In this article, we’ll explain how transfer switches work, why electrical codes require them, the main differences between manual and automatic models, and how to select the right switch for an industrial standby generator.
Key Takeaways
- A generator transfer switch is an electrical device that safely routes power between the utility and a backup generator, so they never feed the building at the same time. This isolation prevents dangerous backfeeding into utility lines and keeps your facility’s electrical system protected during outages.
- For commercial and industrial standby power systems, especially those using diesel and natural gas generator sets, a properly sized and installed transfer switch is needed. It allows automatic or manual changeover to generator power, supports code compliance, and helps critical loads stay online with minimal interruption.
- Transfer switches are generally required by electrical codes when a generator is tied into building wiring. Article 702 of the National Electrical Code (NEC) outlines requirements for optional standby systems and emphasizes the use of listed transfer equipment to prevent parallel connection to the grid.
- Choosing the right transfer switch means matching its voltage and amperage rating to your generator and load, deciding between manual and automatic operation, and working with qualified professionals to ensure correct installation and ongoing maintenance.
- Partnering with a knowledgeable supplier like Depco Power Systems ensures your standby generator and transfer switch are properly matched, installed, and supported so your facility has reliable backup power when it matters most.
What Is a Generator Transfer Switch?
A generator transfer switch is a permanently installed electrical switching device that connects your building’s load to either the normal utility source or a backup generator. Its main job is simple but necessary: it prevents the utility and generator from ever feeding the system at the same time.
In normal conditions, the transfer switch keeps your facility connected to utility power and isolates the generator. When an outage occurs, the switch disconnects from the grid and connects selected circuits or the entire facility to generator power instead. Once utility service is restored and stable, the switch returns the load to the grid and takes the generator offline.
For commercial and industrial facilities running large diesel or natural gas standby generators, the transfer switch is the control point that makes backup power usable at the panel level. Instead of relying on extension cords or ad hoc connections, the switch lets the generator safely energize hard-wired equipment such as lighting, HVAC, process loads, data center equipment, and life safety systems.
Without a transfer switch, tying a generator directly into a building’s electrical system is both unsafe and typically not code compliant. The switch functions as the gatekeeper that makes backup power safe, orderly, and manageable.
How Generator Transfer Switches Work
Although designs vary by manufacturer and application, most generator transfer switches follow the same basic sequence.
In a typical automatic standby configuration, the transfer switch constantly monitors the incoming utility voltage and frequency. When it detects a loss of utility power or conditions outside acceptable tolerances, it sends a start signal to the generator. As the generator reaches proper speed and voltage, the switch opens the connection to the utility, pauses momentarily, then closes the connection to the generator output.
This “break-before-make” action ensures the two sources are never tied together. During an outage, the switch keeps the building connected to the generator until utility power returns and stabilizes. When conditions are normal again, the switch reverses the process, moving the load back to the grid and issuing a cool-down stop command to the generator.
In manual systems, a person performs the same basic steps by operating a handle or mechanism on the switch. The operator manually starts the generator, verifies voltage, and moves the switch from a utility position through a neutral off position into a generator position. When the outage ends, the operator reverses the process.
In either case, the transfer switch:
- Provides clear, physical separation between sources
- Routes power through properly protected circuits
- Gives facility staff a single, defined point of control for backup power
For facilities using large diesel and natural gas standby units sourced from Depco, the transfer switch, often a Service Entrance Rated ATS in new construction or utility-side installations, is integrated into a broader system that may include controls, metering, and load management to prioritize critical circuits.
Infographic: How Generator Transfer Switches Work
Common Types of Generator Transfer Switches
Most transfer switches serving commercial and industrial generators fall into two primary categories: manual and automatic. Within these categories, there are several variations.
Manual Transfer Switches
Manual transfer switches are mechanically operated devices that require an operator to move a handle or lever to change the power source. They are commonly used with portable generators or smaller standby systems where brief downtime is acceptable.
