Automatic transfer switches for standby power · Withstand and closing rating by size, 3-cycle vs 30-cycle, open vs closed transition, bypass-isolation, service-entrance rating and phases
An automatic transfer switch (ATS) moves a load from the normal supply to a standby source such as a generator when the normal supply fails, and back again when it returns. Ask three things first: the load current and the available fault current at the switch, since the switch must close onto and withstand that fault; whether the transfer may interrupt the load (open transition) or must parallel the sources briefly (closed transition); and whether the switch also serves as the service entrance or needs a bypass for maintenance.
Automatic transfer switches for standby and emergency power in buildings, industry and homes, low voltage and medium voltage, including bypass-isolation switches and dual-power (PC and CB class) switches. Manual transfer switches, uninterruptible power supplies and generator paralleling switchgear are outside this page except where compared.
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01
Russelectric's sales guide tabulates UL 1008 closing and withstand ratings in RMS symmetrical amperes at 480 V AC: 3-cycle ratings of 42,000 A for 100–400 A switches, 65,000 A for 600–800 A, 85,000 A for 1,000–1,200 A, 100,000 A for 1,600–3,000 A and 125,000 A at 4,000 A, and 30-cycle ratings of 30,000, 42,000, 65,000, 85,000 and 100,000 A for the same sizes, the last to be confirmed with the factory. It lists UL 1008 and 1008A listed designs, 30-cycle rated ATSs and bypass-isolation switches, and withstand ratings applicable with any maker's circuit breakers. [1]
Generac's residential standby transfer switches show the smaller end: automatic switches of 100, 150 and 200 A, offered as service-entrance rated (some with integrated meter, load centre or surge protection) or non-service-entrance rated, in single- and three-phase and 50 Hz versions, alongside manual transfer switches. [2]
Editorial calculation: at 60 Hz, 3 cycles is 50 ms and 30 cycles is 500 ms; for a 400 A switch, a 42,000 A 3-cycle rating is about 105 times its rated current. Compare the rating and its duration with the available fault current and the clearing time of the upstream protective device. [1]
Give the supplier the system voltage, phases and frequency, continuous load current, available fault current and the upstream protective device and its clearing time, number of poles (switched neutral or not), transition type (open, delayed, closed), bypass-isolation needs, service-entrance use, controller functions (voltage and frequency sensing, time delays, exerciser, communications), enclosure type and rating, listing (UL 1008 or IEC 60947-6-1), and quantity.
02
Russelectric offers open, closed and programmed transitions, a mechanical interlock for open transition, an over-centre mechanism, an external manual operator and an outdoor-rated controller with colour display. Its systems cover emergency generator control with load control or utility paralleling, prime power, cogeneration and microgrids, at low voltage (600 V) and medium voltage (5 kV and 15 kV), with emergency or maintenance bypass switches and UL 1558 or 891 low-voltage construction for paralleling gear. [1]
Editorial note: ask the supplier which transition types the switch and its controller support, what the transfer time is at your settings, how the bypass switch is operated during maintenance, and whether closed transition needs approval from your electricity utility.
ABB's UL solution guide for its TruONE ATS, aimed at OEMs and panel builders, notes that automatic transfer switches for legally required standby systems must meet the requirements for transfer switches in emergency systems, with UL 1008 as the applicable safety standard; a builder offering its own listed product must either obtain authorisation to use the maker's UL file data and its withstand and closing rating file, or construct, test and list the complete solution itself. TruONE integrates the controller with a detachable HMI in the switch, which ABB says replaces up to 20 connection wires and cuts installation time by up to 80%, and covers 200–480 V AC with ±20% tolerance. [3]
Editorial note: if a panel builder will house the ATS, agree at quotation who holds the UL 1008 (or local) listing for the finished assembly and which WCR file applies with your upstream breaker. [3] [1]
03
Suppliers linked to this page list automatic transfer switches; each card links to the page reviewed.
Listings include PC-class and CB-class dual-power ATSs, solid-state transfer switches, manual and automatic transfer switches, and transfer switch equipment; ask each for the withstand and closing rating, utilisation category, transfer time, pole count and listing.
