What exactly are Source 1 and Source 2?
Utility, generator, inverter output, or another supply—plus voltage, phase, frequency, grounding, and control interface.
Automatic transfer switch buying guide
What to Know Before You Buy an Automatic Transfer Switch
Do not buy an ATS from its ampere rating alone. Define the two sources, backed-up loads, voltage and phase, fault duty, poles and neutral, transition sequence, enclosure, controls, and destination-market evidence before approving a model.
2 sources, 1 defined loadMap normal, alternate, and load terminals on the project one-line.
Ratings stay togetherKeep current, voltage, utilization, fault duty, poles, and standard with one catalog number.
Faster is not always betterTransition behavior must fit the load, source, and approved operating sequence.
Evidence before priceCompare exact drawings, certificates, protection conditions, controls, and included accessories.
“Generator Transfer Switch Full.” Photo by Robert.Harker / Wikimedia Commons, CC BY-SA 3.0; cropped and darkened for layout, with this adaptation shared under the same license. The image does not prove any rating, pole count, transition mode, or certification.
Quick purchase gate
Four facts must exist before a quotation can be compared
If one fact is unknown, mark it “confirm before approval.” Do not let a seller fill a design gap with a marketing assumption.
Which loads transfer, and what can wait?
List running demand, inrush, duty, restart order, shedding, and the maximum acceptable interruption.
What must the switch carry and withstand?
Record continuous current, utilization, maximum fault duty, alternate-source protection behavior, and enclosure conditions.
Which exact records support the ordered part?
Require the order code, standard, certificate or listing record, ratings, drawing, protection conditions, and controller manual.
Direct answer
Buy the ATS as part of a defined transfer system
The right automatic transfer switch is the exact model that fits the approved two-source architecture and passes every required rating, control, environmental, assembly, and market-access check. The largest ampere number or fastest transfer time does not make a model suitable.
Start with a load schedule and one-line diagram. Then select the product standard and system arrangement. Only after that should you compare switch class, current, voltage, poles, transition mode, short-circuit data, controller, enclosure, terminals, accessories, and price.
Practical buying rule
If a proposal cannot show how the exact order code fits the one-line, upstream protection, neutral design, operating sequence, and target market, it is not ready for purchase approval.
Scope: SENTOP can support product matching, options, drawings, certificate documents, OEM details, samples, and supply. The qualified project team remains responsible for the load calculation, fault study, earthing, protection, service arrangement, installation, settings, testing, and authority approval.
Choose the architecture first
A standard ATS normally transfers one load between two defined sources
IEC 60947-6-1:2026 covers complete transfer switching equipment within its scope. It does not cover multi-source, load-shedding, bus-tie, hybrid, overlapping-neutral, or static-transfer arrangements as standard TSE categories. Do not assume one conventional ATS or one controller certificate proves the complete source combination.
Conventional ATS certification does not evaluate inverter anti-islanding, grid interconnection, export control, power-control functions, energy management, or battery-system safety. Under IEC, static transfer systems use the IEC 62310 family instead. UL 1008 excludes solid-state/static transfer equipment; other UL routes can apply to defined solid-state ATS, source-interconnection, and distributed-energy systems. Never connect an ATS to raw battery DC unless the exact poles, polarity, making and breaking duty, isolation, and complete-system approval cover that DC application.
Define Source 1
Record system voltage, phase, frequency, available fault current, earthing, and upstream protective device.
Define Source 2
Document generator or inverter operating mode, capacity, start interface, neutral arrangement, and protection limits.
Define the ATS duty
State product standard, class, transition, current, poles, fault data, controls, enclosure, and terminals.
Define the outcome
Identify critical circuits, interruption tolerance, load shedding, restart order, and commissioning acceptance.
The 12-spec purchase check
Keep each rating with its standard and conditions
A model passes only when all required fields are known. A catalog family headline cannot fill a blank cell.
Product standard and market
Choose the applicable route, such as IEC 60947-6-1 or UL 1008, plus local installation and assembly rules.
