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How to Wire a Step Down Transformer

G.K. Bayne

Step down transformers are used to reduce alternating current. One connection of the transformer, called the high side, inputs a high voltage electrical current. The output side then reduces the electricity into a useful voltage for such things as controlling electrical devices for industry. Step down transformers are individually rated by the manufacturer. The ratings can be found on the identification tag of the transformer. As not all transformers are wound the same, the individual schematic on the tag should be followed. By adhering to a basic procedure and process, regardless of the schematic, a step down transformer can be safely and quickly terminated.

  1. Observe and identify the schematic and rating of the step down transformer to be installed. For the large amperage type, adequate circuit protection must be in place before any wiring practice is performed. The same holds true for the smaller step down control transformer.

  2. Remove all electrical power for the transformer circuit. Be sure that both sides of the circuit protection are in the “off” position. When power is reapplied, only one side can be energized at a time. In other words you will only be energizing the high side power feed of the transformer first.

  3. Remove the terminal connection box cover placed at the lower side of the transformer. Only the high amperage types will have this enclosure, while lower powered transformers will have an exposed screw terminal. The bolt size will vary by manufacturer and types of bolts may vary from metric to SAE.

  4. Know termination identification follows for all step down transformers: H1, H2, H3 and H4 signify the high voltage side or power feed end of the transformer. This holds true regardless of the size of the transformer. Interconnection of the transformer will vary depending on the manufacturer and voltage used for feeding the transformer. Follow the individual schematic for the type of transformer to be installed.

  5. Terminate the feed power wires first by cutting the wires to length. If you are using large wire lugs be sure to take into consideration the length of the lug and the amount of wire that can be inserted into the female crimp area. The same holds true for smaller wires on the smaller control transformer for the use of eye rings.

  6. Strip back the outer insulating of the wires with the pocketknife or wire strippers.

  7. Insert the eye ring or wire lug over the bare copper wire and crimp the connection device, using the appropriate-size crimper, permanently to the wire.

  8. Terminate the high side, high voltage of the step down transformer. If the high side terminals are bolts, be sure to follow any torque requirements that are listed by the manufacturer.

  9. Terminate the low side, low voltage of the transformer. Note these terminals will be identified by X1, X2, X3 and X4. Again follow the manufacturer's individual schematics for that particular type of transformer. Note that on small control transformers there will only be an X1 and X2. X1 is the power or “hot” side and X2 is generally the grounding and neutral portion of the low voltage.

  10. Follow the same termination process for the wires as described in step 7 and 8 above.

  11. Terminate the small control transformer for X1 and X2. X1 will go directly to the control circuit after passing through a small fuse that is rated for the circuit. X2 will be terminated not only to the neutral side of the control circuit, but the grounding safety as well. In other words, the X2 side of the small control transformer must be tied to the grounding system of the electrical circuit.

  12. Replace all covers on the transformer and any enclosures that protect you from electricity.

  13. Apply the high voltage to the transformer by switching on the feeder power circuit.

  14. Turn on the low side safety circuit control.

  15. Use a volt meter to test for proper voltage on the step down side of the transformer. It should be the same that is listed on the specs tag provided by the manufacturer.

  16. Warning

    Follow all national, state and local codes when installing any type of electrical device.

    Follow all manufacturer's schematics when terminating step down transformers--connections may vary depending on applications of the transformer.