Both sources matter
Normal and backup feeds can differ in size, ground plan, fault current, quality and start plan.
Engineering selection · Commercial and industrial power
How to Choose a Three-Phase Automatic Transfer Switch
Start with the one-line, not an ampere label. List both sources and each load state. Add the neutral and ground plan, fault current, source-change goal, controls, site and target rules. Then match one switch and all limits on its label.
CHECK 01Electrical limitsVoltage, frequency, phase order, wires, current and load response.
CHECK 02Poles and neutralChoose 3-pole or 4-pole from the grounding study, not a shortcut.
CHECK 03Fault dutyMatch fault current, current fed by motors and the marked breaker or fuse basis.
CHECK 04Product proofCheck product class, rules, controls, case, tests and service plan.
Generator transfer-switch example; the photo does not establish phases, rating or transition type. Photo: Robert.Harker, Wikimedia Commons, CC BY-SA 3.0. Cropped for presentation.
Direct answer
What is the right three-phase ATS?
The one that fits the full system, not just its run current.
Choose the switch with nine linked checks.
Check both sources, the load, current, poles, fault duty, source-change mode, product class, controls, case and rules. Log each choice on the one-line and in the quote request. Do not buy from a fixed spare rate, a building type or a “fast transfer” claim.
Current is not one number
Check each phase, neutral current, motor or transformer inrush, UPS charge and load steps.
Read the full label
Run current, withstand-and-closing rating (WCR), short-circuit current rating (SCCR), breaker interrupting rating and short-time withstand do not mean the same thing.
Plan each state
Include source change, return, test, bypass, service, source loss and loss of both feeds.
Scope: this guide helps form a spec. It is not a wiring plan. The skilled project team must use the local code, device listing, grid rules, safe-work plan and current maker manual.
What the system does
Follow the event from bad source to accepted backup power
In a genset system, the switch works with more than one device. Its moving parts cause only part of the power gap.
Sense normal power
The control unit uses set limits and time lags before it calls the normal feed bad.
Ask the genset to start
The switch sends a remote-start request through an approved contact or link.
Build backup power
The genset controls crank the engine and manage fuel, speed, voltage and protection functions.
Check the source
The switch checks backup voltage, frequency and each source limit that has been set.
Change and restart
The main poles change source. The load may ride through, reset, draw inrush or need a staged start.
The ATS does not crank the engine. Its start request is one input to the genset controls. Full power-return time has sensing, time lags, engine start, voltage and frequency build-up, source checks, pole change and load restart.
One-line first
Use the project drawing as the source of truth
Show both feeds, poles, neutral, ground points, breakers or fuses, bypass, loads, controls and field links.
A product sketch cannot replace the project one-line. Final wiring must follow the signed-off plan and the exact switch manual.
Sample teaching diagram, not a SENTOP product drawing, build plan or fault study. Diagram: Dmitry G, Wikimedia Commons, CC BY-SA 3.0.
Input map
Review single-phase vs three-phase basicsSet the power limits before the catalog search
Use test, study and nameplate data. “Three-phase, 400 A” is not a full spec.
| Input | Record this | Why it changes the switch | File to attach |
|---|---|---|---|
| Source voltage | Line-to-line and line-to-neutral voltage for both feeds. | Sets insulation and sense limits. The two feeds may not be a true match. | One-line, transformer and genset data. |
| Frequency and phase | Rated frequency, allowed range and phase sequence. | A wrong phase sequence can reverse a motor. It may harm the driven gear or process. Closed or in-phase change needs matched live feeds. | Source study, site test and control settings. |
| Wire system | Three-wire or four-wire, neutral use, protective earth (PE) path and bond sites. | Drives pole count, neutral steps, ground-fault sense and possible parallel neutral paths. | Ground study and signed-off one-line. |
| Load current | Highest phase root-mean-square (RMS) current in each run state. | Phase mismatch, load steps and new states can make one phase the limit. | Load list, meter data and device data. |
| Load type | Motors, transformers, UPS, variable-frequency drive (VFD), rectifier, heater and nonlinear loads. | Inrush, left-over voltage, harmonics and restart response affect the rating and source change. | Nameplates, curves and maker notes. |
| Fault current | Max fault current from each source plus current fed by motors. | Sets the valid marked fault rating and its upstream breaker or fuse limits. | Fault study and breaker or fuse list. |
| Site | Indoor or outdoor use, heat, site height, damp, dust, salt, fumes and access. | Changes the case, spacing, cooling, cable entry and safe service space. | Site spec and case plan. |
Current and load response
Open the transfer-switch calculatorUse the formula as a first check, not the final rating
The balanced three-phase math helps check current. A real plant also needs phase, neutral, inrush and run-state data.
