Contactor load-current equation
Three-phase motor input current is calculated before applying a duty margin and utilization category.
I = Pout / (√3 × VLL × pf × η)Iselection = I × Fdutywhere:
- I
- Estimated load current[A]
- Pout
- Motor output power[W]
- η
- Motor efficiency
- Fduty
- Preliminary operating-duty margin
Final AC-1 or AC-3 selection uses the exact manufacturer voltage and kW table.
How to use the Contactor Selection Calculator (AC-1 / AC-3)
Estimate a contactor current class from load type, power, voltage, utilization category, and operating frequency. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Three-phase load AC-3 applies to normal starting and switching off a running squirrel-cage motor A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Three-phase load
- AC-3 applies to normal starting and switching off a running squirrel-cage motor
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. Frequent inching, plugging, reversing, capacitor, transformer, or lighting duty needs the matching utilization category and manufacturer tables. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Three-phase load
- AC-3 applies to normal starting and switching off a running squirrel-cage motor
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Frequent inching, plugging, reversing, capacitor, transformer, or lighting duty needs the matching utilization category and manufacturer tables.