Transformer Principles

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Transformers are electrical devices that are used to change or "transform" the voltage of an electrical current. They are composed of copper windings which in either series or parallel either step up or step down the incoming voltage of the transformer. The incoming voltage is called the primary the outgoing voltage is called the secondary.
The common voltages in North America are as follows:

• Single Phase
• 120 volts
• 120/240 volts 1ø, 3w
• 240 volts

• Three Phase
• 120/208 volts 3ø, 4w
• 480 volts
• 277/480 volts 3ø, 4w
• 600 volts

Also, the higher voltages that are commonly used by the utility companies for power transmission are as follows:

• 2400/4160 volts 3ø, 4w
• 12,470volts 3w
• 13,800volts 3w

Copper Windings

The voltage, amperage and impedance are determined by the number of turns on the primary and secondary windings of the transformer. Typically the impedance remains constant. Therefore the output voltage and amperage is determined by the number of turns on both the primary and secondary sides of a transformer.

A transformer's windings may be connected in a number of ways. The most commonly used transformer connections are Delta and Wye:

When using a wye-delta system, the higher voltage is connected in series and the lower voltage is connected in parallel.

Single Phase vs. Three Phase Output

Single Phase output from a transformer typically involves 120/240 volts 1ø, 3w. In commercial applications, this type of power is typically seen in hotel or condominium units, as those resemble a residential environment. This type of single phase power is also seen in certain industrial applications, such as a millwork shop where the equipment must be rated a minimum 230 volts.

Single phase 120/240v output is neither a wye nor delta system. This means that on the secondary side of the transformer there will be 120volts between either one of the phases:

In order to size the transformer and the overcurrent protection that the phases are connected to, click here.

Three phase transformers are more complicated than single phase. Click here to read about Three Phase Transformers.

Three Phase 120/240v Delta-Connected System (High Leg)

If you have been installing or designing this type of system, I promise you won't like what I have to say. I do not recommend using this system in any application. At one time this system was considered the "cheap" three phase. But the danger and potential damage caused by having unbalanced loads would far exceed the upfront cost to bring another phase conductor into the building. For more information about this, review NEC article 450 to read about the pitfalls of 120/240v 3ø, 3w systems.

The basics of transformers have been discussed in this section. For more information, return to the main Transformers page to see the other relevant transformer data.

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