Balanced-load current
I = 1000 × kVA ÷ (√3 × VLL)I is line current in amperes. VLL is line-to-line voltage. Use this only for a balanced three-phase load. Check the actual power factor when converting between kW and kVA.
Starting and residual voltage
Record locked-rotor or start current, acceleration time, sequencing and the risk of out-of-phase reconnection.
Magnetizing inrush
Closing angle and residual flux can create high inrush. Use transformer and switch data, not a generic multiplier.
Recharge and bypass
Check input current, battery recharge, bypass state, harmonics and how the load reacts to a source step.
Highest phase and neutral
Use the highest RMS phase current. Check triplen harmonics and neutral duty for nonlinear line-to-neutral loads.
No single spare rule fits all jobs. Use a maker rating that meets the local code, product file, heat and site-height limits, run-duty guide and each load state. Do not add a blanket 80%, 100% or 125% rule.
Load check
A motor can change more than the ampere rating
Large motors can add start current and feed fault current. The voltage left in a spinning motor can also matter in a fast source change.
State which motors run at once, how they restart and which loads drop first. Review pole duty and change logic with the switch maker.
The image shows a three-phase motor only; it does not prove power, start duty or switch fit. Photo: KishanMalaviyaatCHETAK ELECTRICALS, Wikimedia Commons, CC BY-SA 4.0. Cropped for presentation.
Pole and neutral choice
Open the full 3-pole vs 4-pole guideChoose 3-pole or 4-pole from the ground plan
A genset or a neutral wire does not answer this point on its own.
Phases change; neutral stays solid
Check where each source is bonded and how ground-fault current flows back. A solid neutral can form an unwanted second path in some systems.
Check the scheme for each source and bypass state.
Phases and neutral change
Check if the source has its own bond and how the switched neutral affects ground-fault sensing. The fourth pole needs the right run and fault rating.
Check the neutral sequence too. Same-time, break-before-make and overlap designs do not act the same.
Write down the choice. Mark neutral-to-ground bonds, protective earth (PE) paths, ground-fault sensors and neutral current on the one-line. Add line-to-neutral load and triplen-harmonic current when they apply.
Fault duty
Match the marked fault-rating set
The run rating tells how much current the switch can carry in normal use. It does not prove that the switch can stand or close on the fault current at the site.
Fault current at site
Find the value from each source at the switch. Add fault current fed by motors under the project method.
Do not treat the grid and genset values as the same.
Exact breaker or fuse
Record breaker maker, catalog or frame, trip unit and settings. For a fuse, state its class and max rating.
Check the clear time or short-time rule named on the switch.
WCR and SCCR
UL 1008 uses a withstand-and-closing rating (WCR). The short-circuit current rating (SCCR) of the built gear may rely on the marked WCR and breaker or fuse set.
Neither term means the breaker interrupting rating or switch run rating.
PC or CB class
IEC class PC and CB have unlike fault and trip roles.
Read the exact product data. A class label does not replace the fault study or the way the parts work as a set.
Closed overlap
Do not add or ignore source fault current by rote when both sources overlap.
Use the maker's test or label basis and the project engineer's written method.
Known load-side fault
A WCR test does not tell the control unit to change onto a known fault.
Fault block, trip and switch logic depend on the model and full system.
For more detail, see the PC class vs CB class ATS guide. Use it only after you check the exact model and local rule.
