, transformers should be sized to the maximum amperage protection allowed by code, the larger the transformer, the more potential for this to occur.

Power Wires Routed over the core and coils
The are being ventilated through the core and coils can be very hot, in excess of 100oC. This can cause wire insulation failure.

Power Wires terminated in the bottom of the transformer compartment
Conduit should not be terminated in the bottom of the transformer with a grated floor. The grated floor is needed to provide airflow to cool the transformer but the grates provide a poor surface to mount a coupling and may also violate NEC code.

Missing Vibration Pads or Vibration Isolators
All transformers vibrate at 120 hz because of the electromagnetic field in the core. These vibrations and audible noise can transfer through the floor, vibration pads and isolators help to minimize this issue in commercial applications.

Missing Drip Shields
While all outdoor applications need a minimum of a NEMA 3R enclosure, even indoor applications near sprinklers would require a minimum NEMA 2S enclosure and therefore drip shields.

Transformer Harmonic Heating
Due to the prevalence of non-linear loads and the harmonics they produce, transformers can overheat if not specified properly. As a rule of thumb, if a load contains 25-50% non-linear sources, use K=4, if a load exceeds 50% non-linear sources use K=13.

Transformer Ambient Heating
Transformers need to be placed in locations that allow proper ventilation to remove the heat they produce during normal operation.

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