If your machine or control panel runs from a three-phase supply at 208 V, 380 V, 400 V or 480 V, the component that steps that power down to a safe level for contactors, PLCs, solenoids and instruments is a three phase control power transformer. Unlike a single-phase unit, it converts all three lines together and keeps the control network properly fed even when phase-to-phase loads such as motor-operated valves or heater banks switch on.
This guide covers what three-phase control transformers are, where they are used, how to size them with the correct kVA formula, and which wiring and standards questions to ask before you buy.
A three-phase control power transformer is a low-voltage, dry-type transformer that supplies control and auxiliary circuits from a three-phase network. Its primary connects to the line voltage (for example 3 × 380 V or 3 × 480 V), and its secondary provides a lower, isolated voltage such as 3 × 220 V, 3 × 110 V or 3 × 24 V for the control loop. The magnetic core carries three sets of primary and secondary windings on a common limb, so all three phases are transformed together.
Three things separate these units from general power transformers:
Ratings are usually expressed in VA or kVA rather than kW, because the load is a mix of resistive and inductive devices with a low and shifting power factor.
Anywhere a three-phase installation needs a dependable low-voltage control supply, a three-phase control transformer is a candidate. Common applications include:
In each case the transformer is sized for the sum of the control loads plus the inrush they draw when energized, not for the main power that the motor or heater consumes.
The choice between the two usually comes down to the supply available at the panel and whether the control loop itself must be referenced to three phases. We have covered the general decision in detail in our single-phase vs three-phase control power transformer guide; the table below summarizes the points that matter when a three-phase unit is under consideration.
| Aspect | Single-phase control transformer | Three-phase control transformer |
|---|---|---|
| Supply connection | One line-to-line or line-to-neutral pair | Three lines (delta or wye) |
| Typical secondaries | 120 V, 110 V, 24 V, 12 V | 3 × 220 V, 3 × 110 V, 3 × 24 V (often with neutral) |
| Sizing formula | VA = V × A | kVA = (VLL × A × √3) / 1000 |
| Typical ratings | 50–2500 VA | Usually 1 kVA and up |
| Balancing | Not applicable | Loads should be balanced across phases; allow margin for unbalance |
| Common failure symptom | Dropping voltage under coil inrush | Neutral shift and overheating of one winding when phases are unbalanced |
A three-phase unit can also supply single-phase loads by connecting across one phase and neutral, but only a fraction of its rated VA is then usable, so a dedicated single-phase model is usually the better value.
Sizing starts from the load, not from the nameplate of the machine. For three-phase circuits the correct relationship is:
kVA = (line-to-line voltage × full-load current × 1.732) / 1000
The factor 1.732 (√3) appears because in a balanced three-phase system the phase voltages and currents are not all at their peak at the same instant, so apparent power is higher than the single-phase product of voltage and current.
| Load type | Sizing note |
|---|---|
| Contactor and solenoid coils | Size for simultaneous inrush VA; sealed VA alone under-sizes the unit |
| Resistive heaters and lamps | Straight VA sum, no inrush allowance beyond cold-filament lamps |
| Small motors and actuators | Keep motor starting load within roughly 60% of rated capacity as a general practice |
| Electronics (PLC, instruments) | Add 20–25% margin and check for harmonic content in the load |
| Mixed panel | Sum by phase, balance the phases, then apply the worst-case inrush step |
If the terminology around VA, inrush and voltage regulation is new to your team, our control transformer VA rating guide explains each term with worked examples you can adapt to a three-phase calculation.
The most common arrangement for control and small power transformers is a delta-connected primary with a wye (star) secondary, designated Dyn11 or Dyn1 depending on the phase shift. A wye secondary provides a neutral point, which is useful when the control loop needs both 3 × 220 V and a 220 V phase-to-neutral supply. Yyn0 configurations, where both sides are wye, are also used when a neutral is required on the primary side.
Frequency and tolerance matter as much as voltage. A unit rated for 50/60 Hz operation is safe on either network, while a 50 Hz-only winding run at 60 Hz will typically have higher core loss. Confirm the available taps (commonly ±5%) so field voltage variations can be corrected at installation.
For compliance, look for products built and tested against the standards family that applies to your destination market: the IEC 61558 series and the UL 5085 series for low-voltage transformers, CSA C22.2 for Canada, and CE marking with RoHS compliance for the European Union. Ask the supplier for the actual test certificates (ratio, no-load loss, dielectric withstand and temperature-rise reports) rather than accepting a marketing claim.
When you shortlist suppliers, walk through this checklist:
For single-phase control loads on the same machines — which most panels also contain — the BK series control transformer (50–2500 VA), the JBK3 machine-control series and the BKJ5 series cover the most common 110 V, 220 V and 24 V control circuits. If your project needs a three-phase configuration, specify the vector group and kVA and confirm the build with the manufacturer before ordering; standard and customized units follow the same factory test routine.
A three-phase control power transformer is a small but critical part of any three-phase machine or panel: it supplies the control loop reliably, isolates operators from the line, and absorbs the inrush of coils and actuators that would otherwise drag the supply down. Size it from the real load inventory using the √3 kVA formula, add inrush and environmental margin, and verify the vector group, frequency and certifications before purchase. If you are unsure whether your application needs a single-phase or three-phase unit, or want help confirming a kVA calculation, talk to our engineering team with your load list and supply voltage and we will recommend a configuration that fits.
It supplies low-voltage control and auxiliary circuits — contactors, solenoids, relays, PLC and instrument supplies — from a three-phase line such as 380 V or 480 V, while isolating the control loop from the main supply.
Use the three-phase apparent power formula: kVA = (line-to-line voltage × current × 1.732) / 1000. Size for the steady-state VA plus the worst-case simultaneous inrush of coils and actuators, then select the next standard rating above the result.
A control transformer is optimized for the inrush of electromagnetic control devices and tight secondary voltage regulation, while an isolation transformer focuses on galvanic separation and noise attenuation. Many control transformers provide isolation as a secondary benefit. See our control transformer vs isolation transformer comparison for the full breakdown.
Technically one or two windings can be energized from a single-phase feed, but the usable output drops to a fraction of the nameplate rating and the remaining windings can overheat under load. If your supply is single-phase, choose a single-phase control transformer instead.
Typical primaries cover 208 V, 230 V, 380 V, 400 V, 415 V and 480 V at 50 or 60 Hz, often with ±5% taps. Confirm the exact vector group, frequency and tap range on the datasheet for the unit you plan to buy.
