Control Transformer Load Calculator
Compare continuous control-circuit VA and simultaneous startup burden with a user-entered control transformer rating.
Nameplate and load data
Use the manufacturer or project design limit. This is not a built-in derating recommendation.
Sum the actual simultaneous continuous coil, relay, and control loads from equipment data.
Enter the additional startup VA for loads that start together. Use manufacturer inrush data; do not count a startup load twice.
Control transformer load
About this calculator
Enter the nameplate VA rating and voltages, a usable capacity limit from the manufacturer or project design, continuous control burden, and additional startup burden for loads that start together. The calculator compares the summed VA demand with the usable rating and reports rated primary and secondary current.
Use manufacturer VA data for the connected devices. This tool does not infer inrush from horsepower, coil type, or contactor size.
Method and assumptions
Usable transformer capacity = nameplate VA × entered usable-capacity percentage. Simultaneous demand = continuous load VA + additional simultaneous startup VA. Headroom = usable capacity − simultaneous demand. Rated winding current = nameplate VA ÷ winding voltage.
The calculation treats the entered VA values as additive and assumes the entered startup value is additional to the continuous load. It does not model voltage regulation, time-current behavior, or loads that start in a sequence.
Interpret the result
A demand within the entered capacity means only that the arithmetic total fits the user-specified usable rating. Verify pickup voltage, transformer thermal duty, control-circuit voltage drop, simultaneous operation, protective devices, and manufacturer requirements separately.
FAQs
Where should the load VA values come from?
Use control-device and transformer manufacturer data for the actual coils, relays, contactors, and connected equipment. Distinguish sealed VA from inrush VA and identify which loads start at the same time.
Should I enter inrush VA as the full startup load?
Enter only the additional startup burden that occurs above the continuous load already counted. Follow the manufacturer’s method for simultaneous loads and do not count the same coil twice.
Does a passing VA comparison guarantee proper operation?
No. The worksheet does not calculate voltage sag, transformer regulation, coil pickup voltage, wiring drop, thermal duty, or control sequence. Verify those conditions using the equipment documentation and project design.
Does this size the transformer or its protection?
No. It compares entered load VA with an entered usable capacity and reports rated winding current. It does not select a transformer, conductor, fuse, or circuit breaker.
Sources
- Use the transformer nameplate and manufacturer documentation for VA rating, winding voltage, temperature limits, and usable capacity.
- Use connected control-device manufacturer data for sealed VA, inrush VA, pickup requirements, and simultaneous operating conditions.