How to size a control transformer for contactor coils, relays and auxiliary loads: inrush, VA rating, derating and wiring tips for panels.
A control transformer is the small power source that feeds a panel's control circuit: contactor and relay coils, indicator lamps, timers, PLC I/O and small auxiliaries. It isolates the 24 V, 110 V or 220 V control side from the main 400 V/480 V distribution and keeps the coil supply steady when the power circuit is heavily loaded or starting a motor. Without a correctly sized control transformer, coils drop out under inrush, PLCs reset, and a single phase imbalance can take a whole machine offline.
The rating that matters is volt-amperes (VA), not watts. A control transformer carries two VA figures you must respect:
Size the transformer so its continuous VA exceeds the summed steady load with about 25% headroom, and its inrush VA covers the largest simultaneous coil pickup. If several contactors close together, add their inrush, not just one.
| Device | Typical steady VA | Typical inrush VA |
|---|---|---|
| AC contactor coil (small, 220 V) | 40–70 VA | 300–600 VA |
| AC contactor coil (large, 220 V) | 100–200 VA | 800–1500 VA |
| Intermediate relay coil | 3–8 VA | 10–20 VA |
| Panel indicator lamp (LED) | 1–3 VA | negligible |
| Small PLC / sensor supply | 20–60 VA | negligible |
Most panels use a single-phase primary taken from two lines of the 400 V system (or 230 V line-to-neutral). A three-phase primary is chosen when the control load is large or phase-loss immunity is required: if one phase fails, a three-phase transformer still delivers reduced but usable secondary voltage, while a single-phase unit goes dead. For typical panels up to a few hundred VA, a single-phase control transformer is cheaper and simpler. Above about 1 kVA, or where continuity matters, choose a three-phase primary.
The secondary voltage decides how sensitive your control circuit is to noise and drop. 24 V AC or DC is the global default for safety and PLC compatibility, but at the same power it draws higher current, so conductor cross-section and voltage drop over long runs matter more. 110 V (or 120 V) control reduces current and is common in North American and some industrial panels; it tolerates longer cable runs with less drop but needs better isolation from the mains. 220 V secondary is rarely used for control but still appears in legacy machines. Match the secondary to the coil voltage of every device you listed in the sizing step — mixing 24 V and 110 V on one transformer requires a dual-secondary unit.
A transformer in a sealed, sun-exposed or densely packed enclosure runs hot. Roughly every 10 °C above the rated ambient halves insulation life, and continuous overload quietly ages the winding. If your panel routinely exceeds 40 °C, derate by 10–20% or specify a higher VA class. Vibration and humidity also matter: in dusty or coastal sites choose a vacuum-impregnated or resin-encapsulated unit.
Most control transformers use stacked EI laminations: cheap, robust, easy to mount on a DIN rail, and tolerant of intermittent overload. Toroidal types are smaller and quieter with lower stray field, but cost more and need care when fusing. For harsh sites — high humidity, vibration, dust or corrosive atmosphere — choose a resin-encapsulated or vacuum-impregnated unit so the windings cannot absorb moisture or loosen. Encapsulation also improves short-circuit withstand and reduces audible hum, which matters in control rooms.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.