An automatic transfer switch for emergency systems is one of the most important pieces of equipment in a building’s electrical backbone. When the normal power source fails, this device detects the loss, signals the backup source to start, and moves the emergency loads to that source without requiring anyone to operate a switch by hand. Emergency systems exist to protect life and property, so the transfer equipment has to work automatically, reliably, and within the short time limits that codes demand.
This guide walks through what an emergency system ATS is, how it is configured, which loads belong on it, how to select and size one correctly, the standards that govern it, how it is applied across different facility types, and the common problems that cause it to fail. Engineers, control-panel builders, facility managers, and electrical contractors will find the specification details and troubleshooting guidance they need to design and keep an emergency power system running.
What Is an Automatic Transfer Switch for Emergency Systems?
An automatic transfer switch, or ATS, is a device that transfers a common electrical load between two power sources: a normal source, typically the electric utility, and an emergency source, typically a generator, second utility feed, or another standby source. In an emergency system, the transfer must happen automatically whenever the normal source is lost or degrades beyond acceptable limits.
What Is the Typical ATS Configuration for Emergency Systems?
Utility Power + Generator + ATS
This is the standard emergency standby configuration. The utility feed serves as the normal source and the generator is the emergency source. A single ATS sits between them and carries the emergency loads, which stay on utility power under normal conditions.
When the utility fails, the ATS senses the loss, starts the generator, waits for the generator to reach acceptable voltage and frequency, and then transfers the load. The change typically happens within 5 to 10 seconds for generator systems. This arrangement is simple, cost-effective, and appropriate for most commercial buildings, smaller hospitals, and industrial plants.
The generator can serve the emergency loads through one ATS, or a larger generator can feed several ATSs, each serving a different load group. If one generator supplies multiple loads, the number of ATSs depends on how the emergency loads must be separated from legally required standby and optional loads.
What Loads Should Be Connected to an Emergency ATS?
Life Safety Loads
Life safety loads are the loads that protect occupants during an emergency and support safe egress. In the context of an emergency system, life safety loads include exit signs, egress and emergency lighting, fire alarm systems, and related equipment that must function to keep people safe.
These loads have the highest priority and, in healthcare, are defined by NEC Article 517 as the life safety branch. They must transfer to the emergency source within the code-required time, typically 10 seconds, and remain energized for the required duration. Under NFPA 110, these are Level 1 loads because a failure could result in loss of life or serious injury.
Life safety loads are the reason an emergency system exists. They are connected to the emergency ATS as a priority, and the system is designed so that these loads restore first in a multi-load transfer sequence.
Critical and Essential Loads
Critical and essential loads are the equipment whose continued operation matters for the mission of the facility. In a hospital, these include equipment that directly affects patient care when the failure of the equipment could place a patient at risk. In other buildings, critical loads may include security systems, communications, and process equipment.
In healthcare, NEC Article 517 defines a critical branch that covers lighting for patient care areas, nurse call systems, and essential patient care equipment. These loads transfer with the life safety branch or shortly after. They are separate from the equipment branch, which handles mechanical power and may transfer later.
For non-healthcare facilities, the equipment loads that are truly essential for the operation are connected to the emergency or legally required standby system based on the building’s needs and the code classification of those loads.
Emergency Lighting
Emergency lighting illuminates paths of egress so occupants can leave the building safely when normal lighting fails. The NEC requires emergency lighting to be automatically energized within 10 seconds of the power loss and to remain on for at least 90 minutes, in line with NFPA 101 Life Safety Code requirements.
Emergency lighting loads belong on the emergency ATS because they are life-safety loads. They include fixtures serving exit access corridors, stairways, ramps, and exit locations. The circuits supplying emergency lighting must be dedicated to emergency use and kept separate from normal loads.
Because transfer time is critical, the design must ensure the ATS and its control settings bring emergency lighting up within the 10-second window. The 90-minute runtime requirement means the generator or battery source must be able to sustain these loads for that period.
Fire Protection and Fire Alarm Systems
Fire alarm systems are emergency system loads under NEC Article 700 and must stay powered during an outage, since a fire emergency is more likely to be detected or to require coordination during a utility failure. Fire alarm panels, annunciators, and the notification appliances they drive are connected to the emergency source.
Fire pumps and fire protection equipment that require power to operate the fire suppression system are also served by the emergency or standby power system. These are life-safety-related loads that must keep the building protected during an outage.
