To wire a 3-phase 45 kVA 480V Delta to 208Y/120V Wye transformer, land the primary phases (Brown, Orange, Yellow) on terminals H1, H2, and H3, and the secondary phases (Black, Red, Blue) on X1, X2, and X3. The secondary White neutral must land on X0 and be bonded to the transformer ground bus via a System Bonding Jumper. This guide assumes standard US NEC color codes and a 45 kVA mixed commercial load.

Decision Path: Sizing and Selecting Your 3-Phase Transformer

Before stripping wire, you must match the transformer topology to your supply and load. The most common botch in 3-phase transformer installations is selecting the wrong winding configuration for the downstream panel. Use this decision tree to lock in your hardware.

Supply Voltage Downstream Load Requirement Required Transformer Topology Concrete Hardware Pick
480V 3-Phase 3-Wire 208V 3-Phase motors + 120V single-phase receptacles Delta Primary to Wye Secondary 45 kVA Delta-Wye (e.g., Eaton V12T45T or Square D EE45T3H)
208V 3-Phase 4-Wire 480V 3-Phase industrial HVAC or machinery Wye Primary to Delta Secondary 30 kVA Wye-Delta Step-Up
480V 3-Phase 3-Wire 480V 3-Phase motors only (no 120V/277V loads) Delta Primary to Delta Secondary 45 kVA Delta-Delta (Isolation)
Default Recommendation: If you are feeding a standard commercial subpanel with mixed 208V 3-phase and 120V single-phase loads from a 480V utility or main switchgear, choose a 45 kVA Delta-Wye transformer with 75°C rated copper/aluminum lugs and a removable System Bonding Jumper (SBJ). This is the industry workhorse for commercial tenant spaces.

Tools, Materials, and Conductor Sizing

Transformer sizing is governed by NEC Article 450, while conductor sizing relies on NEC Article 310. For a 45 kVA transformer, we calculate the full load amps (FLA) to determine wire gauge and overcurrent protection.

The Math (45 kVA Load)

  • Primary FLA (480V): 45,000 VA / (480V × √3) = 54.1 Amps.
  • Secondary FLA (208V): 45,000 VA / (208V × √3) = 124.9 Amps.

Materials List

  • Primary Conductors: 4 AWG THHN Copper (Rated 85A at 75°C). Note: If your lugs are only rated 60°C, you must upsize to 3 AWG.
  • Primary Breaker: 70A 3-pole molded case circuit breaker (MCCB).
  • Secondary Conductors: 1/0 AWG THHN Copper (Rated 150A at 75°C).
  • Secondary Breaker: 150A 3-pole MCCB.
  • Neutral Conductor: 1/0 AWG THHN Copper (White or Gray). Must match phase conductor size for Wye secondaries per NEC 220.61.
  • Equipment Grounding Conductor (EGC): 6 AWG Copper (Green) for primary; 6 AWG Copper for secondary (per NEC Table 250.122 based on 150A breaker).
  • System Bonding Jumper (SBJ): 8 AWG Copper (per NEC Table 250.102(C)(1) for 1/0 AWG secondary conductors).

Required Tools

  • CAT IV True-RMS Digital Multimeter
  • Calibrated torque screwdriver and torque wrench (inch-pound rated)
  • Non-contact voltage tester (NCVT)
  • Wire strippers and rotary cable cutter for 1/0 AWG
  • Phase rotation meter

Mandatory Mains Safety and LOTO Protocol

WARNING: LETHAL VOLTAGE. A 480V primary supply carries immense arc flash and electrocution risk. Working on energized 3-phase gear is strictly prohibited for unqualified persons and requires a documented energized electrical work permit even for qualified electricians.
  1. De-energize the upstream 480V feeder breaker.
  2. Apply a physical Lockout/Tagout (LOTO) device to the breaker handle (OSHA LOTO standard 1910.147).
  3. Test your CAT IV meter on a known live source to verify the meter works.
  4. Verify dead: Measure phase-to-phase and phase-to-ground on the incoming 480V lines. The meter must read 0.00V.
  5. Test your meter on the known live source again to ensure it didn't blow a fuse during the dead test.

NEC-style guidance; your local Authority Having Jurisdiction (AHJ) or facility safety officer has final authority on arc flash PPE requirements.

Step-by-Step Termination: Primary and Secondary

With the system verified dead, proceed with terminations. Strip insulation to the exact length required by the transformer lug manufacturer (usually 1.25 inches for 1/0 AWG). Do not nick the copper strands.