04
The US Tri-Service Electrical Working Group's TP-9 (2008) says an ATS is continuously exposed to system full-load current and must carry the maximum continuous load; it suggests considering one rated as large as its upstream protective device. Designs with integral overcurrent protective devices might be limited to 80% of the ATS rating, and above 40 °C ambient the ATS might require derating. TP-9 cites UL 1008 as requiring an ATS to withstand inrush currents of 20 times its continuous current rating; for total system loads, incandescent lamps cannot exceed 30% of that rating. [4]
For motor loads in open transition, TP-9 sets residual motor voltage below 0.25 per unit before connecting the other source: 1 to 2 seconds of decay suffices, but it advises a conservative 5 to 10 second delay, or a sequenced motor restart. It advises against phase angle monitors, which are not guaranteed to transfer in phase, and specifies four-pole ATS designs so the neutral is switched for ground fault protection. [4]
Editorial calculation: for a 400 A switch, the UL 1008 inrush requirement cited by TP-9 is 20 × 400 = 8,000 A; compare the X/R ratio at the switch with the one used in its certification test. [4]
05
This is a functional reference built from the sources cited; it is not one maker's specification.
| Element | Role | Fix in the order |
|---|---|---|
| Switching mechanism | Over-centre mechanism moves contacts. [1] | Poles, rating. |
| Interlock | Mechanical interlock for open transition. [1] | Interlock type. |
| Controller | RPCTS controller with colour display. [1] | Sensing, delays, comms. |
| Manual operator | Transfers by hand if needed. [1] | External or internal. |
| Bypass-isolation (option) | Allows maintenance without outage. [1] | Bypass type. |
| Enclosure | Indoor or outdoor. [1] [2] | Rating, service entrance. |
06
An ATS quotation needs values for your installation. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Listing standard | UL 1008 for automatic transfer switches. [3] | Listing for the finished assembly. |
| Rated current (A) | 100–200 A residential; 100–4,000 A C&I. [2] [1] | Continuous rating. |
| Rated voltage (V AC) | 480 V table; LV 600 V and MV 5/15 kV systems; 200–480 V ±20% in one integrated ATS. [1] [3] | Voltage. |
| Transfer time (s) | Not stated in reviewed sources. | Time at your settings. |
| Switching capacity (kA) | WCR 30–125 kA by size and duration. [1] | WCR with your breaker. |
| Operating temperature (°C) | Not stated in reviewed sources. | Range. |
| Degree of protection | Outdoor-rated controller option. [1] | Enclosure rating. |
| Mechanical endurance (cycles) | Not stated in reviewed sources. | Cycles. |
| Electrical endurance (cycles) | Not stated in reviewed sources. | Cycles. |
| Control voltage (V AC/DC) | Integrated controller in one design. [3] | Supply. |
| Frequency (Hz) | 50 Hz versions offered (Generac). [2] | 50 or 60 Hz. |
| Number of poles | Single- and three-phase versions. [2] | Poles, switched neutral. |
| Weight (kg) | Not stated in reviewed sources. | Weight. |
07
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
System: [voltage; phases; frequency] Load: [continuous current] Fault: [available fault current; upstream device] Transition: [open, delayed, closed] Options: [bypass-isolation; service entrance; switched neutral] Controller: [sensing; delays; exerciser; comms] Listing: [UL 1008 or IEC 60947-6-1]
These companies list automatic transfer switches.
Lists PC-class and CB-class automatic transfer switches.
Product scope checked 2026-10-11
Lists automatic transfer switches with circuit breakers.
Product scope checked 2026-10-11
Lists automatic and manual transfer switches.
Product scope checked 2026-10-11
Lists an automatic transfer switch series.
Product scope checked 2026-10-11
Lists dual-power automatic transfer switches.
Product scope checked 2026-10-11
Lists a solid-state automatic transfer switch among solid-state relays.
Product scope checked 2026-10-11
Lists automatic transfer switches with surge protectors.
Product scope checked 2026-10-11
Lists dual-power automatic transfer switches.
Product scope checked 2026-10-11
Lists automatic transfer switch equipment with breakers and fuses.
Product scope checked 2026-10-11
Lists automatic transfer switches with circuit breakers.
Product scope checked 2026-10-11
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Russelectric lists 3-cycle closing and withstand ratings from 42 kA (100–400 A switches) to 125 kA (4,000 A) at 480 V; compare them with the available fault current at the switch. [1]
Russelectric offers open, closed and programmed transitions, with a mechanical interlock for open transition. [1]
Generac offers residential ATSs rated for use as the service entrance, some with an integrated meter or load centre, alongside non-service-entrance models. [2]
ABB's TruONE UL guide cites UL 1008 as the safety standard for automatic transfer switches; switches in legally required standby systems must meet the emergency-system requirements. [3]
The US Tri-Service Electrical Working Group's TP-9 says motor residual voltage should fall below 0.25 per unit; 1 to 2 seconds is enough, but it advises a 5 to 10 second delay to be conservative. [4]
Sources checked 2026-10-12. Russelectric (Siemens) and Generac make transfer switches and describe their own ranges under UL 1008; IEC markets use IEC 60947-6-1 classes and terms.
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