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Reverse feeding medium voltage transformers

Eaton does not recommend reverse feeding medium voltage transformers because of the below:

  • Eaton recommends not back-feeding transformers with voltages above 600 volts or above 75 kVA three-phase or above 25 kVA single-phase 
  • Transformers generally 3 kVA and below have winding resistance that can cause voltage drops up to 5%. Designers compensate the winding ratios to account for this voltage drop. When a transformer is back-fed, the compensated winding ratio works in reverse adding to the resistive voltage drop resulting in lower output voltages.
  • All transformers have an inrush current when energized lasting for a few cycles and generally 4-12 times the transformers full load amperage (FLA) rating. This occurs because the magnetic field has not fully formed during the first cycle and provides no mitigation to current flow. Back-fed transformers, regardless of manufacturer, can have significantly larger inrushes sometimes over 40 times FLA. Inrush can cause nuisance tripping of circuit breakers and fuses, voltage drops and stress the insulation if high currents are encountered. The potential inrush problem can increase if the transformer is k-rated, low temperature rise or high efficiency and is independent of load. Larger kVA sizes are not recommended to be back-fed because of this issue. System issues caused by back-fed transformer inrush are not covered by warranty.
  • A neutral can not be derived from a typical back-fed transformer; standard transformers do not have four wire primaries. Back-fed transformers may also develop large fault currents in the event of a catastrophic fault due to lower impedance. When back-feeding a delta-wye transformer, the ground strap normally provided between the XO terminal and the case ground must be removed which potentially voids the warranty. Eaton does not recommend the back-feeding of delta-wye transformers and can provide transformers built specifically for step-up applications.
  • Some inspectors may not allow the installation of back-fed units. Eaton can not guarantee how the inspectors will interpret local regulations.

For a selection of Eaton's dry type transformers please see our Catalog Volume 2 Tab 2