Key characteristics:
- Lower upfront cost and simpler construction
- Require someone onsite to start the generator and move the switch
- Suitable for applications where outages are infrequent and non-critical
- Often used to feed a subset of circuits via a dedicated emergency subpanel
Manual switches are still required to meet listing and code requirements and must be installed by qualified professionals. When used correctly, they provide safe isolation and backfeed protection, just without automation.
Automatic transfer switches (ATS)
Automatic transfer switches are the standard choice for most permanent standby generator installations in commercial and industrial settings. An ATS monitors utility power, starts the generator when necessary, and transfers the load automatically.
Key characteristics:
- Continuous monitoring of utility power quality
- Automatic start and stop of the generator
- Fast transfer times, often meeting the 10-second startup requirement defined in NEC Article 700 for emergency systems
- Programmable time delays and logic to avoid nuisance transfers
- Integration with generator controls and building management systems
Automatic switches come in a variety of configurations, including open-transition designs that introduce a brief break between sources and more specialized closed-transition models that minimize even momentary voltage dips in certain applications.
For many facilities investing in standby diesel or natural gas generators, an ATS paired with a properly sized generator set from Depco delivers the expected level of reliability and automation.
Table: Types of Transfer Switches: Manual vs. Automatic
| Type | Response Time | Cost | Best For | Features |
| Manual | Operator-dependent (1-5 min) | Lower | Portables, small businesses | Simple lever, code-compliant isolation |
| Automatic (ATS) | <10 seconds | Higher | Hospitals, data centers, factories | Power monitoring, generator exercising, load shedding; open/closed transition |
Service Entrance Rated vs. Non-Service Entrance Rated ATS
When talking with customers, one of our first questions is whether they need a service‑entrance rated or non‑service‑entrance rated ATS, because this decision determines placement and code compliance.
Service Entrance Rated ATS
A service‑entrance rated transfer switch is the first device between the utility meter and the building. It includes an integral service disconnect and over‑current protection (via a circuit breaker or fuse), so it can act as the facility’s main disconnect. This configuration reduces the number of components needed in new installations and simplifies compliance with NEC requirements.
Non‑Service Entrance Rated ATS
A non‑service entrance rated ATS has no built‑in over‑current protection. It must be installed downstream of a separate service disconnect panel, making it appropriate where an upstream breaker or disconnect already exists.
How to Choose
New construction often favors service‑entrance rated ATS because they reduce equipment count. Retrofits or sites with an existing main breaker may only require a non‑service‑entrance rated ATS. Consult a licensed electrician and local codes to ensure the correct choice.
Safety, Code Requirements, and Backfeeding Risks
Safety is the primary reason transfer switches are required in standby power systems. When a generator is connected to a building’s wiring, electrical codes require listed transfer equipment, often a Service Entrance Rated transfer switch when the switch serves as the main disconnect, to ensure that utility and generator sources cannot energize the system at the same time.
The National Electrical Code (NEC) addresses optional standby systems in Article 702, which outlines the need for transfer equipment that prevents the inadvertent connection of normal and standby sources. Similar requirements appear in Articles 700 and 701 for emergency and legally required standby systems.
The danger they are designed to prevent is backfeeding. Backfeeding occurs when generator power unintentionally flows through a transformer and energizes utility lines. If a generator is connected without proper transfer equipment, power can travel from a building’s panel back into utility lines that crews expect to be de-energized. This creates significant safety hazards for utility personnel and can damage both utility infrastructure and onsite equipment.
Using a listed transfer switch addresses several key safety and compliance requirements:
- Physical separation between utility and generator sources
- Proper handling of neutral and grounding conductors
- Integration with overcurrent protection and disconnects
- Compliance with product standards such as UL 1008 for transfer equipment
For these reasons, transfer switch installation should always be performed by a qualified, licensed electrician. Most jurisdictions require a permit, inspection, and a licensed electrician to perform the work. A licensed electrician ensures the transfer switch is properly sized, wired, labeled, and integrated with the generator and service equipment in accordance with code.
In many industrial and healthcare environments, additional standards such as NFPA 110 govern how quickly emergency power must become available and how transfer equipment is configured. In those cases, a properly selected automatic transfer switch is essential for meeting system performance and compliance expectations.