Source change
Compare ATS and static transfer switchesChoose the change mode from the load and source study
No single “best” mode or time target fits all jobs.
| Mode | What it does | Where it can fit | What must be checked |
|---|---|---|---|
| Open transition | Breaks the first source before it makes the second. | Common grid-genset and dual-feed use where a short gap is fine. | Load ride-through, motor left-over voltage, inrush and restart steps. |
| Delayed transition | Adds a planned dead time between sources. | Jobs that need more time for motor voltage to fall or a load process to stop. | Allowed power gap, exact time and the reason for the delay. |
| In-phase monitored | Uses product-specific open-transition logic. It waits for the controller's permitted phase-angle and frequency relationship between two live sources. | Some motor or transformer jobs where the source angle matters. | Control method, allowed range, max wait and fall-back plan. |
| Closed transition | Makes the second source before it breaks the first. The two sources overlap for a short time. | Planned change or return between two live sources that are acceptable, compatible and within the selected controller's synchronization limits. | Sync logic, grid approval, interconnection protection, overlap limit and fail-to-open response. |
Closed transition cannot bridge genset start time. If normal power has failed, there is no live source to overlap. The switch must wait until the alternate source meets its acceptance criteria, then change with no closed overlap.
Controls and service
State what the control unit must sense, command and log
More features help only when the team sets their task, limits and pass test.
Source checks
List voltage, frequency, phase loss and phase-sequence checks. Log setpoints, time lags and reset values.
Change and return
Set start request, warm-up, source change, load shed, return, cooldown and stop-request logic.
Genset and controls
Name each dry contact, input, data link, building management system (BMS) point, alarm and control-power feed.
Events and settings
State the log rate, time source, alarm list, setting backup and access roles.
Maintenance path
Decide whether bypass-isolation is needed. Map each possible feed and interlocked operating state.
Site limits
Match the full case to water, dust, heat, site height, salt, fumes, access and cable entry.
Parts and access
Check safe access, parts that can be changed, spares, tools, staff skills and support life.
Linked controls
If remote access is used, set the owner, network line, log-in rules, logs and change control.
Bypass-isolation is not a full power cut. The load may stay on through the bypass while source, load, control or sense wires stay live. The work and safe cut-off plan must cover each state.
Multi-source safety
“Off” at the control unit is not safe isolation
The switch can stay live from normal power, the genset, a bypass, load-side backfeed, UPS or control power. Stored electrical and non-electrical energy and an unwanted start request also need control.
Before work, qualified persons must follow the site's written energy-control procedure. Identify every normal, alternate, bypass, backfeed, control-power, stored-electrical-energy and stored-non-electrical-energy source. Then disconnect the equipment from every required energy source.
Apply locks and tags to each energy-isolating means as the procedure and applicable law require. A tag-only method is permitted only where the governing rule allows it and equivalent additional safeguards are provided. Preventing automatic generator start is an additional control, not the sole means of isolation. Release or block stored energy.
A qualified person must use properly rated test equipment to verify that every exposed part is de-energized and to check for backfeed or induced voltage. An HMI command, selector switch, control contact or interlock is not an energy-isolating device.
Work scene from 2011; it does not prove device phase, rating, safe state or a current work method. U.S. Air Force photo by Senior Airman Kasey Close, public domain, Wikimedia Commons, Public Domain Mark 1.0. Cropped for presentation.
Nine-check workflow
Open the transfer-switch selection hubTurn the one-line into a clear buy spec
Close each check with a file and a named owner.
Map both sources
Log voltage, frequency, phase order, size, ground plan and fault current. Add the remote-start link and genset steps.
Build the load list
List current by phase and state. Add motors, transformers, UPS, variable-frequency drives (VFDs), nonlinear loads and restart needs.
Set the current rating
Use the local design method and exact maker limits. Check heat, site height, case and lugs.
Choose poles and neutral
Use the grounding study. Define the fourth-pole rating and switching sequence when a neutral is switched.
Match fault duty
Check site fault current, current fed by motors, the exact breaker or fuse and the marked fault rating.
Choose the source change
Set the allowed gap, live-source overlap, in-phase logic, motor decay time and grid rules.
Define controls
List sensing, timers, start request, permissives, load shed, alarms, BMS points and event records.
Plan service and the case
Set bypass needs, safe cut-off lines, access, site limits, spares, service and staff training.
