For mission-critical data centers, automatic transfer switches are the first line of defense when utility power fails. The top picks for most facilities are LayerZero eSTS, Eaton PDI TFA, Raritan (Legrand) hybrid rack transfer switch, Schneider Electric ATS, and ABB, with LayerZero and Eaton leading for large-load static transfer applications and Raritan standing out for in-rack redundancy. Your selection criteria should center on five factors: transfer time (in milliseconds), switching technology (static vs. electromechanical vs. hybrid), communications protocol support, UL 1008 or IEC certification, and local serviceability.
Quick shortlist by use case:
- Large Tier III/Tier IV enterprise data centers: Schneider Electric or Eaton PDI TFA (broad ecosystem integration, front-access serviceability)
- Sub-4 ms static transfer for sensitive IT loads: LayerZero eSTS (fiber-optic control logic, <4 ms transfer, triple modular redundancy)
- High-density in-rack redundancy: Raritan hybrid rack transfer switch (fast transfer times on the order of a few milliseconds, outlet-level metering, SCR+relay design)
- Advanced monitoring and BMS integration: ABB (Modbus TCP, Ethernet/IP, PROFINET)
- Generator-integrated and utility-backed systems: GE Power or Siemens
- Specialized or tailored topologies: Equinox
NFPA 110 governs emergency and standby power systems in the United States, and UL 1008 certification is the baseline compliance requirement for any ATS installed in a U.S. data center. Procurement teams that skip transfer-time waveform verification or overlook 4-pole switching requirements for separately derived systems often discover those gaps during commissioning, not before.
Table of Contents
- How do the top ATS options for data centers compare?
- What does each leading ATS vendor actually offer?
- How do you choose the right ATS for your data center?
- What are the engineering trade-offs between switching technologies?
- How does ATS selection affect generator runtime and fuel planning?
- Key Takeaways
- The part of ATS procurement most teams get wrong
- Fuel logistics and runtime support for ATS-backed backup power
- Authoritative sources, datasheets, and further reading
How do the top ATS options for data centers compare?
The table below compares the leading automatic transfer switch options across the dimensions that drive data-center procurement. Transfer times marked as “typical” reflect published vendor specs or representative market ranges; price and lead time are indicative estimates. Request vendor datasheets and factory acceptance test (FAT) waveforms to verify all performance claims before issuing an RFQ.
Methodology note: Specs were drawn from published vendor datasheets, product pages, and the LayerZero, Eaton, Raritan, and ABB reference sources cited throughout this article. Where a vendor does not publish a specific figure, the cell shows “—” and the procurement guidance below the table explains what to request.
| Vendor | Best for | Switching tech | Transfer time | Nominal ratings | Redundancy | Communications | Certifications |
|---|---|---|---|---|---|---|---|
| LayerZero eSTS | Large mission-critical loads needing sub-4 ms transfers | Static (STS) | <4 ms typical | 100–3,000 A, 208–480 V, 3- and 4-pole | Triple modular redundancy, fiber-optic control | Modbus TCP, Ethernet/IP | UL 1008, NEMA |
| Eaton PDI TFA | High-availability data centers needing fast transfer and front-access service | Static (STS) | ≤½–¾ cycle | 100–3,000 A, 208–480 V | Redundant control logic, modular design | Modbus, Modbus TCP, Ethernet/IP | UL 1008 |
| Raritan (Legrand) | High-density rack deployments, in-rack redundancy | Hybrid (SCR + relay) | 4–8 ms | Rack-level, 16–32 A per inlet | Two single-throw relays (no single double-throw SPOF) | SNMP, Modbus | UL listed |
| Schneider Electric | Enterprise data centers, colocation, DCIM-integrated facilities | Electromechanical ATS / STS (family-dependent) | Varies by product family | Wide range, low-voltage | Redundant controls, broad portfolio | Modbus, Ethernet/IP, EcoStruxure | UL 1008, IEC |
| ABB | Facilities prioritizing advanced monitoring and BMS integration | Electromechanical ATS / STS | — | Low-voltage, broad frame range | Integrated monitoring | Modbus TCP, Ethernet/IP, PROFINET | IEC, UL |
| Siemens | Large facilities with standardized industrial controls | Electromechanical ATS | — | Low- and medium-voltage | Industrial-grade redundancy | PROFINET, Modbus | IEC, UL, CE |
| GE Power | Utility-backed and generator-integrated systems | Electromechanical ATS | — | Medium- and high-voltage frames | Industrial pedigree | Modbus, proprietary | UL, IEEE |
| Equinox | Specialized or custom data-center topologies | ATS (application-specific) | — | Application-specific | Tailored configurations | — | — |
Where public specs are missing: ABB, Siemens, GE Power, and Equinox do not publish transfer-time figures or full protocol lists on their public product pages. For any of these vendors, request the following in your RFI: transfer-time test reports under rated load, communications protocol list with firmware version, and MTBF/MTTR data from the factory.
