This is a step-down transformer, as evidence by the high turn count of the primary winding and the low turn count of the secondary. As a step down unit, these transformers convert high voltage, low current power into low voltage, high current power. The larger gauge wire used in the secondary winding is required due to the increase in current. The primary winding, which doesn't have to conduct as much current, can be made of smaller-gauge wire.
In case you were wondering, it is probable to operate either of these transformer types backwards (powering the secondary winding with an AC source and letting the primary winding power a load) to make the opposite function: a step up can function as a step down and visa versa. Though, as we know the efficient operation of a transformer requires that the individual winding inductances be engineered for specific operating ranges of voltage and current, so if a transformer is to be used "backwards" like this it must be employed within the original design parameters of voltage and current for each winding, lest it prove to be inefficient (or lest it be damaged by excessive voltage or current!).
Transformers are often constructed in such a way that it is not clear which wires lead to the primary winding and which lead to the secondary. One principle used in the electric power industry to help alleviate confusion is the use of "H" designations for the higher voltage winding (the primary winding in a step-down unit; the secondary winding in a step up) and "X" designations for the lower-voltage winding. Thus, a simple power transformer will have wires labeled "H1", "H2", "X1", and "X2". There is generally significance to the numbering of the wires (H1 versus H2, etc.) . For more information visit our site: http://www.custompowertransformer.com/