Ask for: exact certificate, listing, or declaration record.Source architecture
Name both sources and confirm the complete, manufacturer-supported arrangement. A two-source ATS is not a generic multi-source controller.
Ask for: approved one-line and sequence.Voltage, phase, and frequency
Match both sources, controller sensing, power terminals, pole connection, and AC/DC duty. Never infer a DC rating from an AC marking or use a standard AC ATS on raw battery DC.
Ask for: Ue, frequency, phase, and wiring diagram.Operational current and load
Use the calculated design load and applicable correction rules. For IEC selection, keep Ie with Ue, utilization category, and stated environment; frame size or Ith alone is not enough.
Ask for: rated operational current under stated conditions.Switch class and protection
Class PC and Class CB describe different short-circuit functions under IEC. Neither label is a universal quality ranking.
Ask for: class, protective device, and trip responsibility.Fault-current duty
For UL, match the exact WCR and its protective-device conditions. For IEC, use only the declarations that apply to the exact model, voltage, and configuration: Icw with its time, Icm as a peak value, and Iq only with the named SCPD. For Class CB, also verify breaking capacities, releases, and settings.
Ask for: applicable value, time, voltage, device conditions, and configuration.Poles, neutral, and earthing
Choose solid, switched, or separately validated overlapping-neutral behavior from the system design. Never switch protective earth or treat a PEN as an ordinary neutral.
Ask for: pole sequence, bonding, and RCD/GFP/IMD diagram.Transition sequence
Open transition—including delayed and in-phase open-transition variants—or closed transition changes interruption, motor behavior, source requirements, and approvals.
Ask for: timed sequence and failure behavior.Controller and interfaces
Define sensing, control supply, start contacts, delays, manual modes, alarms, communications, and safe state.
Ask for: I/O ratings and controller manual.Enclosure and installation
Specify temperature, humidity, altitude, pollution, corrosion, dust/water exposure, mounting, cable entry, and service access.
Ask for: exact IP or NEMA Type and derating data.Terminals and assembly fit
Confirm conductor metal, size, class, quantity per terminal, lug/ferrule, strip length, torque, bend space, and heat-rise limits.
Ask for: terminal matrix and assembly drawing.Lifecycle and supplier proof
Compare spare parts, controller support, change notices, traceability, packing, documents, warranty scope, and service route.
Ask for: deliverable list tied to the PO.Class and transition are separate decisions
Know what the switch must do during normal and fault conditions
Use the exact standard edition and datasheet. The plain-language table below is a buyer map, not a substitute for the rating record.
| Decision | What it means | What the buyer must verify | Common error |
|---|---|---|---|
| IEC Class PC | Transfer equipment capable of making and withstanding short-circuit current, but not intended to break it. | Required SCPD and applicable ratings—such as Icw with duration, Icm, and any Iq only with the specified SCPD—plus complete-TSE type-test and marking evidence. | Assuming “PC” includes overload and short-circuit tripping. |
| IEC Class CB | Transfer equipment intended to make, withstand, and break short-circuit current and provided with overcurrent releases. | Breaker settings, breaking data, selectivity/backup tables, protected poles, and the complete assembled configuration. | Calling it safer without checking the actual protection study. |
| Open transition | Break-before-make. The load is disconnected before the second source connects. | Acceptable interruption, motor/inrush behavior, transfer time, and load restart sequence. | Calling the interruption “seamless” or UPS-like. |
| Delayed transition | Open transition with a defined neutral interval before the second source closes. | Purpose of the delay, adjustable range, motor decay, process tolerance, and commissioned setting. | Adding delay without a documented load reason. |
| In-phase open transition | A break-before-make transfer timed to an acceptable phase relationship. The sources are not paralleled, and the load still sees an interruption. | Load and motor behavior, sensing limits, timing window, fallback behavior, and exact product test data. | Calling it closed transition or guaranteed no-break transfer. |
| Closed transition | Make-before-break during a planned transfer while both live sources meet synchronization limits. Sudden loss of the connected source still forces an open transfer. | Synchronization, overlap time, failed-parallel protection, combined fault contribution, source compatibility, and utility approval where applicable. | Assuming it gives no-break power after a sudden outage. |
| Bypass/isolation | An additional switching path can maintain supply while the ATS is isolated for approved service operations. | Bypass sequence, interlocks, ratings, energized boundaries, access, training, and maintenance procedure. | Assuming every internal part becomes de-energized during bypass. |
Important: IEC 60947-6-1:2026 requires complete TSE within its scope to be manufacturer type tested and marked. Two breakers, a motor operator, and a separate controller should not be presented as a compliant complete transfer switch by inference. For a deeper class comparison, see the guide to PC-class vs CB-class ATS selection.
