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How to Connect An Automatic Transfer Switch (ATS) to A Generator?

Author : Automatic Transfer Switch PowerHome | Published On : 17 Sep 2026

The Automatic Transfer Switch (ATS) is the core component connecting the generator. Connect the mains power, generator, and load to their respective terminals on the ATS, and connect the generator start signal from the ATS to the generator control system. When a mains power failure occurs, the ATS detects the power anomaly and sends a start signal to the generator. Once the generator starts and reaches a stable output, the ATS automatically switches the load from mains power to generator power. After the mains power is restored and stabilized, the ATS switches back to mains power and sends a shutdown signal.

Steps for Connecting an ATS to A Generator

1. Disconnect All Power Sources

Before connecting, turn off the mains power, generator output, and related control power, and ensure that the lines are not energized. An ATS typically connects to both mains and generator power simultaneously; therefore, incorrect wiring can cause serious electrical hazards. Installation and commissioning should be performed by qualified electrical personnel.

2. Confirm the Three Main Connections of the ATS

Before installation, review the ATS wiring diagram and confirm the following three terminal connections:

  • Main Power Input: Connect to the mains power supply.
  • Generator Input: Connect to the generator output.
  • Load Output: Connect to the load that needs to be automatically switched to power by the ATS.

Some ATS systems also have independent control terminals for connecting the generator start signal.

3. Connect the Mains Input

Connect the mains power supply to the ATS's mains input. Wiring methods differ for single-phase, three-phase, and different voltage systems. Connect the corresponding lines according to the ATS's rated voltage and terminal markings. Do not determine the wiring method solely based on terminal locations; refer to the equipment wiring diagram.

4. Connect the Generator Output

Connect the generator's AC output to the ATS's backup power input. The generator's output voltage, number of phases, and frequency must match the ATS's allowed backup power specifications. For three-phase systems, the phase sequence must be specifically confirmed to avoid incorrect phase sequence after generator connection. After connection, check that the wiring between the generator output and the ATS backup input is secure and confirm that the generator output has not been mistakenly connected to the mains input.

5. Connect the ATS Load Terminal

Connect the load requiring automatic power switching to the ATS load output terminal. Under normal circumstances, the load is powered by mains electricity. When the mains power fails and the generator reaches stable output conditions, the internal switching mechanism of the ATS transfers the load to the generator side. It is crucial that the ATS load capacity meets the actual load requirements, especially for equipment with high starting currents such as motors, air conditioners, and water pumps.

6. Connect the Generator Start Signal

Connecting the generator start signal is one of the most important control circuits when connecting the ATS to the generator. The ATS typically sends a start signal to the generator controller through a separate generator start terminal. When the mains power is normal, the ATS remains in normal operation. Upon detecting a mains power failure, the ATS issues a start command, and the generator controller receives the signal and starts the engine.

Common systems use dry contact start signals, but different generators may employ different start control methods, such as two-wire start, three-wire start, or other control interfaces. Therefore, it cannot be assumed that all generators use the same start wiring.

Before connecting, the following must be confirmed:

  • ATS generator starting terminal definition
  • Generator remote starting terminal definition
  • Starting signal type
  • Control voltage requirements
  • Permitted starting methods of the generator controller

If the ATS and generator control interfaces are incompatible, a manufacturer-specified interface module or relay must be used for conversion.

7. Check Neutral and Grounding

After completing the main wiring connections, the neutral and grounding methods must be confirmed. ATS may use different pole numbers and neutral switching methods, and generator systems may use different grounding configurations. The connection of the grounding conductor and neutral conductor must comply with the equipment instructions and local electrical codes. Especially in stand-alone generator systems, the system grounding and neutral point configuration must be confirmed to avoid incorrect multiple neutral grounding points.

8. Check All Wiring

Before formal startup, conduct a comprehensive check of the connection between the ATS and the generator. Key checks:

  • Is the mains power connected to the normal power source?
  • Is the generator connected to the backup power source?
  • Is the load connected to the load terminal?
  • Is the generator start signal connected to the correct control terminal?
  • Is the three-phase system phase sequence correct?
  • Do the neutral wire and grounding conform to the system configuration?
  • Are the terminal screws and wiring secure?
  • Do the wiring specifications meet the rated current requirements?

Also, confirm that there is no prohibited direct parallel connection between the mains power and the generator. The function of a typical ATS is to switch two power sources, not to allow two independent power sources to operate in parallel simultaneously.

Testing Steps After ATS Connection

First, ensure the mains power is normal and that the ATS is in the normal power position and the load is functioning normally.

Then simulate a mains power failure. After detecting the mains power anomaly, the ATS should send a start signal to the generator. After the generator starts and reaches the specified output conditions, the ATS switches the load to the generator side.

Next, restore the mains power. The ATS should detect the mains power restoration and switch the load back to mains power after the specified stabilization time. After the switchback is complete, the generator will typically continue to run for a cooling period before stopping.

If any step does not work as expected, stop the test and check the ATS control wiring, generator remote start settings, and power input status.

Common Wiring Errors

The most common problem when connecting an ATS to a generator is not the ATS's switching mechanism itself, but rather incorrect connection of the power supply and control terminals.

  • Reversed Mains and Generator Inputs: The normal power supply and backup power supply must be distinguished according to the ATS terminal markings; physical location alone is insufficient.
  • Ignoring the Generator Start Signal: Connecting only the generator power line without correctly connecting the start control circuit may prevent the ATS from automatically starting the generator in the event of a mains power failure.
  • Failure to Confirm Start Signal Type: The remote start interfaces of the ATS and generator are not entirely the same; the control method must be determined according to the instruction manuals of both devices.
  • Incorrect Three-Phase Sequence: After connecting a three-phase generator to the ATS, ensure the phase sequence meets system requirements.
  • Incorrect Neutral Line Handling: The neutral line handling method differs depending on the number of poles in the ATS and the generator grounding system; wiring must be done according to the specific system configuration.
  • No simulated power outage test performed: After wiring is completed, it is not enough to only test the normal state of the mains power. The complete process of mains power failure, generator start-up, load transfer, mains power restoration and switching back should also be tested according to the manufacturer's specifications.

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