What Is an Automatic Transfer Switch and Why Commercial Buildings Need One
A standby generator without an automatic transfer switch is like a fire extinguisher stored in the back room with a locked door and no one on-site with a key. The equipment exists and is theoretically capable of addressing the problem, but the conditions required to actually use it may not be present when the problem occurs. An automatic transfer switch (ATS) is the component that connects the generator to the building electrical system and initiates the transfer sequence when utility power fails. Without it, a commercial standby generator cannot restore building power automatically during an outage.
This distinction matters most for Indianapolis commercial buildings that experience power interruptions during Indiana’s summer storm season, when the outage may occur at 2 AM on a Saturday, or during a severe storm when dispatching someone to a commercial building to manually connect a generator is neither safe nor practical. The commercial generator installation and the automatic transfer switch are two parts of the same system, and installing one without the other leaves the commercial building with incomplete backup power capability.
Commercial standby power is only as good as its transfer switch. Call Mission Mechanical at 317-733-8686 or schedule a generator and ATS assessment for your Indianapolis commercial building.
How an Automatic Transfer Switch Works
Monitoring Utility Power
The ATS continuously monitors the voltage and frequency of the incoming utility power supply. Most commercial ATS units are programmed with voltage and frequency thresholds that define acceptable utility power quality. When measured utility voltage drops below the programmed minimum, or when utility power ceases entirely, the ATS recognizes this as an outage condition requiring generator transfer. A brief time delay, typically a few seconds, prevents the ATS from responding to very short transient events that resolve before the generator can be started.
Starting the Generator and Completing the Transfer
After the time delay confirms the outage, the ATS sends a start command to the generator control system. The generator engine starts, warms up, and reaches stable speed and output voltage within 10 to 30 seconds for most commercial generators. The ATS monitors generator output voltage and frequency, waiting for both to stabilize within acceptable limits. Once generator output is verified as stable, the ATS opens the utility connection and closes the generator connection, transferring the building electrical load to generator power.
Returning to Utility Power
When utility power is restored, the ATS detects the return of stable utility voltage and frequency. A retransfer time delay, typically one to five minutes, allows utility voltage to stabilize before initiating the return transfer. The ATS then opens the generator connection, closes the utility connection, and transfers the building load back to utility power. After the retransfer is complete, the ATS sends a cool-down stop command to the generator, which continues running unloaded for a programmed cool-down period before shutting down. The ATS then returns to its monitoring standby mode.
Types of Commercial Automatic Transfer Switches
Open Transition (Break-Before-Make)
Open transition is the standard ATS configuration for most commercial standby generator applications. The ATS breaks the connection to one source before making the connection to the other. This produces a brief interruption, typically a fraction of a second to a few seconds, during the transfer. For most commercial loads including lighting, HVAC, and general office equipment, this brief interruption is acceptable. Open transition ATS equipment is the standard used in most Indianapolis commercial generator installations.
Closed Transition (Make-Before-Break)
Closed transition ATS briefly parallels both the utility and generator sources before opening the utility connection, resulting in no power interruption during the transfer sequence. This configuration requires that both sources be synchronized to the same voltage, frequency, and phase angle before the parallel connection is made. Closed transition ATS is appropriate for Indianapolis commercial buildings with sensitive equipment that cannot tolerate even a brief power interruption: data centers, medical imaging equipment, and precision manufacturing processes. Closed transition ATS is more complex and more expensive than open transition, and is specified when the application genuinely requires uninterrupted transfer.
Critical-Load Transfer Switches
An ATS can be configured to transfer only a subset of the building’s electrical circuits to generator power, with the generator sized to support those critical loads only. Critical-load configurations connect the generator to circuits serving servers and networking equipment, HVAC for essential areas, security and access control, emergency lighting, and point-of-sale systems, while leaving non-essential circuits on the utility source. A critical-load ATS paired with an appropriately sized generator maintains essential commercial operations through extended outages at a lower investment than a full-building generator.
NFPA 110 and Code Requirements for Commercial Transfer Switches
The NFPA 110 Standard for Emergency and Standby Power Systems establishes the design, installation, testing, and maintenance requirements for emergency and standby power systems in commercial buildings. Healthcare facilities, certain assembly occupancies, and other building types defined by the building code are required to have emergency power systems including automatic transfer switches that meet NFPA 110. For commercial occupancies installing optional standby systems, NFPA 110 provides the industry standard framework that Mission Mechanical follows for all Indianapolis commercial generator and ATS installations.
