Quick answer
An automatic transfer switch (ATS) is a self-acting switch that watches utility voltage, signals a standby generator to start when the power fails, moves the home’s circuits to the generator, and moves them back after utility power returns.
It does the same safety job as any transfer switch, keeping the two sources apart, but no person has to be home to do it. That is what makes standby backup hands-free.
Quick facts
The key facts about Automatic Transfer Switch (ATS), with sources:
- Listing standard
- UL 1008, which covers automatic and manual transfer switches 1
- Usual partner
- A home standby generator on natural gas or propane 2
- Exercise
- Generac standby systems run a scheduled self-test, typically weekly for 5–12 minutes depending on model 2
- During the exercise
- Power usually is not transferred to the house 2
- Placement of generator
- Generac cites NFPA 37: 5 ft from openings such as windows and doors 3
Key takeaways
- An ATS is the brain and the gate of a standby system: it senses the outage and moves the load.
- It never lets utility and generator power meet, which protects line workers and the generator.
- It pairs with a standby generator or, in some systems, a battery gateway.
- A service-rated ATS can also act as the home’s main disconnect, which saves panel work in some installs.
- The transfer switch page covers the manual type; this page is about the automatic one.
What an ATS is
An automatic transfer switch is a box of heavy contacts plus a small controller. The controller watches the voltage coming in from the utility. When the voltage drops out, it waits a moment to make sure the outage is real, then tells the generator to start.
Once the generator is up to speed, the controller moves the contacts from the utility side to the generator side. When utility power is steady again, it moves them back, and the generator runs on for a short cool-down before it stops.
All of this follows one rule from NEC 702.5: transfer equipment must be built and installed so the normal and backup sources cannot be connected by accident 1. UL 1008 is the listing standard that tests the switch for this job 1.
How the automatic sequence works
Each maker sets its own timers, but the order is the same.
The switch senses loss of utility voltage, waits through a short delay so a flicker does not start the engine, starts the generator, waits for it to warm up, and transfers.
When the grid comes back, it waits again to be sure the grid is stable, transfers back, and lets the engine cool down.
The ATS also manages the weekly or periodic exercise. Generac says its standby systems run a scheduled self-test, and that power is usually not transferred to the house during it 2.
- SenseUtility voltage drops out
- DelayShort wait to ignore flickers
- StartSignal the standby generator
- TransferContacts move to generator once it is stable
- RetransferUtility returns and holds steady; contacts move back
- Cool downEngine runs unloaded, then stops
- Back to step 1
Types of automatic transfer switches
| Type | What it switches | When it is used |
|---|---|---|
| Whole-house, service-rated | The entire service; can serve as the main disconnect | Generator sized for the whole home |
| Whole-house with load management | Entire service, but sheds big loads like AC when the generator is near its limit | Generator smaller than the full home load |
| Partial-home ATS | A subpanel of chosen circuits | Smaller standby units |
| Battery gateway / backup switch | Home or a critical loads panel to a battery | Solar-plus-storage systems 7 |
| Meter-mounted automatic | Whole home at the meter socket | Only where the utility allows 5 |
Example: what happens in a real outage
Here is a timeline for a home with a standby generator and a service-rated ATS. The timers are illustrative; each model sets its own. The exercise figures come from Generac’s published support notes 2.
| Moment | What the ATS does | What you notice |
|---|---|---|
| Grid fails | Senses loss of voltage, starts a short delay | Lights go out |
| A few seconds later | Signals the generator to start | Engine starts outside |
| Generator is stable | Transfers the load | Lights return |
| Grid returns | Waits for steady voltage, then retransfers | A brief blink |
| After retransfer | Generator cools down and stops | Engine quiets |
| Next scheduled test | Runs the exercise, usually without moving the house load | Engine runs 5–12 minutes 2 |
The cost of the gap matters for some homes. Our outage cost tool puts a number on lost food, work and comfort.
Where the ATS sits
Most home ATS units mount outdoors near the meter or indoors next to the main panel. On a standby job quote it often appears bundled with the generator as “generator with ATS,” plus wiring, gas line and permit.
If the ATS is service-rated, it may carry the “service disconnect” label and serve as the home’s main shutoff. Under the 2023 NEC, one- and two-family homes need an outdoor emergency disconnect, readily accessible and marked “EMERGENCY DISCONNECT” 4.
