Breaker accessory control-power equations
Operating demand and continuous holding demand are calculated separately before applying supply margin.
VApeak = max(VAtrip event, VAmotor event)VAcontinuous = ΣVAholding + VAotherVAsupply >= max(VApeak, VAcontinuous) × FmarginI = VA/Vwhere:
- VApeak
- Governing short-duration operating demand[VA]
- VAcontinuous
- Continuous control-supply demand[VA]
- Fmargin
- Entered supply design factor
The power supply's transient overload curve and each accessory's exact pickup waveform still govern selection.
How to calculate breaker accessory control power
Enter the exact operating and holding VA from each accessory datasheet. Choose whether all operating loads can coincide or only the largest credible event occurs, then add continuous loads and the supply margin.
- Confirm the common control voltage and AC/DC type.
- Enter shunt-trip operating VA.
- Enter undervoltage-release pickup and holding VA separately.
- Enter motor-operator and continuous auxiliary loads.
- Check both transient and continuous supply capability.
Peak VA versus continuous VA
Shunt trips and motor operators usually create short operating demands, while undervoltage releases can impose continuous holding demand. A control transformer or DC power supply must support the governing event without excessive voltage dip and also carry the steady load thermally.
VAsupply >= max(VApeak, VAcontinuous) × design margin
I = VA/V
Accessory compatibility checks
Matching voltage is not enough. The shunt trip, undervoltage release, motor operator, auxiliary contacts, mounting hardware, terminals, and wiring must be approved for the exact breaker series and frame.
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Breaker accessory power example
If the combined trip and undervoltage pickup event is 200 VA while a motor operator needs 300 VA, largest-event mode uses 300 VA. A 125% margin produces a 375 VA supply reference, while the undervoltage holding coil and auxiliaries remain a separate continuous-load check.
Assumptions
- Entered VA values come from exact accessory datasheets at the selected control voltage
- Largest-event mode assumes motor operation does not coincide with the combined trip/release event
- Power-supply transient capability is represented by VA only as a first screen
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Check AC/DC type, voltage tolerance, pickup duration, inrush waveform, duty cycle, wiring voltage drop, fuse protection, and power-supply overload curve.
- Every accessory must match the exact breaker series, frame, mechanism, mounting, and terminal arrangement.