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Best for: documenting ATS/MTS timing on a UPS-protected bus with on-site generation, or screening residential interlock vs automatic transfer switch amp classes.

Not ideal for: utility interconnection protection, arc-flash studies, or selective coordination—those need project-specific PE deliverables.

Quick answer: how do I plan ATS transfer time and size? #

Transfer time (UPS bridge): Sum generator crank + stabilize + ATS delay + written operations margin. Enter that same bridge in the UPS Runtime Calculator and Generator UPS Calculator—not the ATS contactor datasheet alone.

Size the switch (residential screening):

Setup Typical hardware Notes
Portable generator, few circuits Interlock kit or 6–10 circuit manual transfer switch Match inlet amps (often 30 A) and panel ampacity
Standby generator, whole panel Automatic transfer switch (ATS) 100–200 A class Generator controller + ATS must be a listed pair when required
UPS-protected critical bus + site gen ATS on UPS input (or building) + UPS bridge minutes See sequence tables below

For plug-in hour-scale backup without a generator, compare portable power station vs UPS instead of forcing an ATS.

Use this guide with the Transfer Switch Size Calculator, Generator UPS Calculator, Generator Size Calculator, UPS Runtime, and the UPS calculator hub / Power calculator hub. Decide interlock vs ATS/MTS in interlock kit vs transfer switch.

Screen amp frame → Transfer Switch Size Interlock vs transfer switch

Transfer switch sizing chart (residential screen) #

Generator / service cue Screen switch / inlet class Typical path
Portable ~3–5 kW, few circuits 30 A inlet + interlock or small MTS Attended backup
Portable ~7.5 kW 30–50 A class Fridge + sump + lights—not whole HVAC
Standby on 100 A service 100 A ATS (match service) Unattended
Standby on 200 A service 200 A ATS Whole-panel standby
Selected-circuit MTS 6–10 circuit / 30–50 A Labeled essential loads

Open the Transfer Switch Size Calculator for generator-kW, whole-house, or select-circuit modes. Chart is educational screening—not a stamped panel schedule.

How do I choose manual vs automatic transfer? #

Factor Manual (MTS) / interlock Automatic (ATS)
Who is home? Someone can start gen and flip switch Unattended homes, storms, vacation
Cost / complexity Lower Higher (controller wiring, commissioning)
Transfer speed Human minutes Seconds after gen stable
UPS role Still need UPS for IT blinks while you set up UPS bridges crank → ATS close
Code / listing Interlock must be listed for your panel Prefer generator OEM–matched ATS when specified

Example — 7.5 kW portable gen on a 200 A house panel: Many jurisdictions allow a listed interlock plus a 30 A inlet. Budget UPS minutes only for devices you keep plugged into UPS; the rest wait until you transfer manually.

Example — 22 kW standby gen: Plan a 100–200 A ATS, document crank + warm-up in the MOP, and size UPS Ah for that bridge on critical IT—not for running the whole house on batteries.

What transfer switch amp / circuit count do I need? #

Screening only—final selection is panel schedule + electrician:

  1. List must-run circuits (fridge, boiler controls, sump, network, medical).
  2. Sum continuous amps at the feeder voltage; apply NEC continuous-load factors where required.
  3. Pick hardware class:
Class Typical rating Shop (Amazon search)
Interlock kit Matches 100–200 A main panel Generator interlock kit
Manual 6–10 circuit 30–50 A generator inlet common Manual transfer switch 30 A
Automatic whole-panel 100–200 A ATS Automatic transfer switch 200 A

After hardware class is clear, size amps with Transfer Switch Size, the generator with Generator Size, and the UPS bridge with Generator + UPS. For interlock vs ATS/MTS decisioning, see interlock kit vs transfer switch.

Worked example: 200 kW critical load bridge budget #

Given: 200 kW IT/critical bus, double-conversion UPS, site MOP:

Step Policy time
Utility loss → gen crank 10 s
Engine start to stable 35 s
ATS delay (BBM) 0.2 s
Operations warm-up margin 60 s
Total bridge ~105 s (~1.8 min)

Planning actions:

  1. Set UPS Runtime Calculator target ≥ 2 min at 200 kW (round up from 1.8 min).
  2. Enter same bridge in Generator UPS Calculator for kVA screening.
  3. Document BBM—UPS carries 100% kW during the 0.2 s break.

If warm-up policy is 3 min instead of 60 s margin, bridge becomes ~3.5 min—battery minutes must follow written MOP, not ATS nameplate alone.

What “transfer time” means on a UPS feed #

On many industrial one-line diagrams:

  1. Utility feeds a UPS input (rectifier) and sometimes a bypass path.
  2. UPS output feeds a critical bus (PDUs, PLCs, IT).
  3. An ATS upstream or downstream selects utility vs generator for the UPS input or whole building depending on architecture.