Primary Side (480V Delta)

  1. Phase A: Land the Brown 4 AWG THHN wire on terminal H1. Torque to the manufacturer's specification (typically 120 in-lbs for 4 AWG).
  2. Phase B: Land the Orange 4 AWG THHN wire on terminal H2. Torque to spec.
  3. Phase C: Land the Yellow 4 AWG THHN wire on terminal H3. Torque to spec.
  4. Ground: Land the Green 6 AWG EGC on the transformer chassis ground lug (usually marked with a ground symbol). This is not a neutral; the Delta primary has no neutral.

Secondary Side (208Y/120V Wye)

  1. Phase A: Land the Black 1/0 AWG THHN wire on terminal X1. Torque to spec (typically 180 in-lbs for 1/0 AWG).
  2. Phase B: Land the Red 1/0 AWG THHN wire on terminal X2. Torque to spec.
  3. Phase C: Land the Blue 1/0 AWG THHN wire on terminal X3. Torque to spec.
  4. Neutral: Land the White 1/0 AWG THHN wire on terminal X0. Torque to spec.
  5. System Bonding Jumper (SBJ): Install the 8 AWG Green (or bare) jumper directly from the X0 terminal to the transformer's equipment ground bus bar. This is a separately derived system requirement per NEC Article 250.30. Without this, your 120V circuits will float and destroy electronics.
  6. Secondary Ground: Land the Green 6 AWG EGC (running to the downstream panel) on the transformer equipment ground bus bar.
  7. Earth Ground: Run a Grounding Electrode Conductor (GEC) from the transformer ground bus to the building's main grounding electrode system (e.g., cold water or ground rods).
Pro Tip: Never land the SBJ and the downstream panel neutral on the same lug. The SBJ bonds X0 to ground *inside* the transformer. The downstream panel must have an isolated neutral bar; the neutral and ground must remain strictly separated in the subpanel fed by this transformer.

Verify and Test: Expected Meter Readings

Remove LOTO, clear the area, and re-energize the primary 480V breaker. Set your True-RMS multimeter to AC Volts (minimum 600V rating).

Primary Verification (Incoming)

  • Measure H1 to H2: Expect 480V (acceptable range 456V - 504V).
  • Measure H2 to H3: Expect 480V.
  • Measure H3 to H1: Expect 480V.

Secondary Verification (Outgoing)

  • Line-to-Line (208V): Measure X1-X2, X2-X3, X3-X1. Expect 208V on all three combinations.
  • Line-to-Neutral (120V): Measure X1-X0, X2-X0, X3-X0. Expect 120V on all three combinations. If you read 208V here, your X0 neutral is missing or broken.
  • Neutral-to-Ground: Measure X0 to the chassis ground bus. Expect 0.0V to 1.5V. If you read 120V here, your System Bonding Jumper is missing or loose.

Phase Rotation Check

Connect your phase rotation meter to X1, X2, and X3. The meter should indicate Clockwise (ABC) rotation. If it reads Counter-Clockwise (CBA), de-energize and swap any two secondary phase conductors (e.g., swap X1 and X2) to correct the rotation for 3-phase motors.

The Most Common Botch: Floating the X0 Neutral

The single most frequent and destructive error when wiring a 3-phase Wye secondary is failing to install the System Bonding Jumper (SBJ) between X0 and the ground bus, or failing to run a dedicated Grounding Electrode Conductor to earth.

The Symptom

You energize the transformer and measure 208V line-to-line perfectly. However, when you measure line-to-neutral at the downstream subpanel, your 120V receptacles read randomly between 90V and 145V. When a large 208V motor kicks on, the 120V lights flicker violently, and sensitive IT equipment throws overvoltage/undervoltage faults or suffers fried power supplies.

The Physics of the Failure

Without the SBJ bonding X0 to ground, the neutral point of the Wye secondary is "floating." It is no longer locked to 0V earth potential. The capacitive coupling between the transformer windings and the unbalanced single-phase 120V loads cause the neutral point to shift wildly away from the electrical center of the phasor diagram. By installing the 8 AWG SBJ from X0 to the ground bus, you force the neutral point to remain at 0V relative to earth, locking the line-to-neutral voltage at a stable 120V regardless of how unbalanced the downstream single-phase loads become. Always verify the SBJ is torqued to spec before closing the transformer enclosure.