In addition to NFPA 110, commercial transfer switch installations must comply with NEC Article 700 for emergency systems, NEC Article 702 for optional standby systems, and applicable OSHA electrical standards for commercial workplace electrical installations. Permit requirements vary by Indianapolis jurisdiction, and Mission Mechanical manages all permit applications and inspection coordination as part of commercial generator and ATS installation projects.
Why Commercial Buildings Cannot Skip the Automatic Transfer Switch
A manual transfer switch requires someone to go to the building, connect the generator, and transfer the load manually, in sequence and in the correct order, every time an outage occurs. This means commercial buildings need a qualified person available at any hour, willing to travel to the building during a storm or at 2 AM, and capable of performing the transfer safely. The alternative, manually connecting a portable generator or running extension cords from a portable generator to the building, is both a code violation and a serious safety hazard that can back-feed voltage onto utility lines and endanger utility workers restoring power.
The commercial electrical panel infrastructure needed to support a properly installed commercial generator and ATS creates a safe, code-compliant backup power system that operates without human intervention when it is needed most. An automatic transfer switch eliminates the dependency on someone being available, willing, and capable to operate the system manually during the least convenient possible circumstances.
A commercial generator without an automatic transfer switch is incomplete. Contact Mission Mechanical at 317-733-8686 or request a generator and ATS installation assessment.
Frequently Asked Questions: Automatic Transfer Switches for Commercial Buildings
What exactly does an automatic transfer switch do for a commercial building?
An automatic transfer switch continuously monitors the building’s utility power supply. When it detects that utility voltage has dropped out of acceptable range or ceased entirely, it sends a start signal to the standby generator, waits for the generator to reach stable operating speed and voltage, and then switches the building electrical load from the utility source to the generator output. When utility power is restored and verified as stable, the ATS transfers the load back to utility power and sends a stop signal to the generator. This entire sequence is automatic, requiring no human intervention.
How long does it take for an automatic transfer switch to restore power to a commercial building?
The transfer time from utility outage to generator power delivery has two components: generator startup time and transfer switch operation time. Most commercial standby generators reach stable output within 10 to 30 seconds of receiving the start signal. Open transition automatic transfer switches add a brief additional time to verify generator output quality before transferring the load. Total time from utility outage to restored power through an automatic transfer switch typically ranges from 10 to 30 seconds for most commercial standby systems.
What is the difference between an open transition and closed transition automatic transfer switch?
An open transition (break-before-make) ATS disconnects the load from the utility source before connecting it to the generator source. This produces a brief power interruption of a few cycles during the transfer. Open transition is the standard for most commercial standby applications. A closed transition (make-before-make) ATS briefly parallels both sources before opening the utility connection, resulting in no power interruption during transfer. Closed transition ATS is used in commercial facilities with sensitive equipment that cannot tolerate even a brief power interruption, including data centers and certain medical applications.
Can an existing commercial generator be retrofitted with an automatic transfer switch?
Yes. An automatic transfer switch can be added to an existing commercial standby generator installation that currently uses manual transfer or no transfer switch. The ATS must be rated appropriately for the generator output amperage and voltage and compatible with the generator control system. Mission Mechanical evaluates existing commercial generator installations in Indianapolis and surrounding areas and installs automatic transfer switches where the generator is functional but the transfer capability is manual or absent.
What happens if the automatic transfer switch itself fails during a power outage?
An ATS that fails during an outage means the generator starts but the building load is never transferred to generator power, leaving the building dark despite the generator running. Transfer switch failures can result from worn or corroded contacts that fail to make the connection, control board failures that prevent the transfer sequence from initiating, or mechanical failure of the switching mechanism itself. Regular ATS testing and maintenance identifies these conditions before they prevent the transfer switch from performing during an actual outage.
Does NFPA 110 require automatic transfer switches for commercial buildings?
NFPA 110, the Standard for Emergency and Standby Power Systems, establishes requirements for emergency and standby power systems in commercial occupancies. For Level 1 emergency power systems, which are required in certain occupancies such as healthcare facilities, automatic transfer switches are required. For optional standby systems in commercial occupancies where NFPA 110 compliance is chosen, automatic transfer switches are also the standard. Mission Mechanical designs and installs commercial generator and ATS systems in compliance with applicable code requirements for Indianapolis commercial properties.
How is a commercial automatic transfer switch sized?