5 Tips for Choosing and Maintaining a Transfer Switch
Installing a transfer switch is the first step. To keep your backup power system reliable for years, it is important to choose the right device and include it in your maintenance plan.
1. Match the Switch Rating to Your Generator and Load
Start by making sure the transfer switch’s voltage and amperage ratings match your generator and distribution system, and determine whether the application requires a Service Entrance Rated transfer switch.
When sizing your transfer switch, determine whether it needs to be Service Entrance Rated. If the ATS will serve as your facility’s main disconnect, it must be service‑entrance rated and include a breaker or fuse. If a separate service disconnect already exists, a non‑service‑entrance ATS may be appropriate.
The switch must be able to handle the generator’s maximum output current and the connected load.
For example, a three-phase 480-volt generator will require a switch rated for that voltage and appropriate ampacity. Many commercial systems use standard switch sizes such as 100, 200, 400, or 800 amps. An experienced supplier like Depco can help match switch ratings to the specific diesel or natural gas generator sets you are evaluating.
2. Decide Between Manual and Automatic Operation
For smaller facilities or non-critical loads, a manual transfer switch may be adequate, especially when paired with a portable generator. In most commercial and industrial environments, however, an automatic transfer switch is the better choice.
An ATS reduces downtime by starting the generator and transferring the load without waiting for someone to arrive on-site. It also standardizes the sequence of events, which can be important for sensitive equipment and safety systems. When evaluating options, consider how often outages occur, how quickly power must be restored, and who will be available to operate the system.
3. Work with a Licensed Electrician and Follow Local Requirements
A transfer switch connects directly to your main electrical service and to the generator output. When it is installed as the first point of disconnect after the utility meter, it must be a Service Entrance Rated transfer switch.
For this reason, a licensed electrician should handle installation, testing, and any subsequent modifications. The electrician will pull the appropriate permit, coordinate with the authority having jurisdiction, and confirm that the installation passes inspection.
Following local requirements ensures the system is safe, insurable, and compliant. It also reduces the risk of operational issues caused by mis-wired neutrals, undersized conductors, or improperly configured control circuits.
4. Include the Transfer Switch in Routine Inspections
Transfer switches are robust, but they are still mechanical and electrical devices that can be worn over time. Include the switch in your regular generator system inspections. With power safely isolated as part of planned maintenance, check for signs of overheating, loose connections, moisture, or corrosion inside the enclosure.
For automatic switches, operate the test function periodically so the mechanism transfers under controlled conditions. Many facilities combine generator load testing with transfer switch operation to verify that the entire standby system performs as expected.
5. Partner with a Supplier Who Understands Complete Systems
Standby power is more than just a generator. The generator, transfer switch, controls, and distribution all must work together. Choosing a supplier that understands complete systems can simplify decisions and reduce integration risks.
Depco Power Systems helps customers pair the right automatic or manual transfer switch with the right generator set and accessories. Whether you are evaluating a new diesel generator package or upgrading an existing natural gas unit with modern transfer equipment, working with a knowledgeable partner helps ensure your system is properly sized, installed, and supported over its full life cycle.
Conclusion: Safer, More Reliable Backup Power with the Right Transfer Switch
A generator transfer switch may not be the most visible component in your backup power system, but it plays a critical role in safety, code compliance, and reliability. By keeping utility and generator power isolated, it prevents backfeeding, protects line workers and equipment, and allows your standby generator to support hard-wired loads throughout your facility.
For industrial and commercial operations, pairing the right transfer switch with the right generator is essential. A properly sized, installed, and maintained switch gives you confidence that when the next outage hits, your diesel or natural gas generator set will take over smoothly and safely.
If you are planning a new standby power project or updating existing equipment, Depco Power Systems can help you evaluate options and put a complete solution in place. Explore our current inventory of new and used generator sets, including diesel units and natural gas units.
For questions about transfer switches or turnkey packages, contact Depco to speak with an experienced backup power specialist.