The panel is a separate approval boundary: A compliant TSE does not by itself verify the completed ATS panel. Where IEC 61439 applies, the assembly maker must use IEC 61439-1 with the relevant assembly part, often IEC 61439-2. It must keep design-verification evidence and perform routine verification on every completed assembly. In North America, a UL 1008-listed open-type transfer device does not by itself establish the SCCR, service-equipment suitability, or listing of the finished switchboard or panel.
Load and source matching
The ATS rating does not size the backup source
An ATS may be rated to carry a feeder while the generator or inverter is intentionally sized for a smaller managed load. That can be a valid architecture only when the backed-up loads, shedding logic, restart sequence, protective devices, and conductors are designed together.
- Record steady and starting demand. Motors, compressors, transformers, power supplies, and grouped electronic loads can affect the source and transition choice.
- Separate must-run and managed loads. State which loads are blocked, shed, delayed, or manually restored. Validate load-management logic separately; IEC ATSE compliance does not prove that function.
- Check both fault extremes. Utility supply can create high fault current; a generator or inverter may supply much lower fault current and change trip performance.
- Do not use a universal sizing margin. Apply the governing load, feeder, generator, equipment, and local rules rather than an invented 125% or 150% ATS rule.
For a generator project, continue with SENTOP's generator transfer switch solution and generator and standby system pathway. Use the separate method to calculate ATS amperage from the load.
Short-circuit purchase gate
Current rating and fault rating answer different questions
The ATS must carry the planned load and also fit the fault-current and protective-device conditions at its installation point. WCR is not SCCR or an OCPD interrupting rating.
Every-source duty
Find the prospective fault current from each source. Include motor and other load-side contributions. For closed transition, include the combined contribution while sources overlap. Keep the voltage, configuration, and applicable X/R or power-factor basis.
UL WCR or IEC data
Use the term and test framework shown for the exact model. Do not translate WCR, Icw, Icm, Iq, SCCR, or breaking capacity by name alone.
Named conditions
Confirm breaker/fuse type, rating, setting, clearing time, and any series or conditional relationship required by the manufacturer.
Assembly and OCPD proof
Do not copy the ATS WCR into the assembly record. Separately verify the panel or switchboard SCCR, or the applicable IEC Icw/Ipk or conditional rating and design evidence. Check every OCPD interrupting rating and alternate-source clearing behavior.
IEC terms in plain language: Icw is an rms short-time withstand current and is inseparable from its stated time. Icm is a peak short-circuit making capacity. Iq is an rms conditional short-circuit current that is valid only with the manufacturer's specified SCPD. Keep each value with its voltage, configuration, device conditions, and exact catalog number.
Poles, neutral, and service position
Do not select 3-pole or 4-pole from a product filter alone
The required pole arrangement depends on phase, TN/TT/IT or North American bonding topology, whether the alternate source is separately derived, neutral current, isolation requirements, and local rules. The exact product diagram must show which poles are switched and protected and how RCD, ground-fault, or insulation-monitoring functions behave.
Overlapping-neutral equipment needs its own validated product and system evidence; IEC 60947-6-1:2026 does not cover it as a standard TSE category. Use the engineering guide on when to specify a 3-pole or 4-pole ATS for the next design check.
Never switch protective earth or a PEN conductor. Do not create required common operation by combining unrelated single-pole devices. Neutral switching and bonding belong on the approved one-line.
