Selecting Transformer
Before Selecting a drytype Transformer
 Determine is it three phase or single phase.
 Determine power line voltage and frequency it will be primary voltage for your transformer.
 Determine voltage for your application it will be secondary voltage of the transformer.
 To determine the capacity required in voltamperes.
This is where our VoltAmperekWatt calculator will be very handy.
Put load current (amperes), load voltage choose single or three phase and you get capacity of the required transformer kVA (kilovoltamperes). Always select the transformer larger than the actual load. This is done for safety purposes an allows for expansion.This is not all, you'll have to consider when choosing a transformer:
 Primary voltage: delta or wye (usually dictated by the utility).
 Secondary voltage (480V for small facilities) or deltawye.
 3,4,or 5 wire configuration (4 is the most common).
 Impedance.
 Temperature rise.
 Location, space requirement.
 Indoor or outdoor.
 Accessories (including taps) and instrumentation.
 Basic impulse level, which is the withstand rating in kV.

If you've got to design the power distribution for an entire building, you'll have to select the service transformer and the distribution transformers. First you must determine the largest load you can expect at any given time. Example, you don't normally run electric heat and air conditioning at the same time. so your calculations would include only the larger of the two loads. You might not operate all equipment simultaneously either, however you would run some loads at the same time. Determine what those loads arebased on operational requirements, then determine the kVA each will draw. Once you done the individual calculations and added them up to arrive at the maximum load, you should add in factors for reasonable unplanned load growth (20% is common) and for planned addition load.
Based on these criteria, you can select the correct transformer for your job.
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