Skip to main content

What is Core Loss?

Losses in watts caused by magnetization of the core and its resistance to magnetic flux when excited or energized at rated voltage and frequency. Also referred to as excitation loss or no-load loss.

  • Can wire or cable be located above a transformers core and coil

      HPS recommends side or bottom entry for conduit into a type 3R, 12, 4 and 4X enclosure. Neither U.L. or NEC addresses this specifically with transformers. C.S.A. C22.2 No. 47 does address this and disallowed cable entry from the top of the enclosure. The issue with insulated cables is a transformer cools by convection with air moving from the bottom to the top of the coils. The air exiting from the top of the coils can be heated significantly higher than ambient. Cables which enter through the top of the enclosure will experience this heated air which causes three issues:

      • Cable insulation damage within the enclosure.
      • Hot air entering the conduit can cause insulation damage with the conduit.
      • Hubs must be used to prevent water intrusion

      Because of these issues, HPS does not recommend that cable enter through the top of a transformer’s enclosure in any application. Note that some of these issues can be mitigated if using non-jacketed hard bussing instead of cable.

  • 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.

      More troubleshooting

  • Can conduit be installed through the top of an enclosure

      HPS recommends side or bottom entry for conduit into a type 3R, 12, 4 and 4X enclosure. Neither U.L. or NEC addresses this specifically with transformers. C.S.A. C22.2 No. 47 does address this and disallowed cable entry from the top of the enclosure. The issue with insulated cables is a transformer cools by convection with air moving from the bottom to the top of the coils. The air exiting from the top of the coils can be heated significantly higher than ambient. Cables which enter through the top of the enclosure will experience this heated air which causes three issues:

      • Cable insulation damage within the enclosure.
      • Hot air entering the conduit can causing insulation damage with the conduit.
      • Hubs must be used to prevent water intrusion

      Because of these issues, HPS does not recommend that cable enter through the top of a transformer’s enclosure in any application. Note that come of these issues can be mitigated if using non-jacketed hard bussing instead of cable.

  • What is the difference in enclosures for indoor and outdoor non-hazardous applications?