Neutral remains connected
May fit some non-separately-derived arrangements. Verify source bonding, fault paths, sensing, and local requirements.
Neutral transfers with phases
May be required in a separately derived or other defined system. Check sequence, pole rating, overlap, and protection.
Exact assembly approval
For North America, require the complete assembly to be listed and marked for use as service equipment at the installed ratings. Confirm disconnecting, overcurrent-protection, bonding, utility, and AHJ requirements. A component listing or “service capable” claim is not enough.
Load-side arrangement
Record the upstream main, feeder protection, neutral path, cabinet integration, working space, and cable route.
Controls and maintainability
Write the sequence before the controller is ordered
A controller feature has value only when its input, output, timing, failure state, and owner are defined. Request a plain-language sequence of operations and an I/O schedule.
- Source sensing: monitored phases, voltage/frequency limits, phase loss/sequence, confirmation delay, and recovery rules.
- Alternate-source interface: contact type and rating, start/ready feedback, warm-up, cool-down, and failed-start behavior.
- Transfer and retransfer: permissives, time delays, manual mode, test mode, source priority, and unstable-source response.
- Loads: shed outputs, restart order, external contactors/controllers, feedback, and what happens if communication fails.
- Service: event log, alarms, local indication, remote protocol, cybersecurity responsibility, spares, and backup of settings.
Use the separate ATS wiring guide only after the approved architecture and exact model are known.
Purchase-to-commission workflow
Approve the evidence in six controlled steps
The PO should preserve the assumptions used to select the switch.
Freeze system inputs
Issue the one-line, load schedule, source data, fault values, earthing decision, location, and market requirements.
Output: approved requirements sheet.Match the exact model
Compare class, ratings, poles, transition, controls, terminals, enclosure, dimensions, and accessories.
Output: complete order code.Review documents
Check drawing, datasheet, short-circuit conditions, certificate/listing, manual, I/O, and bill of materials.
Output: approved submittal.Verify each shipped assembly
Review the design evidence, then obtain the routine-verification record for each completed assembly. A sample or type-test report does not replace this shipped-unit check.
Output: traceable routine-verification record.Install and commission
Qualified personnel follow the approved design, instructions, settings, safe-isolation plan, and test script. Check nameplate and drawing match, phase sequence, neutral/bonding/PE, controller settings and I/O, both-source loss and recovery, start/ready, transfer/retransfer and delays, failure modes, interlocks, load shedding/restart, alarms, communications, manual/test modes, bypass sequence, and closed-transition synchronization/overlap where provided.
Output: signed site acceptance, test, and settings record.Hand over lifecycle data
Keep final drawings, manuals, certificates, settings, test results, spares, contacts, and maintenance plan.
Output: owner record set.Test boundary: An agreed FAT does not replace routine verification of each shipped assembly or the site acceptance test. Never create a site fault to “prove” WCR, SCCR, or short-circuit withstand.
RFQ worksheet
Send enough data to compare one build
SENTOP can review a model, photo, drawing, BOM, or requirement and support ATS component matching. Send the same input package to every bidder and require each proposal to list its assumptions and exclusions.
Compare installed value
A low switch price can hide a different system scope
Normalize every quotation before comparing cost. One bidder may include the controller, breakers, cabinet, lugs, bypass, monitoring, testing, and documentation while another quotes only a switching mechanism.
| Quote line | Confirm it includes | Why it changes cost or risk |
|---|---|---|
| Power path | Complete transfer mechanism, interlocks, poles, neutral arrangement, OCPDs if included, terminals, and internal conductors. | Different assemblies can share one ampere headline but have different protection and installation work. |
| Controller | Sensing, control supply, display, I/O, communication module, accessories, firmware/version, and documentation. | Missing interfaces create external relays, control power, panel space, programming, and test work. |
| Cabinet | Enclosure rating, dimensions, mounting, gland plates, heating/ventilation if needed, labels, and cable access. | The wrong cabinet can force redesign or lose the required environmental rating after field modification. |
| Proof and service | Certificates, drawings, manuals, design-verification evidence, shipped-unit routine records, agreed FAT, site acceptance/commissioning records, packing, spares, warranty scope, and change notice. | Documentation and lifecycle gaps often appear after the lowest component quote is accepted. A sample or type test does not replace verification of the delivered assembly. |
Avoid these purchase shortcuts
Eight mistakes that make bids look comparable when they are not
Buying by generator kW
kW does not define ATS current, voltage, fault duty, poles, utilization, or system protection.