Column leaders at a glance:
- Fastest published transfer time: — LayerZero eSTS (<4 ms) and Eaton PDI TFA (≤½–¾ cycle, approximately 8–10 ms at 60 Hz)
What does each leading ATS vendor actually offer?
LayerZero eSTS
LayerZero builds static transfer switches specifically for mission-critical uptime. The eSTS product line achieves transfers in under 4 ms using fiber-optic control logic that isolates the control plane from electrical noise. Triple modular redundancy in the control system means no single component failure can prevent a transfer. The 4-pole configuration switches the neutral, which is the correct specification when the ATS feeds a separately derived system. Front-access enclosures support IR scanning and module replacement without rear clearance.
- Typical ratings: 100–3,000 A, 208–480 V, 3- and 4-pole configurations
- Request from vendor: FAT waveform reports, fiber-optic control logic documentation, bypass procedure guide
Eaton PDI TFA Static Transfer Switch
Eaton’s PDI TFA uses a patented VSS (Voltage Source Switching) algorithm to achieve transfer times of ≤½ to ≤¾ cycle under load. True front-access enclosures allow installation, maintenance, and IR scanning from a single service face, which matters in high-density white space where rear access is restricted. Redundant control logic and modular construction support field serviceability without a full unit replacement.
- Typical ratings: 100–3,000 A, 208–480 V
- Request from vendor: VSS algorithm documentation, front-access mechanical drawings, local service contact and parts lead time
Pro Tip: When evaluating Eaton PDI TFA, ask for a real-load transfer-time test report, not just a specification sheet. The VSS algorithm’s performance under inrush conditions is the figure that matters for IT loads.
Raritan (Legrand) hybrid rack transfer switch
Raritan’s rack-level unit combines SCRs with electromechanical relays to deliver 4–8 ms transfers at the rack, alongside outlet-level metering and remote switching. The two single-throw relay design eliminates the single double-throw relay as a single point of failure, improving reliability for out-of-phase transfer scenarios. Inlet and outlet metering provides granular visibility into per-rack load without additional monitoring hardware.
- Typical ratings: Rack-mount, 16–32 A per inlet, single- and three-phase options
- Request from vendor: Transfer-time test data under full rack load, SNMP MIB files, outlet-level metering resolution specs
Schneider Electric (APC/Schneider)
Schneider Electric’s ATS portfolio spans low-voltage electromechanical and static transfer switch families, with deep integration into the EcoStruxure Power Monitoring and DCIM ecosystem. For enterprise data centers and large colocation facilities, that integration means ATS events feed directly into power management dashboards without custom middleware. The breadth of the portfolio is both an advantage and a complexity: specifying the right product family requires careful alignment with the facility’s power architecture.
- Request from vendor: EcoStruxure integration documentation, transfer-time specs by product family, UL 1008 test reports
ABB
ABB’s ATS line emphasizes communications breadth, supporting Modbus TCP, Ethernet/IP, and PROFINET natively. That protocol coverage makes ABB a strong fit for facilities that need to integrate ATS events into building management systems or cloud-based monitoring platforms. Published transfer-time figures are not listed on ABB’s public product pages; request them directly alongside IEC test reports.
Siemens
Siemens brings industrial-grade build quality and compliance with major international standards, including IEC and CE certifications. The platform suits large facilities that run standardized Siemens industrial controls across the site, since PROFINET integration with existing SCADA or BMS infrastructure is straightforward. Transfer-time specs are not publicly listed; request them with PROFINET configuration documentation.
GE Power
GE’s industrial ATS solutions are built for utility-backed and generator-integrated configurations, particularly at medium- and high-voltage frames. The industrial pedigree is the primary differentiator: GE has decades of experience with generator-integrated controls and utility interconnection. For data centers that operate large diesel generator plants, GE’s generator-ATS integration experience is worth evaluating alongside transfer-time and communications specs.
