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D r y - T y p e D i s t r i b u t i o n T r a n s f o r m e r s

Transformer Selection Steps

Single Phase Loads

1. Determine electrical load

Voltage required by load.

Amperes or kVA capacity required by load.

Frequency in Hz (cycles per second).

Verify load is designed to operate on a single phase supply.

All of the above information is standard data normally obtained from equipment nameplates or instruction manuals.

2. Determine supply voltage

Voltage of supply (source).

Frequency in Hz (cycles per second).

The frequency of the line supply and electrical load must be the same. Select single phase transformer designed to operate at

this frequency, having a primary (input) equal to the supply voltage and a secondary (output) equal to the voltage required by

the load.

3. If the load nameplate expresses a rating in kVA, a transformer can be directly selected from the charts. Choose from a

group of transformers with primary and secondary voltages matching those you have just determined.

Select a transformer with a standard kVA capacity

equal to or greater than

that needed to operate the load.

Primary taps are available on most models to compensate for line voltage variations.

(Refer to question #2 in the Transformer Questions and Answers Section on page 6.)

When load ratings are given only in amperes, tables 1 and 2 or the following formulas may be used to determine proper kVA size

for the required transformer.

(1)

To determine

kVA

when volts and amperes are known:

kVA =

Volts x Amps

1000

(2)

To determine

Amperes

when volts and amperes are known:

Amps =

kVA x 1000

Volts

Single Phase Example

Question:

Select a transformer to meet the following conditions. Load is single phase lighting using incandescent lamps. Each fix-

ture requires 1.3 amps @ 120 volts, 1 phase, 60 Hz, power factor of unity. The installation requires 52-100 watt fixtures. The desired

circuit distributing power to the light fixtures is 120/240 volt, three wire, single phase. The supply voltage is 460 volt, 3 phase.

Answer:

Compute the kVA required.

1.3 Amps x 120 V

= .156 kVA for each lighting fixture

3 phase

Always use amps x volts to compute VA, never use lamp wattage. .156 kVA/Fixture x 52 Fixture = 8.11 kVA. The two sizes (kVA)

nearest 8.11 kVA are 7.5 kVA and 10 kVA. Use the 10 kVA. This will not overload the transformer and allows some capacity, 1.89

kVA, for future loads. Since the supply is 460 V (not 480 V) use the 456 V tap. This will produce approximately 120 volts on output. If

the tap is not used, the output will be 115 V compared to the desired 120 V. Note the transformer selected is single phase but the

supply is 480 V, 3 phase. Single phase is obtained by using any 2 wires of the 3 phase supply.