ATS current and architecture rules
ATS screening begins with maximum load current and design margin, then selects class, poles, transfer architecture, and short-circuit rating requirements.
IATS = Iload × KdesignATS WCR/SCCR ≥ available fault currenttoutage = detection + source availability + transferwhere:
- IATS
- Minimum ATS continuous-current basis[A]
- Iload
- Maximum transferred load current[A]
- Kdesign
- Entered design factor
- WCR/SCCR
- Short-circuit withstand/closing or equipment rating under specified conditions[kA]
Switch movement time is only one part of an outage. Generator start, voltage and frequency stabilization, intentional delays, and load ride-through must be considered separately.
How to size an ATS
Enter maximum transferred load current and the justified design factor, then identify supply conductors, neutral switching, integrated protection, alternate source, load ride-through, and available fault current.
- Calculate required continuous current.
- Choose PC or CB class.
- Determine 2P, 3P, or 4P switching.
- Match transfer architecture to load tolerance.
- Verify WCR/SCCR and upstream protection.
PC class versus CB class
PC class is primarily a transfer device that makes and withstands short-circuit current while coordinated upstream protection clears faults. CB class uses breaker-based switching and can provide fault interruption within the complete device rating.
Transfer time and critical loads
Mechanical transfer in hundreds of milliseconds can suit lighting, HVAC, pumps, and general distribution. PLCs and IT equipment may require local hold-up, UPS, or STS support. A mechanical ATS cannot bridge generator start time or provide no-break power alone.
Pole and short-circuit checks
Neutral switching depends on source bonding, separately derived source treatment, earthing, RCD behavior, and local requirements. The ATS WCR/SCCR must be valid at available fault current with the exact upstream protective device and clearing conditions.
Assumptions
- Low-voltage open-transition ATS is the normal starting architecture
- Maximum load current already reflects realistic demand
- WCR/SCCR and protective-device coordination are checked from product data
Important Warnings
- Switching time is not total outage time; generator start and stabilization usually dominate restoration time.
- Closed transition, source paralleling, neutral switching, WCR/SCCR, and selective coordination require engineered approval.