Equinox
Equinox occupies a niche position, offering ATS configurations tailored to specific data-center topologies rather than a broad catalog. For specialized deployments where standard product families do not fit the power architecture, Equinox is worth an RFI. Public technical specifications are limited; a direct technical consultation is the right starting point.
How do you choose the right ATS for your data center?
Getting the ATS selection right requires working through sizing, certification, serviceability, and procurement process in sequence. Here is a structured approach.
Sizing and specification
- Determine nominal current and voltage. Calculate the maximum continuous load current at the ATS terminals, then apply a derating factor for ambient temperature and altitude. Size the ATS frame to at least 125% of the calculated load current.
- Specify 3-pole or 4-pole switching. If the ATS feeds a separately derived system (transformer-isolated or generator-derived), specify 4-pole switching to switch the neutral and eliminate circulating currents and neutral potential differences that damage sensitive equipment.
- Set the transfer-time requirement. For IT loads on double-conversion UPS systems, electromechanical ATS transfer times (typically 100–200 ms) are often acceptable. For loads on line-interactive or offline UPS, or for direct IT loads, specify static transfer switch technology with transfer times under 4 ms.
- Confirm frame size and enclosure type. Verify that the selected frame fits the available floor space and that front-access enclosures are specified where rear clearance is limited.
Procurement checklist: what to request in an RFI/RFQ
- Transfer-time test reports under rated load (FAT waveforms, not just specification sheets)
- UL 1008 or IEC certification documentation
- 4-pole switching option confirmation and neutral switching methodology
- Front-access enclosure mechanical drawings
- MTBF and MTTR data from the manufacturer
- Local service contacts and spare-parts lead times
- Communications protocol list with firmware version
- Warranty terms and scope (parts, labor, on-site response time)
- Factory acceptance test (FAT) and site acceptance test (SAT) checklists
- Published case studies or reference deployments in comparable data-center environments
NFPA 110 requires that ATS installations in emergency and standby power systems be performed by licensed electrical contractors and that regular maintenance and testing be documented. Build those requirements into your procurement scope from the start.
Questions to ask vendors
- What is the measured transfer time under full rated load, and can you provide FAT waveform data?
- How does the synchronization algorithm handle out-of-phase transfers?
- What is the warranty scope, and does it include on-site labor?
- What is the typical spare-parts lead time for control boards and switching modules?
- What is the firmware update policy, and does a firmware update require a service interruption?
Red flags to watch for
- No published transfer-time spec, or a spec stated only at no-load conditions
- Single double-throw relay design with no redundancy path
- Missing UL 1008 or IEC certification documentation
- Communications limited to dry contacts with no Modbus or Ethernet option
- No local service network or unclear warranty response time
For a broader view of redundant power strategies and how ATS selection fits into overall facility resilience, the Data Center Fuels redundancy guide covers the architectural context in detail.
What are the engineering trade-offs between switching technologies?
Static transfer switches (STS)
Static transfer switches use power semiconductors (SCRs or IGBTs) with no moving parts. The result is transfer times under 4 ms, which is fast enough to protect loads that cannot tolerate even a half-cycle interruption. LayerZero’s eSTS achieves this with fiber-optic control logic; Eaton’s PDI TFA uses its patented VSS algorithm. The trade-off is cost: STS units carry a higher price point than electromechanical alternatives, and semiconductor-based designs require careful thermal management.
SCRs handle high surge currents better for high-power applications; IGBTs are better suited where high-frequency switching or energy efficiency is the priority. That semiconductor trade-off is worth raising explicitly with vendors during the RFI process, particularly for large-frame STS units feeding high-inrush IT loads.
Electromechanical ATS
Electromechanical units use contactors or relays and typically transfer in 100–200 ms. For data centers with double-conversion UPS systems, that window is within the UPS hold-up time and is often acceptable. Schneider Electric, Siemens, and GE Power all offer electromechanical ATS families with strong industrial credentials. The lower cost and simpler maintenance profile make electromechanical ATS the right choice for utility-to-generator transfers where the UPS provides the ride-through.
Hybrid rack transfer switches
Raritan’s hybrid design combines SCRs for the initial fast transfer with electromechanical relays that carry the steady-state load. The result is 4–8 ms transfer times at rack level, with the thermal efficiency of relay-based steady-state conduction. Two single-throw relays replace the single double-throw relay, removing a common single point of failure. Outlet-level metering adds per-outlet visibility without a separate PDU.
