Introduction #

When this guide fits: You must document automatic transfer switch (ATS) timing for a UPS-protected bus that also has on-site generation—crank, warm-up, transfer, and re-transfer back to utility.

When it is not suitable: You need utility interconnection protection, arc-flash incident energy, or selective coordination studies—those require project-specific PE deliverables.

Transfer time is not a single number on a datasheet. It is a sequence of events where the UPS must keep the load alive while the generator becomes the stable source. If your maintenance and operations team (MOP) uses brochure 10 s start assumptions but the plant policy is 3 min warm-up, battery minutes will fail on the first real outage.

Use this guide with the Generator UPS Calculator, UPS Runtime Calculator, and UPS calculator hub so timing budgets and kVA land in the same workbook.

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.

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.
  2. Run Generator UPS Calculator and UPS Runtime Calculator with the same kW and bridge minutes.
  3. Schedule a loaded transfer test at partial design kW before SAT sign-off.
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 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.