The selection depends mainly on:

1. Load type: Resistive (constant resistance, variable resistance) or inductive (transformer).
2. Firing mode: Zero-cross for standard resistive loads; phase-angle for transformers and non-linear resistance elements.
3. Number of phases: Single-phase, three-phase 2-leg (open delta), or three-phase 3-leg (full three-phase).
4. Current rating: At least 20–30 % above the full-load current, considering cold-resistance inrush.
5. Control mode: Decide if you need V², I², P control, or simple open-loop.
6. Communication/protection requirements: Modbus, Profibus, heater break alarm, etc.

Our application engineers can help you size the perfect unit.