Matching only the panel amp label
The location, calculated load, transition, source limits, and fault study still control the selection.
Calling any fast transfer “seamless”
Open transition interrupts the load; closed transition has strict source and approval conditions.
Choosing 4-pole because it looks safer
Neutral switching is an earthing and bonding decision, not a universal upgrade.
Ignoring the upstream OCPD
A fault rating often depends on a named breaker/fuse type, setting, or clearing time.
Equating NEMA Type and IP
They are different rating systems. Specify the required system and fully installed enclosure condition.
Accepting family-level certificates
Verify the exact model, variant, rating, factory, standard, file, and conditions of use.
Skipping the test and handover scope
Transfer, retransfer, alarms, load shedding, settings, training, and final records must be planned before delivery.
Before exposed work: Identify utility, generator, inverter, battery, PV, load-side backfeed, control power, remote commands, autostart, and stored mechanical/electrical energy. Qualified personnel must follow the governing safe-isolation or lockout/tagout procedure, disconnect every relevant energy source, release or restrain stored energy, apply the required locks and tags, and verify de-energization with suitable test equipment—including possible induced voltage and backfeed. Bypass position, selector switches, push buttons, software commands, and interlocks are not energy-isolating devices.
For a separate installation-planning boundary, read how to plan an ATS installation safely. For inverter or battery architecture, use the dedicated guide to planning an ATS with an inverter.
Continue the selection
Use the next page for the next decision
SENTOP automatic transfer switches
Review available product routes after the system duty is defined.
Explore the ATS rangeTransfer switch selection workflow
Compare broader transfer-switch families and selection inputs.
Use the selection guideSolar and energy systems
Plan product matching around the approved inverter and backup architecture.
Review energy-system supportStandards and certificate support
Request exact, scoped documents for the ordered product and target market.
Review document supportOEM/ODM service support
Coordinate model matching, labels, packaging, quantities, and project delivery.
Review OEM/ODM supportBuildings and infrastructure
Connect ATS requirements to the wider protection and distribution BOM.
Open the industry pathwayLow-voltage protection selection
Keep the ATS, OCPDs, distribution, residual-current, and surge functions distinct.
Review protection rolesATS manufacturing support
Review factory, documentation, OEM, and project-supply capabilities.
Review manufacturing supportFrequently asked questions
Automatic transfer switch buying FAQ
These answers support a purchase review. The exact product records, project design, and local approval still control.
What size automatic transfer switch should I buy?
Start with the approved system location and calculated load, then verify rated operational current, voltage, phase, frequency, utilization, conductors, temperature, enclosure, and fault duty. Do not choose from generator kW or a panel amp label alone. An ATS may be sized for the full normal-source feeder or service load while load management keeps the alternate source within its capacity. That sizing does not by itself make the ATS suitable for use as service equipment.
What is the difference between a PC Class and CB Class ATS?
Under IEC 60947-6-1, Class PC transfer equipment can make and withstand short-circuit current but is not intended to break it. Class CB equipment is intended to make, withstand, and break short-circuit current and has overcurrent releases. The class does not rank quality. Verify the complete protection strategy, exact short-circuit data, settings, and approved assembly.
Is open or closed transition better?
Neither is universally better. Open transition separates one source before connecting the other and causes an interruption. Closed transition briefly parallels synchronized live sources during a controlled transfer. It needs purpose-designed equipment, compatible sources, protection, controls, and utility approval where applicable. It does not make a sudden outage transfer behave like a UPS.