Benefits and drawbacks
What it does well
- Works when no one is home, including overnight and during travel.
- Restores power in seconds, which matters for medical devices, sump pumps and freezers.
- Runs the generator’s exercise to keep it ready 2.
- Load-managing models let a smaller generator cover a whole house.
Where it falls short
- Only useful with a standby generator or battery system.
- More parts than a manual switch, and a controller that needs power to work.
- A short gap still occurs; it is not instant like a UPS.
- Needs a licensed electrician, often a gas fitter, and permits.
Limits to know
- Not gap-free for computers. The transfer delay causes a blackout of seconds; desktop PCs restart unless they sit on a UPS.
- Load still matters. An ATS cannot make a small generator carry central air unless it sheds loads.
- Utility rules. Meter-mounted designs need utility approval 5.
- Exercise can’t be skipped. Generac notes exercise cannot be disabled, and turning the panel off may affect warranty 2.
What drives the cost
No prices here. These are the factors behind an ATS quote; the cost tool shows how they add up.
- Amperage rating. A 200-amp service-rated ATS costs more than a small partial-home unit.
- Load management modules. Each shed load adds a relay or module.
- Indoor vs outdoor enclosure. Outdoor-rated boxes cost more.
- Panel changes. Moving circuits to a new subpanel adds labor.
- Permits. Electrical and sometimes gas permits.
- Warranty. Usually tied to the generator’s registered warranty; read its terms.
How an ATS gets installed
A licensed electrician handles the electrical side. Here is the sequence so you know what to expect.
- A load calculation decides whole-house or partial-home design.
- The generator site is chosen; Generac cites NFPA 37 clearances of 5 ft from openings and 3 ft at the front and ends 3.
- Permits are pulled for electrical work and, where needed, the gas line.
- The ATS is mounted and wired between the utility service and the panel.
- The system is tested with a simulated outage, then inspected.
Upkeep
- Leave the generator in automatic mode so the ATS can run the exercise 2.
- Book the generator service the maker sets out, which often includes an ATS check.
- Keep the ATS enclosure clear of plants and water.
- Ask an electrician to test a full transfer once a year if your model does not do it.
Warning signs
Call your installer if the generator runs but the house stays dark, if power does not return to utility after the grid is back, or if the ATS shows a fault light. A buzzing or hot enclosure is a reason to call an emergency electrician.
If you smell gas at any time other than during the exercise, Generac advises calling a dealer or the gas company right away 2.
Do not open the ATS cabinet
An ATS holds utility and generator power at the same time and can switch without warning. Leave all checks inside the enclosure to a licensed electrician. Never try to bypass it to power the house.
Codes and standards
| Authority | Rule | Requirement |
|---|---|---|
| NFPA 70 (NEC) | 2017 ed. as cited by UL, Section 702.5 | Transfer equipment must prevent inadvertent interconnection 1 |
| NFPA 70 (NEC) | 2023 ed., Section 230.85 | Outdoor emergency disconnect at one- and two-family homes 4 |
| UL | UL 1008 | Listing standard for automatic and manual transfer switches 1 |
| NFPA 37 (via Generac) | Engine installation clearances | 5 ft from openings, 3 ft front and ends 3 |
ATS vs other ways to switch
| Option | Switches itself? | Typical gap | Paired with |
|---|---|---|---|
| ATS | Yes | Seconds | Standby generator |
| Manual transfer switch | No | Until you flip it | Portable generator |
| Interlock kit | No | Until you flip it | Portable generator |
| Battery gateway | Yes | Very short; varies by system | Battery storage |
| UPS | Yes | None for the plugged-in device | One device |
Myths
- Myth An ATS gives uninterrupted power.
- Reality There is a gap of seconds while the generator starts. Only a UPS bridges that.
- Myth The ATS starts a generator that is turned off.
- Reality The generator must be left in automatic mode for the ATS signal to work 2.
- Myth Any ATS can go with any generator.
- Reality Ratings, controls and listing must match the generator and service.
Where hands-free backup pays off most
Outage exposure is the state factor. EIA found U.S. customers averaged 11 hours without power in 2024, while South Carolina averaged nearly 53 hours 6. Code rules also depend on which NEC edition your state has adopted.
