Will An Automatic Transfer Switch Continue to Draw Current When the Generator Is Not Running?

This article explains the normal power present at a Generac automatic transfer switch (ATS). It clarifies that the ATS has constant 240 VAC supplied by two 120 VAC lines, and that it also draws a small amount of 120 VAC on the T1 circuit for battery charging. Use this information when troubleshooting questions like “ATS has power,” “T1 circuit voltage,” “charger missing AC,” or low battery concerns, helping ensure reliable starting and operation.

  • Jul 1, 2026
  • Knowledge

Title
Will An Automatic Transfer Switch Continue to Draw Current When the Generator Is Not Running?
URL Name
Will-An-Automatic-Transfer-Switch-Continue-to-Draw-Current-When-the-Generator-Is-Not-Running

Public Answer

Will an automatic transfer switch continue to draw current when the generator is not running?

Yes. An automatic transfer switch (ATS) can remain energized when the generator is not running. In normal standby operation, the electricity present at the ATS comes from utility power, not the generator.

The utility source side of the ATS normally receives 240 VAC, supplied by two 120 VAC lines. The ATS also provides a constant 120 VAC supply on the T1 circuit, which powers the generator battery charger and helps keep the generator battery charged while utility power is available.

Where the ATS gets power

The ATS can receive electricity from either the utility source or the generator source, depending on system conditions.

  • When utility power is available, the ATS is energized by utility power. This is the normal condition when the generator is not running.
  • When utility power fails and the generator starts, the ATS transfers the electrical load so generator power can supply the home or building.
  • If utility power fails and the generator is not running or is not powering the home, the ATS may not have normal source voltage available.

Voltage may be present at the ATS whenever utility power is available or when the generator is running and supplying the home. Current flows through the ATS when connected loads, control circuits, or charging circuits are using power.

Environment

This article applies to Generac home standby generator systems using an automatic transfer switch (ATS) with a T1 battery charging circuit.

See also

Was this article helpful?
Upload Files


Need more help? Get in touch with our support team.


Contact Support