3-pole vs. 4-pole switching
When the ATS is fed by separately derived sources (generator output or transformer-isolated feeds), switching the neutral is required to prevent circulating currents and neutral potential differences. A 4-pole eSTS switches all four conductors simultaneously. Specifying 3-pole switching for a separately derived system is a design error that can damage sensitive IT equipment; confirm the source derivation status before finalizing the ATS specification.
Communications protocols: what to require
- Modbus RTU / Modbus TCP: Standard for integration with SCADA, BMS, and DCIM platforms
- Ethernet/IP: Preferred for Allen-Bradley and Rockwell-based control environments
- PROFINET: Required for Siemens-based industrial control networks
- SNMP: Standard for IT-managed monitoring platforms and data center infrastructure management tools
Specifying Modbus TCP or Ethernet/IP enables cloud-based alarming and waveform capture for post-event analysis. That waveform data is the most useful diagnostic tool after an unplanned transfer event.
Pro Tip: Require vendors to demonstrate live communications integration with your DCIM or BMS platform during factory acceptance testing. A protocol listed on a datasheet and a protocol that works reliably in your environment are not always the same thing.
For a technical primer on data center backup power systems and how switching technology fits into the broader power architecture, the Data Center Fuels backup power guide covers the full stack.
How does ATS selection affect generator runtime and fuel planning?
ATS selection is not just an electrical engineering decision. The transfer behavior, inrush handling, and synchronization approach of the chosen switch directly affect how hard the generator works during an event, which determines fuel burn rate and on-site fuel requirements.
Transfer behavior and generator loading
A fast, clean transfer with proper inrush management reduces the voltage and frequency excursions that cause generators to hunt or shed load. An electromechanical ATS with a slow or poorly synchronized transfer can cause the generator to experience a large inrush spike, which increases fuel consumption in the first minutes of an event and can trip generator protection relays. Static transfer switches with synchronization algorithms (like Eaton’s VSS) reduce that inrush stress, which translates to more predictable generator loading and more accurate fuel burn estimates.
Fuel planning tied to ATS topology
The ATS topology determines the generator run profile, and the run profile determines the fuel contract. Key planning inputs:
- Transfer time and frequency: More frequent testing (monthly vs. quarterly) increases annual fuel consumption. Build testing cadence into fuel contract volumes.
- Generator load acceptance: Specify the ATS’s inrush handling capability alongside the generator’s load acceptance rating. A mismatch here causes frequency dips that can trigger load shedding.
- Minimum runtime scenarios: Plan for at least 24–72 hours of on-site fuel storage for extended utility outages, depending on facility tier requirements and local resupply SLA.
- DEF requirements: Modern Tier 4 Final diesel generators require diesel exhaust fluid (DEF). Include DEF storage and resupply in the fuel plan alongside diesel.
- Seasonal adjustments: Cold-weather starts increase fuel consumption in the first minutes of operation. Adjust winter fuel reserves accordingly.
Fuel-planning checklist for ATS procurement
- Confirm generator load acceptance rating and match it to the ATS inrush specification.
- Calculate expected fuel burn at 75% and 100% generator load for your minimum runtime scenario.
- Specify on-site diesel storage capacity to cover the minimum runtime plus a resupply buffer.
- Include DEF storage capacity and resupply SLA in the fuel contract.
- Define emergency resupply response time with your fuel provider (typically 2–4 hours for critical facilities).
- Schedule combined ATS transfer tests and generator load acceptance tests at commissioning, and document fuel consumption during each test.
- Verify that the fuel provider can support the testing cadence volume in addition to emergency resupply.
For runtime calculation support and on-site fuel storage guidance, the Data Center Fuels fuel calculator and the diesel stockpiling guide cover the planning inputs in detail. The Data Center Fuels emergency power checklist also maps directly to the commissioning and resupply verification steps above.
Commissioning guidance: run the ATS transfer test under full rated load, capture waveforms, and document generator frequency and voltage recovery time. That data becomes the baseline for ongoing maintenance testing and the input for fuel consumption estimates. A generator installation partner with experience in ATS commissioning can help coordinate the electrical and fuel-side acceptance tests in a single site visit.
