New Install Commissioning: Won't Transfer, ATS vs Controller vs Utility Sense Decision Tree

Why this matters

The genset starts on the commissioning test, runs clean, makes voltage and frequency, and then the load never moves. The customer is standing there expecting their lights to swap to generator and nothing happens. On a new install this is almost never a failed part. It is a wiring, sensing, or configuration miss made during the install, and the three suspects are the ATS, the generator controller, and the utility-sense circuit. Working the wrong one first burns the commissioning window and makes the install look amateur. This tree separates the three in the order that costs the least time and verifies each branch with a real measurement.

Symptom presentation

• Engine cranks and runs on the test button or test mode, but utility loss produces no engine start, or the engine starts and the contactor never picks up the generator side.
• Variations: engine starts but ATS stays on utility; engine never starts on simulated utility loss but starts on the controller test; ATS hums or chatters but will not seat; everything works in manual but not in auto.

Note which variation you have. Each one points to a different branch before you lift a cover.

Quick checks

• Confirm the system is in AUTO, not OFF or MANUAL, on both the generator controller and the ATS. A new install left in OFF is the single most common cause.
• Verify the start-signal wiring between the ATS and the generator. On most residential and light-commercial units this is a two-wire start (often labeled 23/194 or equivalent). A swapped or open pair means the ATS calls and the generator never hears it.
• Confirm utility voltage is present and within the sense window on all phases the ATS monitors.
• Check the generator battery voltage at rest (12.6 V for a healthy 12 V system) so a weak battery does not masquerade as a control fault.
• Confirm the ATS control transformer or control power is energized; many switches will not actuate the coil without it.

Isolation tree

Start by deciding whether the engine even gets a start command on a real utility loss.

Branch A: Simulate utility loss (open the utility breaker feeding the ATS) and watch the engine.

If the engine does NOT crank: The fault is upstream of the engine, in utility-sense or the start signal.

• Measure the start-signal terminals at the generator controller while utility is removed. If the two-wire start does not close (no continuity / no expected voltage change), the ATS is not commanding start. Move to the ATS sense circuit.
• At the ATS, confirm the controller registers utility loss. If the ATS still shows utility present with the breaker open, the sensing leads are landed on the wrong source side or the sense fuses are blown. Correct the landing and replace any open sense fuse.
• If the ATS sees the loss and closes the start contacts but the generator still does not crank, the start pair to the generator is open, swapped, or landed on the wrong terminals. Ring it out end to end and reland.
• Confirm the generator controller is in AUTO and that no active fault (low battery, fault lockout from a prior failed start) is inhibiting auto-start. Clear and reset.

If the engine DOES crank and run but the load does not transfer: The fault is in the ATS power section or its transfer logic, not in sensing or start.

• Confirm the generator produces voltage and frequency at the ATS generator-side lugs (verify 120/240 V split-phase or the rated three-phase voltage, and 60 Hz). No power at the lugs means a generator-output or wiring fault, not the ATS.
• If generator power is present at the ATS but the contactor will not move to the generator side, check the transfer time delays. Many controllers hold for a warm-up timer before transfer; wait out the full delay before calling it failed.
• Inspect the ATS for a mechanical interlock or shipping pin not removed at install. New switches sometimes ship with a lock that blocks the generator-side contactor.
• Verify control power to the ATS transfer coil. A missing neutral or open control fuse lets the engine run with no way to actuate.

Branch B: If auto fails but MANUAL transfer works.

Manual working and auto failing isolates the fault to the automatic-control chain: utility sensing, the auto/test logic, or the start signal, never the power contacts (those are proven by the successful manual transfer). Work Branch A's sensing and start-signal steps; the power section is cleared.

Branch C: If the engine starts on the controller TEST but never on utility loss.

The controller and engine are good. The break is in how the ATS tells the generator to start. Confirm the ATS sense window settings (dropout voltage and time delay) are not set so wide that a clean utility drop never trips them, then ring out the two-wire start from ATS to generator.

Confirming diagnosis

• Utility-sense fault: with the utility breaker open, the ATS now displays utility loss and the start contacts close. Confirmed once the sense leads are correctly landed and fuses are good.
• Start-signal fault: continuity from ATS start terminals to the generator controller is correct and the engine cranks on simulated loss. Confirmed.
• ATS power/logic fault: generator power is verified at the ATS lugs, all timers expired, interlock/shipping pin removed, and the contactor seats to the generator side. Confirmed.
• Mode/config fault: setting both devices to AUTO restores full operation on a clean simulated loss. Confirmed.

Remediation

• Reland miswired sense or start leads to the correct source side and terminals, then re-ring the run end to end.
• Replace open sense or control fuses and confirm control power present.
• Remove any shipping pin or mechanical lock left in the ATS at install.
• Set utility dropout and transfer-delay parameters to the design values; do not leave factory-wide windows on a final commissioning.
• Run a full end-to-end commissioning test: kill utility, confirm auto-start, confirm transfer, restore utility, confirm retransfer and cooldown. Log the timings.

References

• NEC Article 702, Optional Standby Systems, and Article 701, Legally Required Standby Systems, transfer-equipment requirements.
• NEC Article 700, Emergency Systems, automatic transfer-switch requirements where applicable.
• NFPA 110, Standard for Emergency and Standby Power Systems, transfer-switch and commissioning/acceptance-test requirements.
• UL 1008, Standard for Transfer Switch Equipment, and the ATS manufacturer installation and commissioning manual.