How to Wire a Three-Phase Transformer for 208V

Introduction

Wiring a three-phase transformer for 208V can seem daunting, but with some basic electrical knowledge and the right materials, it can be a straightforward DIY project. A three-phase transformer allows you to step down higher voltage from the utility company to 208V that can power motors and other equipment.

Benefits of 208V Three-Phase Power

• 208V is an efficient voltage for running industrial equipment like large motors and pumps. It provides more power than 120V circuits while using less amperage than higher voltages.

• Three-phase power allows for constant, uninterrupted power delivery to equipment. If one phase is lost, the other two can continue supplying power.

• Three-phase transformers offer versatility. You can connect equipment to match 208V three-phase or derive 120V single-phase power from the transformer.

• 208V three-phase systems are used in many commercial and industrial settings. Wiring for 208V allows you to power specialized equipment.

Step-by-Step Guide

Wiring a three-phase transformer is not complicated, but attention to detail is required. Follow these key steps:

Select an Appropriate Transformer

• Choose a transformer rated for the voltage and capacity you need. A standard configuration is 480V primary, 208V secondary.

• Use a three-phase transformer with four windings (H1, H2, H3, and X3). This gives you three 208V phases and a grounded neutral.

• Make sure the transformer can supply enough kVA for your intended loads. Allow extra capacity for future expansion.

Install Mounting and Enclosure

• Mount the transformer on a sturdy base or platform near your electrical service panel.

• The transformer must be protected in a weatherproof enclosure if outdoors. Indoors, an open mounting or wire cage is acceptable.

Connect Primary Windings

• The primary side connects to your incoming 480V three-phase utility service or motor generator.

• Use properly sized wiring for the rated transformer amperage. Connect phase 1 to H1, phase 2 to H2, and phase 3 to H3.

• Install a disconnect switch and overcurrent protection on the primary feeding the transformer.

Connect Secondary Windings

• The secondary 208V output will feed your panels and equipment. Connect neutral X3 to ground.

• Run three sets of output wires from H1, H2, and H3. Use colored wire coded for 208V three-phase.

• Connect the secondary wires to lugs on a 208V distribution panel or directly to equipment.

Install Grounding and Bonding

• Ground the transformer enclosure according to local codes. Use a ground rod if mounted outside.

• Bond neutral X3 to ground at the transformer and distribution equipment. This completes the grounding path.

Check Voltages and Make Final Connections

• Check line-to-line and line-to-neutral voltages before energizing. You should read 208V and 120V.

• Verify proper equipment rotation. Correct if needed by swapping two input phase connections.

• Inspect all connections and replace covers on the transformer enclosure before regular operation.

Wiring Diagram

Here is a basic wiring diagram showing how to connect a 208V three-phase transformer:

Conclusion

With the right preparation and safety precautions, wiring a transformer for 208V three-phase power is a manageable project for a qualified electrician. Following the steps for proper selection, mounting, grounding, and phasing is key. The result is reliable power supply for running larger motors and equipment that require the benefits of 208V three-phase. Take time to thoroughly plan and install your transformer wiring to avoid issues down the road.