Do I need a 3-pole or 4-pole automatic transfer switch?
The answer depends on phase, earthing, whether the alternate source is separately derived, neutral current, isolation rules, and the exact device design. Confirm which poles are switched and protected. Never switch protective earth or a PEN conductor, and do not decide from a generic product photo.
What is WCR on an automatic transfer switch?
In the UL 1008 framework, the short-circuit withstand and closing rating describes the transfer switch's ability to withstand and close onto fault current under stated voltage, protective-device, and sometimes time conditions. The design check must include available fault duty from either source, motor and other load-side contribution, and combined contribution during closed-transition overlap. WCR is not the completed assembly SCCR, an OCPD interrupting rating, or a promise that the ATS remains serviceable after a rated fault. Do not treat an IEC value as the same rating by translation.
Can one ATS manage utility, generator, and battery sources?
Not by assumption. A conventional ATS normally transfers one load between two sources. IEC 60947-6-1:2026 does not cover multi-source TSE as a standard product category within its scope. Conventional ATS certification also does not approve inverter anti-islanding, grid interconnection, export control, power-control functions, energy management, or battery safety. A utility, generator, battery, and PV system needs a manufacturer-supported architecture, coordinated controllers, protection, interlocks, operating modes, and system-level approval.
Does an outdoor ATS only need a high IP or NEMA rating?
No. Specify the rating system required by the market and complete installed condition. NEMA Type and IEC IP are not interchangeable labels. Also check temperature, sunlight, condensation, corrosion, pollution, altitude, cable entries, drainage, mounting, ventilation or heaters, working space, and service access.
What documents should I receive before issuing the purchase order?
Request the exact order code, datasheet revision, general arrangement and terminal drawings, one-line/pole diagram, standard and approval record, current and short-circuit conditions, conductor/torque data, controller and I/O manual, sequence of operations, enclosure details, accessory list, design-verification evidence, routine-verification record for each shipped assembly, agreed FAT, site acceptance/commissioning records, packing, warranty scope, spare parts, and change-control process. A sample or type-test report does not replace routine verification of the delivered assembly.
Primary and official sources
Verify the current edition and exact product scope
Standards define test and marking frameworks; they do not approve a project combination by inference.
Current international TSE scope, including ATSE, bypass/isolation, closed transition, and stand-alone controllers, with stated exclusions.
Open the IEC publication pageCurrent North American standard scope for transfer switch equipment up to 1,000 V within its listed categories.
Open the UL standard pageOfficial overview of UL transfer-switch certification categories, including emergency and optional standby equipment.
Review UL certification scopeManufacturer technical overview of transfer sequences, poles, transition, service entrance, bypass/isolation, and WCR.
Read the ATS fundamentalsOfficial manufacturer explanation of IEC Class PC, Class CB, and Class CC terminology.
Open the ABB white paperManufacturer description of open, delayed, and closed transition sequences and application boundaries.
Review transition modesOfficial explanation that IEC IP degree ratings and NEMA enclosure Type designations are not equivalent labels.
Read the enclosure FAQGeneral rules and the relevant power-assembly part create a separate verification boundary for the completed switchgear or controlgear assembly that contains the TSE.
Open IEC 61439-1 · Open IEC 61439-2Static transfer systems use a different IEC family. UL 1008S covers a defined solid-state ATS scope for optional standby systems.
Open IEC 62310-1 · Open UL 1008SMicrogrid interconnect switching, inverter/interconnection equipment, and distributed-energy systems can invoke separate product and system requirements.
Open UL 1008B · Open UL 1741 · Open UL 3001U.S. general-industry boundaries for hazardous-energy control, electrical de-energizing, locks/tags, and qualified-person verification.
Read the LOTO rule · Read the electrical ruleReady to compare models?
Turn the one-line and load plan into one clear ATS requirement
Send source data, load priorities, standard, current, voltage, poles, neutral, transition, fault duty, enclosure, controller, certificates, quantity, and delivery target. We will support product matching without replacing the qualified system design and approval process.
