DRY-TYPE DISTRIBUTION TRANSFORMERS

Steps for Selecting the Proper Transformer

1. SINGLE PHASE LOADS

1. Determine electrical load

A.Voltage required by load.

B.Amperes or KVA capacity required by load.

C.Frequency in Hz (cycles per second).

D.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

A.Voltage of supply (source).

B.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 secondaryvoltages matching those you have just determined.

A. Select a transformer with a standard KVA capacity equal to or greater than

that needed to operate the load.

B. 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.)

C. 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 KVA and volts 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 fixture 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 volts = .156 KVA

1000

For each lighting fixture

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.

TABLE 1

Full Load Current in Amperes–Single Phase Circuits

TABLE 2

Full Load Amperes Single Phase A.C. Motors

When motor service factor is greater than 1, increase full load amps proportionally.

Example: If service factor is 1.15, increase above amp values by 15%

2. THREE PHASE LOADS

1. Determine electrical load

A. Voltage required by load.

B. Amperes or KVA required by load.

C. Frequency in Hz (cycles per second).

D. Verify load is designed to operate on three phase.

All the above information is standard data normally obtained from equipment

nameplates or instruction manuals.

2. Determine supply voltage

A. Voltage of supply (source).

B. Frequency in Hz (cycles per second).

The frequency of the line supply and electrical load must be the same. A three

phase transformer is selected which is 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 the group of transformers

with primary and secondary voltages matching that which you have

just determined.

A. Select a transformer with a standard KVA capacity equal to or greater than

that needed to operate the load.

B. 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.)

C. When load ratings are given only in amperes, tables 3 and 4 or the following

formulas may be used to determine properKVA size for the required transformer.

Three Phase Example

Question: Select a transformer to fulfill the following conditions. Load is a three phase induction motor, 25 horsepower @ 240 volts, 60 Hz and a heater load of 4 kilowatts

@ 240 volts single phase. The supply voltage is 480Y/277, three phase, 4 wire.

Answer: Compute the KVA required. Motor—From table 4 the current is 68 amps.

240 volts x 68 amps x 1.73 = 28.2 KVA

1000

(The KVA can also be obtained from table 4).

Heater — 4 KVA

A three phase transformer must be selected so that any one phase is not overloaded.

Each phase should have the additional 4 KVA rating required by the heater even though

the heater will operate on one phase only. So, the transformer should have a minimum

KVA rating of 28.2 + 4 + 4 + 4 or 40.2 KVA. Refer to the appropriate selection chart.

A 480 delta primary— 240 delta secondary transformer may be used on a 4 wire, 480Y/277 volt supply. The fourth wire (neutral) is not connected to the transformer.

To not overload the transformer, a 45 KVA transformer should be selected.

NOTE: Any two wires of the 240 volts, 3 phase developed by the secondary of the

transformer may be used to supply the heater. Any 2 wires of a 3 phase system

is single phase.

TABLE 3

Full Load Current in Amperes–

Three Phase Circuits.