The UPS covers gaps when:

  • Utility fails before the generator is online.
  • The ATS breaks before it makes (break-before-make).
  • Voltage or frequency is out of window during re-transfer to utility.
Term Typical meaning Planning note
Crank / start Engine reaches stable speed Cold weather adds seconds to minutes
Stabilization Voltage and frequency in band Operations may extend for smoke/load policy
ATS transfer Switch to emergency source Overlap vs break-before-make changes ride-through
Re-transfer cooldown Delay before returning to utility UPS may see voltage step during sync

Standard sequence (write this in your MOP) #

Document each step with owner, max time, and UPS state:

Step Event Example time budget UPS role
1 Utility loss detected 0 s Inverter on battery
2 Generator start command 0–2 s Battery carries full load
3 Engine crank and accelerate 15–45 s Battery carries
4 Generator voltage/frequency stable 30 s–3 min Battery carries
5 ATS transfer to generator 0.1–1 s (device) May see micro-break on some designs
6 UPS on stable emergency feed Steady state Rectifier may recharge battery
7 Utility returns — Monitor sync policy
8 Re-transfer cooldown 5–30 min typical Battery if utility not yet accepted
9 ATS back to utility 0.1–1 s UPS absorbs mismatch briefly

Planning bridge minutes for the UPS should cover steps 1–5 at minimum—often 10–15 minutes in data centers and 5–10 minutes in industrial plants with proven fast start, unless your written MOP proves a shorter window.

Worked example: 50 kW critical bus with ATS #

Assumptions:

  • Critical load 50 kW, PF 0.85
  • Generator crank 30 s, stabilization 90 s, ATS delay 15 s
  • Operations adds 5 min policy margin for cold-start worst case

Time budget to generator stable:

  • Device timing: (30 + 90 + 15) s ≈ 2.25 min
  • With operations margin: plan 7–10 min UPS bridge (not 2 min)

Generator kVA (screening):

  • Use Generator UPS Calculator with 50 kW, 10 min bridge, 5 kW recharge → about 93 kVA required → 100 kVA frame (planning only).

Runtime check:

  • Enter 10 min target in UPS Runtime Calculator with your battery V, Ah, and efficiency—minutes must clear the MOP table, not the ATS nameplate alone.

Break-before-make vs overlap #

Break-before-make (BBM): Load is briefly unsupported by utility or generator—the UPS must carry 100% of critical kW for that interval.

Overlap (closed transition): Requires compatible sync and permissives; reduces UPS stress but adds protection and training requirements. Never assume overlap because the ATS brochure mentions it—verify site license and utility rules.

If BBM is 200 ms at the ATS but the UPS sees a 10 ms output glitch from downstream coordination, log both—the weak link sets runtime acceptance.

Re-transfer and “cooldown” traps #

Re-transfer is where many Sunday tests fail:

  • Generator runs stable for hours; utility returns; ATS waits cooldown; UPS still on battery if input path drops during sync checks.
  • Closed transition back to utility without UPS input may cause DC bus disturbances if rectifier sync is aggressive.

Write cooldown and sync permissives in the same table as crank time. If cooldown is 15 minutes, UPS minutes at commissioning must prove 15 minutes at load, not only 3 minutes to first transfer.

Common mistakes #

  1. Using generator brochure start time without plant warm-up policy.
  2. Ignoring BBM at the ATS when UPS sizing assumed seamless transfer.
  3. Testing at no-load only—transfer at design kW exposes voltage dip and recharge surges.
  4. Splitting ownership—electrical owns ATS, facilities owns generator, nobody owns total bridge minutes.
  5. Buying a 30 A inlet for a 50 A load schedule—circuit count and amps must match the generator breaker and cord.
  6. Skipping the UPS for “we have a generator”—ATS does not remove utility blinks before the engine is online.
  7. Using a portable power station as an ATS substitute for hardwired panels—unsafe and usually non-code; see portable power vs UPS for plug-in loads only.

See also UPS Runtime: Common Mistakes and Complete UPS Sizing Guide Step 7.

Next steps #

  1. Build a one-page MOP table with max seconds/minutes per step and sign-off from operations (or a residential circuit list for MTS/interlock).
  2. Run Generator Size → pick transfer hardware class → Generator UPS Calculator and UPS Runtime with the same kW and bridge minutes.
  3. Schedule a loaded transfer test at partial design kW before SAT sign-off (or a documented homeowner drill for portable setups).
What transfer time should I enter in the Generator UPS calculator?

Use your written planning bridge—often gen start + ATS delay + operations margin in minutes—not the ATS contactor rating alone. Override with the MOP total if it is longer than derived seconds.

Does the UPS recharge during generator operation?

Often yes—the rectifier loads the generator after transfer. Include optional recharge kW in generator sizing. If recharge is deferred by policy, document that assumption.

How is this different from the ride-through guide?

This guide focuses on ATS sizing cues and generator sequence timing. The ride-through guide focuses on UPS battery minutes and how to verify they cover the sequence.

Do I need overlap transfer?

Not always—many industrial sites use BBM with UPS bridging. Overlap requires sync engineering, training, and utility approval where applicable.

Interlock kit vs manual transfer switch—which is safer?

Both can be code-compliant when listed for your panel and installed by a qualified electrician. Interlocks keep the normal panel layout; multi-circuit MTS boxes isolate selected branch circuits. Never backfeed through a dryer outlet.

Can I skip ATS if I only buy a portable power station?

Yes for plug-in loads only. Hardwired panels still need a listed transfer method for a generator. Compare options in Portable power station vs UPS.