Primary winding = used for input into a transformer
Secondary winding = Divided into segments so the output voltage depends on which segments are used.
Single phase transformers may have two or three wire secondaries.
two-wire secondary = One of the wires is a neutral(ground)
three-wire secondary = two segments, one lead at one end of the secondary. second lead connected to the midpoint secondary and grounded
third lead is connected to the other end of the secondary
If 240 volt is required then the first and third lead is used
If 120 volt is required then the ground and only one of the other leads are used
Two basic types of connections:
Wye (Y) = shaped like letter Y
Delta = shaped like a triangle
a neutral connection would be taken from the center of a Wye or midpoint of one of the Delta's secondary windings.
illustratively two parallel lines represent the iron core.
Neutral wire is always grounded for safety
typical system voltages are 120/208 & 270/480, lower number is the line-to-neutral voltage, higher number is line-to-line voltage.
Note ratio of the voltages is 1.73 (square root of 3)
Residential is limited to 120/240 volt single phase systems, from a three wire secondary.
If larger load required (range, AC, refrigerator, etc.) the line to line voltage is used at 240 volts.
Receptacles and switches are at 120 volts
Small commercial: Typically 120/240 volt single phase OR 120/208 three phase service.
Larger commercial: Higher voltage three phase service, like 277/480 volts or 2400/4160 volts.
Typical Secondary Voltages
Single Phase:
120v 2 wire (2 wires)
120/240v 3 wire (3 wires)
Three Phase:
240v Delta (3 wires)
120/240v Delta (4 wires)
480v Delta (3 wires)
208v Wye (3 wires)
120/208v Wye (4 wires)
480v Wye (3 wires)
277/480v Wye (4 wires)
2400/4160v Wye (4 wires)
4160v Delta (3 wires)
4160v Wye (3 wires)
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