Is an ATS right for you?
If someone in the home relies on powered medical gear, if you travel, or if your area sees long storm outages, an ATS with a standby generator is the hands-free option. If outages are short and you are home, a manual switch costs less.
Start with the generator sizing calculator, then compare quotes on our standby generator installation page.
- If You are often away
- Then Automatic
- If Medical equipment at home
- Then Automatic, plus a UPS for the device
- If Short, rare outages and a portable unit
- Then Manual switch or interlock
- If Solar with a battery
- Then The battery’s own automatic gateway
Next step
Size the load first. The ATS rating follows the generator and the service, not the other way round.
Questions about Automatic Transfer Switch (ATS)
How does an automatic transfer switch know the power is out?
It watches the voltage on the utility lines. When the voltage drops below a set level for longer than a short delay, the controller decides the outage is real and signals the generator to start.
The delay keeps a quick flicker from starting the engine. Exact thresholds and timers vary by model and are set by the maker.
How long does an automatic transfer switch take to switch?
Usually a matter of seconds. The switch waits a moment to confirm the outage, the generator starts and warms up, and then the load transfers. Times differ by model. Because there is a gap, computers and some electronics will lose power briefly unless they are plugged into a UPS.
What is the difference between a transfer switch and an automatic transfer switch?
Transfer switch is the whole category. An automatic transfer switch is the type that acts on its own, while a manual transfer switch is thrown by hand. Both must keep utility and generator power apart, under NEC 702.5, and both are listed to UL 1008. Automatic models also start the generator.
Can an automatic transfer switch work with a portable generator?
Usually not. An ATS needs a generator it can start remotely, which most portable units cannot do, and a person still has to roll the portable unit out and fuel it. Portable generators pair with a manual transfer switch or a listed interlock kit, installed by an electrician.
What is a service-rated ATS?
A service-rated ATS is built to sit at the service entrance and act as the home’s main disconnect, ahead of the panel.
That lets it switch the entire house and can save adding a separate disconnect.
It must be listed for that use, and the 2023 NEC also requires an outdoor emergency disconnect at one- and two-family homes.
Does the generator power my house during its weekly test?
Usually not. Generac says that during the scheduled exercise the generator starts and runs to test itself and keep parts lubricated, but power is usually not transferred to the house. Depending on the model, the exercise runs weekly, every two weeks or monthly, for about 5 to 12 minutes.
Can an ATS run my whole house on a smaller generator?
Some can, using load management. The ATS or added modules switch off large loads, such as an air conditioner, when the generator nears its limit and bring them back when there is room. Whether that works for you depends on your loads, which a sizing calculation will show.
Do I need a permit for an automatic transfer switch?
In most places, yes. An ATS ties into your service and adds a second source, so electrical permits and inspection are standard. A standby generator often needs a gas permit too. The NEC edition and local amendments your state adopted decide the details, and a licensed electrician handles them.
Does a battery system use an automatic transfer switch?
Battery systems use a gateway or system controller that does the same job: it senses the outage, isolates the home from the grid and switches to battery power. Enphase, for example, describes whole-home and partial-home backup layouts built around its system controller. The idea is the same as an ATS.
Sources
- IAEI Magazine, UL Question Corner: transfer equipment, NEC 702.5, UL 1008, 2017 NEC, May/June 2017, retrieved .
- Generac Support, What Does the Exercise Timer Do on a Home Standby Generator, retrieved .
- Generac Support, What Should I Know About Clearance Requirements When Installing My Generator, retrieved .
- Larimer County (CO), 2023 NEC Significant Code Changes, retrieved .
- PG&E, Electric Rule No. 31, Customer-Owned Meter Socket Adapter Devices, retrieved .
- U.S. EIA, Today in Energy: hurricanes in 2024 led to the most hours without power in 10 years, retrieved .
- Enphase, Storage System Owner’s Guide (partial and whole home backup), retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Checked against IAEI’s UL column, Generac support articles, Larimer County’s 2023 NEC summary, PG&E Rule 31, Enphase’s owner’s guide and EIA outage data on 2026-10-10.
Suggest a correction. We fix errors and say what changed.