Control Transformer Sizing
Control transformer sizing determines the VA (volt-amp) rating of the step-down transformer that supplies a machine's control circuit — typically 230/400V down to 110V AC or feeding a 24V DC power supply — by totaling the steady-state and inrush loads of every device the transformer must support without nuisance tripping or voltage sag.
- Supplying 110V AC control circuits for contactor coils, relays, and pilot devices.
- Feeding the primary side of a 24V DC SMPS that powers PLC, sensors, and I/O.
- Isolating control circuits from the main power distribution for safety and noise immunity.
- Sizing transformers for machines with multiple simultaneous contactor or solenoid inrush events.
Sizing starts with the sum of continuous VA loads (relay coils, indicator lamps, PLC power supply input), then adds the inrush VA contribution of the largest simultaneously-energized inductive loads, since transformer momentary overload capacity is limited. Transformers are typically sized with 20–25% spare capacity beyond calculated peak load to accommodate future additions and to avoid operating continuously near the transformer's thermal limit. Primary and secondary fusing must also be coordinated with the transformer's rated current and inrush characteristics.
- Sizing only for steady-state load and ignoring simultaneous contactor/solenoid inrush current.
- Leaving no spare capacity, forcing a transformer replacement when a single device is added later.
- Mismatching primary/secondary fuse ratings with the transformer's actual inrush and continuous current.
- Using a single transformer for both control power and a noisy variable-speed drive control circuit without isolation.
- Ignoring voltage drop on long secondary runs feeding remote control stations.
ClusterVise totals the connected control-circuit VA load directly from the generated I/O and device list, applies the standard spare-capacity margin, and proposes a matched transformer and fuse pair — instead of requiring a separate manual load tabulation late in the design.
| Item | Selection | Basis |
|---|---|---|
| Steady-state load | 180 VA (relays, PLC PSU, indicators) | Sum of continuous connected control loads |
| Inrush contribution | +220 VA momentary | Largest simultaneous contactor coil inrush event |
| Calculated peak | 400 VA | Steady-state plus inrush allowance |
| Selected transformer | 500 VA control transformer | Includes ~25% spare capacity margin |
| Fusing | Primary 2A, secondary 5A | Coordinated to transformer inrush and rated current |