Check the proof
Review the exact product file, standard edition, tests, settings, drawings, manuals and final pass script.
Three sample jobs
The same ampere label can need a new switch plan
These cases frame the key checks. They do not name a model.
Grid plus standby genset
Focus: power-return steps, life-safety or standby rules, mixed line-to-neutral loads and fault match.
Check source bonds before you choose 3-pole or 4-pole. Add UPS for loads that cannot ride through genset start.
Large process motors
Focus: highest start group, fault current fed by motors, left-over voltage, load shed and safe restart.
Choose open, delayed or in-phase change from motor and source data. Fast moving parts are not enough.
Genset, UPS and bypass layers
Focus: shared weak points, allowed power gap, bypass states, safe service and proven load response.
The switch can support long loss of grid power. UPS or storage may cover the genset start gap. An STS may serve a lower dual-live-source tier.
Explore genset transfer system parts or compare current ATS models. The project team still owns the final power plan.
Site-test proof
Test the full sequence, not only the switch
Use an approved script to simulate or safely initiate each permitted source-loss, source-return, transfer, retransfer, start-request, alarm, load-shed, bypass and total-source-loss scenario.
Log source values, time, control settings and what the real load did. Verify protection, interlocks and remote points only by the approved methods and within the equipment manual and site safety program.
Sample relay and switchgear check, not an ATS test method or result. Photo: MTA Capital Construction Mega Projects, Wikimedia Commons, CC BY 2.0. Cropped for presentation.
Handover pack
Read the ATS test-planning guideKeep the proof for the next safe test
One good source change does not prove each feed, fault and service state.
Signed-off submittal
Exact part number, ratings, product file, drawings, parts, breaker or fuse limits and test logs.
Control record
Source limits, all timers, change logic, start request, load shed, alarms and access rules.
Test script
Start checks, safe states, meters, steps, pass results, witnesses and stop rules.
State map
Normal, backup, test, bypass, safe cut-off and fault states with allowed staff acts.
How parts work as a set
Fault study, WCR basis, breaker or fuse list, settings and closed-overlap method when used.
Final one-line
Sources, bonds, neutral, PE, bypass, field wires, control power, data links and labels.
Care plan
Checks, tests, clean work, grease if allowed, parts, time plan and safe cut-off steps.
People and rights
Staff roles, switch limits, urgent response, alarms, bypass rules and change control.
Standards and product files
Name the exact standard, edition and product scope
A broad IEC, UL or CE claim does not prove that a switch or full system fits the job.
Transfer switching equipment
The current fourth edition applies to defined transfer switching equipment up to 1,000 V AC or 1,500 V DC. It includes ATSE, stand-alone ATS controllers, bypass/isolation transfer switch equipment and ATSE with closed-transition capability. It excludes static transfer switches covered by the IEC 62310 series.
Transfer-switch equipment
Check the active edition, exact product Listing, stated use, WCR marking and marked upstream protective-device conditions. A listed part does not settle the full system plan.
System and site rules
NFPA 70 and NFPA 110 can affect emergency, legally required and optional standby systems. Use the locally adopted edition and meet the authority having jurisdiction.
Work rules
NFPA 70B, NFPA 70E and OSHA rules can affect service and live-risk work in U.S. jobs. Local law and the site plan control.
Market proof is model-specific. Ask for the file number, standard edition, ratings and use conditions for the exact part number. Then check that the built board, site work and control steps still comply.
Quote-ready input
Send an ATS specificationBuild the ATS RFQ from the one-line
SENTOP can check model fit, switch gear, files, samples and supply needs. The skilled project team still owns the full power plan and approval.
01 UseState the site, load need, allowed power gap, target land, local body and system class.
02 One-lineShow both sources, genset, UPS, breakers, bypass, bonds, neutral, PE, loads and control power.
03 Source dataGive voltage, frequency, phase order, wires, ground plan, source size, pass limits, remote-start link and genset steps.
04 Load dataGive the highest phase and neutral root-mean-square (RMS) current, kW, kVA, power factor, motors, transformers, UPS, VFDs and inrush.