A commercial ATS must be rated for the full output amperage and voltage of the standby generator it is paired with, and must match the voltage configuration of the building electrical system (single-phase or three-phase, 208V or 480V). An undersized ATS will be damaged or fail when it attempts to carry the full generator load. The ATS is also matched to the utility service equipment at the building, typically installed at or near the main service entrance panel. Mission Mechanical sizes ATS equipment as part of the complete commercial generator installation design.
How often should a commercial automatic transfer switch be tested?
NFPA 110 recommends that automatic transfer switch operation be tested monthly as part of the generator exercise program. Monthly exercise runs should confirm that the ATS detects the simulated outage condition (generator exercise mode), initiates the generator startup, and completes the transfer sequence correctly. Annual load testing of the full system confirms performance under actual load conditions. Mission Mechanical includes ATS operational testing in commercial generator maintenance programs for Indianapolis commercial properties.
Does ATS installation require permits in Indianapolis?
Yes. Automatic transfer switch installation involves modifications to the commercial electrical service entrance equipment and connection to the generator system, which requires electrical permits from the applicable Indianapolis or local jurisdiction. Mission Mechanical manages permit applications and inspection coordination for all commercial ATS installations in the Indianapolis area.
What is an exerciser on a commercial automatic transfer switch?
Many commercial ATS units include a built-in exerciser timer that initiates a generator startup and simulated transfer at a programmed interval, typically weekly or monthly. The exerciser run does not transfer actual building load to the generator but verifies generator startup and ATS control function on a regular schedule without requiring manual intervention. Mission Mechanical configures exerciser timers as part of commercial ATS commissioning for Indianapolis commercial buildings.
What signs indicate a commercial automatic transfer switch is failing?
Signs include the generator exercise run completing normally but the ATS failing to transfer during a real outage (which would only be discovered after the fact); ATS status indicators showing fault conditions; audible clicking or chattering from the ATS cabinet that indicates contact instability; and generator startups that do not result in building power restoration during outage events. Thermographic inspection of the ATS cabinet during maintenance can also identify hot contacts that indicate resistance buildup before they fail.
Can a transfer switch be installed for only critical loads in a commercial building?
Yes. A transfer switch can be configured to connect the generator only to specific circuits designated as critical loads, rather than to the full building load. This critical-load ATS configuration allows a smaller generator to maintain essential commercial operations during an outage without requiring a full-building generator capable of supporting all loads simultaneously. Critical-load selection typically includes HVAC for critical zones, server and data infrastructure, security systems, emergency lighting, and point-of-sale systems.
What is the purpose of a time delay on a commercial automatic transfer switch?
ATS units typically include time delays at several points in the transfer sequence. A pickup delay prevents the ATS from initiating a transfer in response to very brief power fluctuations that self-resolve before the generator could be started. A dropout delay ensures utility power has actually failed rather than briefly sagged before beginning the generator startup sequence. A retransfer delay on return to utility power allows utility voltage to stabilize before transferring the load back, preventing a retransfer onto unstable power.
How does an automatic transfer switch know when utility power has been restored?
The ATS continuously monitors utility voltage and frequency. When utility power is restored after an outage, the ATS senses that utility voltage and frequency have returned to acceptable limits. It then waits for the configured retransfer delay period to ensure stability before initiating the transfer sequence back to utility power. After the load transfers back to utility, the ATS sends a cool-down stop signal to the generator, which continues to run for a cool-down period before shutting down.
How can Indianapolis commercial buildings schedule automatic transfer switch installation with Mission Mechanical?
Contact Mission Mechanical at 317-733-8686 or request service online for commercial automatic transfer switch installation or assessment in Indianapolis and surrounding central Indiana communities. Our licensed commercial electricians design and install complete generator and ATS systems for commercial properties, managing permits, coordination, and commissioning.
When to Call Mission Mechanical for Commercial ATS and Generator Service
Mission Mechanical has provided licensed commercial electrical services in Lawrence, Indianapolis, Fishers, and throughout central Indiana since 2002. Our licensed commercial electricians hold Indiana commercial electrical licensing, carry full general liability and workers compensation insurance, and are BBB A+ accredited. Our commercial clients consistently recognize our installation quality and service on Google and Yelp. Mission Mechanical is a MICCS safety member and Indianapolis Colts Small Business Partner 2026. We install and service complete commercial generator and automatic transfer switch systems for Indianapolis commercial buildings. Contact us at 317-733-8686 or schedule service online.
