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STS/ATS Transfer Switch Selection | Engineering Database

STS/ATS Transfer Switch Selection

electrical

Selects static or automatic transfer switches for critical power applications. Filters by device type, ampere rating, voltage, fault current, and manufacturer preference; returns top 3 candidates with loading percentage, heat output, kVA capacity, and cooling requirements.

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Load Amps*

Total continuous load current the transfer switch must carry. Size for maximum expected load, not average. Typical: 100-600A for datacenter PDU feeds, 800-3000A for main switchgear.

Voltage

System operating voltage in volts. Common values: 208, 480, or 600 V for commercial/industrial facilities. If omitted, defaults to 480 V.

Phases

Number of electrical phases. Typically 3 for commercial/industrial, 1 for small branch circuits. Standard: 3-phase for all commercial/datacenter applications. If omitted, defaults to 3.

Device Type

Transfer switch type. STS (Static Transfer Switch) for sub-cycle transfer (~4 ms); ATS (Automatic Transfer Switch) for slower, lower-cost transfer (~100 ms). If omitted, defaults to STS.

Max Transfer Time Ms

Maximum acceptable source transfer time in milliseconds. Defaults to 4 ms for STS and 100 ms for ATS if not specified. Critical IT loads typically require <4 ms.

Available Fault Current Ka

Available symmetrical fault current at the switch location in kA. From short-circuit study. Switch SCCR must meet or exceed this value per NEC 110.9. If omitted, no fault current filtering is applied.

Preferred Manufacturer

Manufacturer name or partial match to filter results. E.g. 'Eaton', 'ABB', 'Schneider'. Case-insensitive substring match. If omitted, all manufacturers are considered.

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Reference data

This calculator pulls live values from the following engineering reference tables in the database.

sts ats specs

About this calculator

STS/ATS Transfer Switch Selection is part of the Engineering Database calculator suite. Each tool runs deterministic calculations against authoritative reference data and code-compliant methods, so results are reproducible and traceable to source standards.

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