How to Wire a Three-Phase Transformer

1. Read the nameplate before you bend a stick of pipe

Everything flows from the nameplate: kVA, voltages, impedance, tap positions, connection diagram, clearances. Photograph it and the diagram inside the wiring compartment. Do the current math now: primary FLA = kVA × 1000 ÷ (480 × 1.732). For 75 kVA: 75,000 ÷ 831 = 90.2 A. Secondary FLA = 75,000 ÷ (208 × 1.732) = 208 A.

Check clearances too — NEC 450.9 requires ventilation openings stay clear, and most dry-types want several inches off the wall per the manufacturer. Mount it where you can actually open the compartment and land conductors.

2. Size the primary OCPD per NEC 450.3(B)

Table 450.3(B) governs transformers 1,000V and under. With primary-only protection, the primary device is capped at 125% of primary FLA: 90.2 A × 1.25 = 112.8 A. Note 1 lets you round up to the next standard size from 240.6(A) — a 125 A breaker. (Because this install also has a secondary OCPD, the table would allow the primary device up to 250% — a hard ceiling with no round-up — but 125% is the usual conservative choice.)

Remember 450.3 protects the transformer, not the conductors — Article 240 governs those. Size the primary feeder to the 125 A device: No. 1 AWG Cu at 130 A (75°C column, Table 310.16), or No. 2 AWG Cu at 115 A under the 240.4(B) next-size-up allowance. Run an equipment grounding conductor per Table 250.122: No. 6 AWG Cu.

3. Size the secondary OCPD and conductors

The primary breaker can't properly protect the secondary conductors of a delta-wye unit, so you need a secondary device. Per Table 450.3(B) it can be up to 125% of secondary FLA: 208 A × 1.25 = 260 A; in practice a 250 A main breaker in the downstream panel is the common choice.

Secondary conductors run under the 10-foot or 25-foot rules in 240.21(C)(2) or (C)(6) — they must terminate in a single OCPD that limits current to their ampacity. Size for the 250 A device: 250 kcmil Cu at 255 A (75°C, Table 310.16). Keep the run short; unfused secondary conductors are a real fault hazard.

4. Establish the separately derived system — NEC 250.30

This is the step that fails inspections. The secondary needs its ground reference built from scratch at the transformer (or the first disconnect — pick one, never both). Three connections land at XO: the secondary neutral, the system bonding jumper, and the grounding electrode conductor.

The system bonding jumper connects XO to the enclosure, sized from Table 250.102(C)(1) based on the secondary phase conductors — No. 2 AWG Cu for 250 kcmil secondaries. It's what gives a downstream ground fault a path back to the source; without it, the breaker never trips. Many units ship with a factory strap from XO to the case — verify it's installed and sized right, don't assume.

The grounding electrode conductor runs from XO to the building or structure grounding electrode system per 250.30(A)(4) — the 2023 NEC dropped the old nearest-electrode preference — and 250.68(C) lets building steel or interior metal water pipe extend that connection. Size it from Table 250.66: No. 2 AWG Cu for 250 kcmil secondaries. Critically: the neutral lands on an isolated neutral bar in the downstream panel — no bonding screw. One bond point, period.

5. Land the primary — delta connections and taps

Kill and verify the primary feeder, then land H1, H2, H3 per the nameplate diagram. A delta primary has no neutral — three phase conductors plus the EGC, which bonds to the enclosure ground lug. Strip to the manufacturer's gauge marks and use listed lugs.

Set the taps before you button up. Most dry-types offer taps in 2.5% steps above and below nominal — follow your nameplate. If measured primary voltage runs 466V instead of 480V, a −2.5% tap brings the secondary back near 208V. Change taps only fully de-energized, set all three phases to the same position, and record it.

6. Land the secondary — wye connections

Land X1, X2, X3 and the neutral at XO per the diagram. Confirm the 250.30 work while the compartment is open: neutral, system bonding jumper, and GEC all at XO, jumper bonded to the case, GEC headed to a real electrode. Route conductors clear of the coils, and use flexible conduit for the last connection — dry-types hum at 120 Hz, and rigid raceway telegraphs that vibration through the building. Maintain the EGC path through or around the flex.

7. Torque every termination to spec

NEC 110.14(D) requires terminations tightened to the manufacturer's torque value with an appropriate tool — a torque wrench, not feel. The values are on the lug, the compartment label, or the instruction sheet; they vary by lug and manufacturer, so use the number printed on your equipment. Torque the primary lugs, secondary lugs, XO connections, bonding jumper, GEC, and EGC, and record the values. Loose terminations are the top cause of transformer connection failures.

8. Pre-energization checks and startup

Before energizing: megger primary-to-ground, secondary-to-ground, and primary-to-secondary against the manufacturer's acceptance values, pull any shipping braces and debris from the enclosure, confirm the secondary main is open, and put every cover back on.

Energize the primary with the secondary unloaded. Expect inrush — a dry-type can momentarily draw many times full-load current. Measure secondary voltage: roughly 208V phase-to-phase and 120V phase-to-neutral on all three legs, balanced within about 1–2%. If it's off, de-energize, lock out, and adjust taps. Then close the secondary main, verify rotation on downstream three-phase equipment, and come back after a day under load to check termination temperatures.

Why is the transformer secondary called a separately derived system?

Because the secondary winding has no direct electrical connection to the primary supply — power transfers magnetically. The secondary has no ground reference until you create one per NEC 250.30 with a system bonding jumper and a grounding electrode conductor.

Where do I bond the neutral on a 480V to 208Y/120V transformer?

At exactly one point — either at the transformer XO terminal or at the first disconnecting means, never both. Most installers bond at the transformer and keep the downstream panel neutral isolated.

Do I need overcurrent protection on the secondary?

For a delta-wye transformer, yes in practice. The primary breaker cannot properly protect the secondary conductors, so you install a secondary device — typically a main breaker in the downstream panel sized up to 125 percent of secondary full-load current per Table 450.3(B).

How do I calculate full-load amps for a three-phase transformer?

Divide the kVA times 1,000 by the line-to-line voltage times 1.732. A 75 kVA unit draws about 90 amps on the 480V primary and about 208 amps on the 208V secondary.

What are transformer taps for and when do I change them?

Taps adjust the primary winding ratio, usually in 2.5 percent steps, to compensate for site voltage above or below nominal. Change them only with the transformer de-energized and locked out, set all three phases to the same position, and re-measure secondary voltage.

Why does the transformer hum, and is it a problem?

The hum is magnetostriction in the core, a normal 120 Hz vibration on a 60 Hz system. A sudden change in the sound, or hum with heat or odor, is worth investigating.