05 Source changeState open, delayed, in-phase or closed duty. Add change, return, load shed and fall-back needs.
06 Fault dutyGive fault current from each source, current fed by motors, the exact upstream breaker or fuse and the proposed marked fault basis.
07 Product and siteState poles, neutral sequence, class, lugs, case, heat, site height, cable entry, access and bypass.
08 Proof and supplyList rules, files, tests, settings, drawings, training, spare parts, order size, ship-to site, labels, pack and due date.
For a first pass, use the transfer-switch sizing tool. Then send the result with the one-line and fault study. Do not send only a current value.
Continue the specification
Related SENTOP resources
Frequently asked questions
Three-phase automatic transfer switch FAQ
How do I size a three-phase automatic transfer switch?
Start with the highest RMS phase current in each real load state. Add load response, heat, site height, wire and lug data, duty and local rules. Do not buy from one kVA sum or a fixed spare rate.
How do I calculate current from three-phase kVA?
For a balanced load, use I = 1000 × kVA ÷ (√3 × line-to-line voltage). For a load that is not balanced, check the highest phase current and the true neutral path. Add motor, transformer, UPS and harmonic data when they apply.
Do I need a 3-pole or 4-pole ATS?
The ground and bond study decides. A 3-pole ATS leaves the neutral solid. A 4-pole ATS switches it. Check source bonds, ground-fault sense, return paths, neutral current, fourth-pole rating and neutral switch steps.
What is the difference between PC class and CB class ATS?
They are IEC switch classes with unlike fault and trip roles. The exact product data and upstream breaker or fuse still control. Neither class label replaces the fault study or full gear review.
What fault-current data must match an ATS?
Find fault current from each source at the switch and add current fed by motors. Match the exact upstream breaker or fuse and its settings to the marked WCR, SCCR or other fault limits for the model.
How fast should a three-phase ATS transfer?
Fast enough for the load and source plan, but no one time fits all jobs. Full genset power-return time has sensing, time lags, engine start, voltage and frequency build-up, source checks, pole change and load restart.
Should I choose open, delayed, in-phase or closed transition?
Choose from load ride-through, motors, transformers, source compatibility and utility rules. Closed transition briefly overlaps two live sources that meet the selected controller's acceptance and synchronization limits. It does not remove the gap while a stopped genset starts.
Does an ATS start and stop the generator?
An ATS normally issues or removes a remote-start request. The generator-set controller handles cranking, fuel, speed, voltage regulation, protection functions, cooldown and shutdown. Document the exact field interface and responsibility for each step.
Which standard applies: IEC 60947-6-1 or UL 1008?
It depends on the target market, equipment and system. IEC 60947-6-1:2026 covers defined transfer switching equipment. UL 1008 covers transfer-switch equipment within its scope. Check the exact model file, standard edition, ratings and local adoption.
What should I send with a three-phase ATS RFQ?
Send the one-line, both source ratings, load list, allowed power gap, pole and neutral plan, fault study, upstream breaker or fuse, source-change and control needs, case, rules, tests, order size and ship-to site.
Main source files
Standards and maker proof
Use the full current rule, local code and exact product file for a final choice.
- IEC 60947-6-1:2026 — scope for transfer switching gear, ATSE, control, bypass and closed-change tasks.
- UL 1008, Transfer Switch Equipment — active North American product rule page. Check the exact Listing and label.
- UL switch services — switch product file and test scope.
- Eaton ATS basics — source-change modes, switch types, neutral and bypass facts.
- Cummins T-011 Transfer Switch Application Manual — source sense, start requests, source checks, motors and ratings. Use the current model manual for design.
- Schneider Electric / ASCO Publication 1128 — WCR facts and marked fault-rating use.
- NFPA 70 (2026) and NFPA 110 (2025) — U.S. power and standby rules. Local use controls.
- NFPA 70B (2026) and NFPA 70E (2024) — service and safe-work rules.
- OSHA 1910.333 — U.S. qualified-person and de-energization requirements for covered work.
Ready for model review?
Send the one-line, not only the ampere rating
SENTOP can match an ATS model and supply pack once the system duty is clear. Add source, load, fault, neutral, source-change, control, case, rules and order data.
