Choose an Automatic Transfer Switch That Matches the Generator and the House
Choose whole-home or essential-load transfer, then verify service amps, generator capacity, controls, poles, neutral, fault rating and enclosure.
An automatic transfer switch (ATS) is the usual connection for a permanently installed standby generator that must operate when nobody is home. It detects a utility failure, starts a compatible generator and transfers the connected loads. When utility power returns, it transfers back and allows the generator to cool down before shutdown (Cummins).
A standard standby-generator ATS is generally not suitable for a portable generator. Generac says its standby ATS relies on controls that portable units do not have and warns that connecting a manually started portable could damage both devices. A portable unit normally needs listed manual transfer equipment selected for that generator and installation (Generac, June 2025). See which manual 200-amp transfer switch you need for that arrangement.
Choose whole-home or essential-load transfer first
| Backup plan | Switching arrangement |
|---|---|
| Permanently installed standby generator; automatic operation | Compatible ATS |
| Only a refrigerator, furnace, sump pump and selected lights need backup | ATS feeding a dedicated essential-load panel |
| Most or all panel circuits should remain available | Whole-panel ATS, with enough generator capacity or automatic load management |
| Portable generator started by hand | Listed manual transfer equipment—not a typical standby ATS |
Both whole-panel and essential-load systems can transfer automatically. Eaton’s residential diagrams show a whole-house switch feeding all branch circuits and an essential-load configuration feeding a separate subpanel (Eaton residential transfer-switch catalogue).
A whole-panel ATS does not mean a smaller generator can run every appliance at once. The connected-load calculation must account for running demand and motor-starting surge. Central air conditioners, well pumps and sump pumps can determine the required capacity. Optional loads may need to be locked out or shed automatically. Use the calculation method in portable generator sizing and runtime, then have the standby-system design verified for the actual equipment.
Verify these six specifications
1. Utility-service amperage
For whole-home coverage, the ATS typically matches the electrical service: a 200A service generally uses a 200A ATS. A lower-rated switch can serve selected circuits when the installation is properly protected (Generac compatibility guide, July 2026).
A 200A ATS does not imply 200A of generator output. Its rating applies to the switch and the service path. The generator has its own output rating and protective breaker. Supplying 200A at 240V would equal roughly 48,000W, but Generac notes that most homes do not require a generator matching the full utility-service amperage. A smaller generator can work with a 200A ATS when the backup load is limited or managed (Generac ATS-breaker explanation).
2. Generator capacity and load management
The generator must handle the selected running loads and the largest relevant starting surges. Load-management modules can temporarily prevent lower-priority equipment from operating during overload. One Generac ATS manual, for example, describes shedding connected loads when generator frequency indicates an overload and then restoring them by priority after a delay (Generac ATS manual). Those thresholds and delays are product-specific.
3. Generator-controller compatibility
Match the ATS to the generator’s approved control method, not just its voltage and amperage. In some systems the generator controller decides when to transfer; in others, logic is in the switch. Generac says this difference commonly makes its ATS incompatible with other manufacturers’ generators, although certain two-wire-start arrangements may work when both manufacturers approve them (Generac compatibility guide). Obtain written model-to-model compatibility before purchase.
4. Voltage, phase, poles and neutral arrangement
The generator and ATS must match the home’s actual voltage and phase. For a 120/240V single-phase service, that means equipment specifically rated for that system—not merely a switch with the desired amp number.
The electrician must also determine the required number of switched poles and whether the neutral should switch. That decision depends on the generator’s bonding and the complete system design. Do not add or remove a neutral-to-ground bond based on a generic diagram.
5. Service-entrance and fault-current ratings
If the ATS will be the home’s service disconnect, it must be listed and marked for service-equipment use. A non-service-entrance ATS belongs downstream of suitable service equipment. The installation also needs an adequate short-circuit current rating for the available utility fault current and the specified protective-device combination.
NFPA’s residential inspection overview calls for checking the service-equipment label, switch capacity, overcurrent protection, conductors, grounding, neutral separation and either a load calculation or automatic load management (NFPA).
6. Enclosure and location
An indoor-rated enclosure cannot simply be mounted outdoors. Match the enclosure rating to the location and preserve required working space. The project may also require utility coordination, an electrical permit and inspection.
Expect a short interruption
An ATS does not provide uninterruptible power. The house goes dark while the system confirms the outage, starts the engine and prepares to transfer.
Timing varies by model and settings. In one Generac system, the documented factory sequence is a 10-second engine-start delay followed by a 5-second warmup. After utility power returns, transfer back occurs after about 15 seconds, followed by roughly one minute of unloaded operation before shutdown (Generac ATS manual). Do not apply those times to another model without checking its manual.
If an installed system fails to transfer, do not open the ATS or force its mechanism. Check only owner-level items identified in the generator manual, such as whether the controller is in AUTO and whether it displays a fault. The cited Generac manual warns that the switch contains high voltage and must never be manually transferred while energized.
Have the complete system designed and tested
A transfer switch isolates generator power from utility power and prevents backfeed, which can damage property and injure utility workers (Electrical Safety Foundation International). Never power house wiring through a receptacle or improvised double-male cord.
Have a licensed electrician select and install the ATS, verify generator compatibility, calculate loads, resolve neutral and grounding details, arrange required permits, and test a simulated outage. For a fuel-fired standby generator, the project must also confirm that the gas meter, regulator, piping or propane supply can support the generator along with other fuel-burning appliances; NFPA’s inspection overview specifically flags existing gas-meter and regulator capacity as an item to check.