Key Takeaways
For most mission-critical data centers, the ATS decision comes down to three variables: transfer time, switching technology, and whether the vendor can demonstrate certified performance under real load conditions.
| Point | Details |
|---|---|
| Static transfer switches lead for sensitive IT loads | LayerZero eSTS achieves <4 ms transfer times, and Eaton PDI TFA achieves ≤½–¾ cycle transfer times, which is approximately 8–10 ms at 60 Hz; both are suitable for loads that cannot tolerate a half-cycle interruption. |
| 4-pole switching is required for separately derived systems | Switching the neutral eliminates circulating currents and neutral potential differences that damage sensitive equipment. |
| Demand FAT waveforms, not just spec sheets | Transfer-time claims must be verified under rated load; a spec sheet figure at no-load is not sufficient for procurement. |
| Communications protocols determine monitoring capability | Specifying Modbus TCP or Ethernet/IP enables waveform capture and cloud alarming, the most useful diagnostic tools after an unplanned transfer. |
| Data Center Fuels connects ATS selection to fuel logistics | Fuel burn rate, DEF needs, and resupply SLAs all depend on ATS topology and generator run profile; plan both streams together. |
The part of ATS procurement most teams get wrong
The conventional wisdom in ATS procurement is to lead with transfer time. That is not wrong, but it is incomplete in a way that creates real operational risk.
Transfer time matters most for the loads that cannot ride through even a brief interruption. For those loads, a static transfer switch with a verified sub-4 ms transfer is the right answer, and LayerZero and Eaton PDI TFA are the vendors that can back that claim with waveform data. But the majority of data center ATS applications involve loads protected by double-conversion UPS systems, where the UPS provides the ride-through and the ATS transfer time is largely irrelevant to load continuity. In those cases, specifying an STS when an electromechanical ATS would serve equally well is a cost decision, not a reliability decision.
What teams consistently underestimate is the serviceability dimension. An ATS that requires rear access in a high-density row is a maintenance liability from day one. Front-access enclosures are not a premium feature; they are a practical requirement for any white-space deployment where rear clearance is constrained. The same logic applies to communications: an ATS with only dry-contact outputs is not a monitoring tool, it is a black box that tells you a transfer happened but nothing about why or how cleanly.
The other gap is the fuel side. ATS selection and generator fuel planning are almost always handled by different teams on different timelines. The result is facilities that have specified a fast, redundant ATS but have not sized their on-site fuel storage to support the testing cadence, let alone a 48-hour extended outage. Coordinating those two procurement streams from the start is the single most practical step a data center manager can take to reduce operational risk.
Fuel logistics and runtime support for ATS-backed backup power
Selecting the right ATS is only half of the backup power equation. The generator behind it needs fuel, and that fuel needs to be there when the transfer happens.
Data Center Fuels, in partnership with Anytime Fuel Pros, provides on-site diesel fuel delivery and DEF logistics for data centers across the United States. Whether you are sizing fuel contracts for a new facility, planning resupply SLAs for extended outage scenarios, or coordinating DEF delivery for Tier 4 Final generator fleets, the team can map your generator run profile to a delivery schedule that keeps your tanks above the minimum runtime threshold.
The practical next step for procurement teams: use the Data Center Fuels fuel services page to request a runtime calculation or get a quote for on-site diesel and DEF delivery. Bring your ATS topology, generator load profile, and minimum runtime requirement, and the team will size the fuel contract to match.
Authoritative sources, datasheets, and further reading
Use these sources to validate specs and request the right documents from vendors during procurement.
- LayerZero 250 A 120/208 V 4-Pole eSTS product page — transfer-time spec, 4-pole switching documentation, fiber-optic control logic overview
- LayerZero 800 A 480 V 3-Pole eSTS product page — high-current STS specs and bypass procedure documentation
- Eaton PDI TFA Static Transfer Switch — VSS algorithm documentation, front-access enclosure drawings, UL 1008 certification
- ABB Automatic Transfer Switches — communications protocol list, IEC certification documentation
- Raritan Intelligent Hybrid Rack Transfer Switch (Legrand) — hybrid SCR+relay design specs, outlet-level metering documentation
- NFPA 110 Standard for Emergency and Standby Power Systems — U.S. compliance baseline for ATS installation, testing, and maintenance
- Automatic Transfer Switch (ATS) definition — TechTarget — foundational ATS definition and data-center context
- Understanding Transfer Switches: Types, Applications and Design Considerations — MissionCriticalEngineers — semiconductor trade-off analysis (SCR vs. IGBT) and switching technology overview
- Eaton Automatic Transfer Switch Fundamentals — low-voltage ATS fundamentals and application guidance
Documents to request from vendors during procurement: FAT waveform reports under rated load, MTBF/MTTR data sheets, front-access mechanical drawings, communications protocol configuration guides, UL 1008 or IEC test certificates, and local service contact lists with response-time commitments.