The fire alarm system and fire pump circuits must have reliable, code-compliant transfer. Because fire protection equipment is mission-critical, it is normally served by an emergency ATS or a dedicated transfer arrangement meeting the applicable code requirements.
Elevators and Essential Building Systems
Elevators are often needed in emergencies, particularly for occupant evacuation and for firefighter access in high-rise buildings. In many cases elevators are classified as legally required standby loads rather than emergency loads, and they are served by a separate legally required standby system and ATS rather than the emergency ATS.
Buildings may require emergency or standby power for elevators, and some configurations provide a pre-signal so the elevator can be positioned before power is lost. Modern elevators may also regenerate power during descent, which can affect the emergency generator, so the design must account for regenerative behavior.
Essential building systems such as sump pumps, security systems, and selected HVAC may be served by the emergency or standby system depending on the building’s needs and code classification. The designer determines which systems are essential and assigns them to the appropriate branch and ATS.
Data Centers and Telecommunications Equipment
Data centers and telecom facilities need power reliability for servers, networking equipment, and communication systems. These loads may be served by an emergency or standby power system and ATS when they are designated as critical loads, but they are not automatic emergency ATS loads in the same way life-safety loads are.
In practice, data center critical loads are often protected by a multilayer approach. A UPS rides through short interruptions, including the time the ATS takes to transfer to a generator, and the generator supplies the UPS and cooling for extended outages. The ATS transfers the facility to the generator, and the UPS covers the milliseconds during the open transition.
The transfer time and the sequence must be coordinated so that the UPS input and the generator output match when the ATS transfers. Closed-transition or synchronized transfer is sometimes used when even a momentary break cannot be tolerated.
Healthcare and Hospital Equipment
Hospitals use a specialized essential electrical system governed by NEC Article 517 and NFPA 99. The system is divided into life safety, critical, and equipment branches, each with its own transfer requirements. The emergency ATS serves the life safety branch, and additional transfer equipment covers the critical and equipment branches.
Life safety loads transfer within 10 seconds. The critical branch also transfers within 10 seconds, while the equipment branch may transfer after a time delay. Hospital generators can serve all three branches, but the systems must be kept separated so a fault on one branch does not take down the others.
Because patient safety depends on power, healthcare ATSs are subject to rigorous testing, including NFPA 110 maintenance and monthly exercise programs. The design must account for the transfer time required by each branch and the load on each branch.
How to Select an ATS for an Emergency System?
A practical selection process works through a defined set of inputs: the sources, the connected load, the required current rating, the number of poles, the voltage and frequency, the motor and inrush demand, the short-circuit and withstand ratings, the required transfer time, and any service-entrance requirements. The table below summarizes the main selection checks.
Selection step | What to determine | Key check |
|---|---|---|
Power sources | Normal and emergency source type, voltage, phase | Both sources compatible with ATS |
Connected load | Total emergency load, demand factors | ATS capacity covers calculated load |
Current rating | Calculated load current plus margin | Rating ≥ load, 20-30% margin typical |
Number of poles | 2P / 3P / 4P based on phase and neutral | Neutral switching and grounding correct |
Voltage and frequency | Match system nominal values | ATS rated for system voltage and frequency |
Motor inrush | Highest starting demand | ATS handles locked-rotor/starting surge |
Short-circuit ratings | WCR and interrupting rating | WCR ≥ available fault current at terminals |
Transfer time | Load sensitivity and code limits | Meets 10-second and application timing |
Service entrance | Main disconnect and fault duty | Meets service-entrance requirements |
Determine the Normal and Emergency Power Sources
The first step is to define both sources that the ATS will switch between. The normal source is usually the electric utility, and the emergency source is a generator, a second utility feed, or another supply. The ATS rating must be compatible with both sources.
Record the nominal voltage, phase configuration, and frequency of each source. For generators, note the rated output and how long it takes to start and reach acceptable voltage and frequency, because this affects the transfer time budget. For utility backup, confirm that both feeds are compatible in voltage and grounding.
The transition type is also decided here. Open transition, closed transition, and delayed transition each fit different applications, and the choice affects the ATS design and how sensitive the loads can be to a momentary interruption.
Determine the Connected Load
The connected load is the sum of the emergency loads the ATS will serve. List every load that will be connected to the emergency panel, including life safety, critical, and any equipment loads assigned to the emergency source. Use the calculated load values, not just nameplate ratings, for accuracy.
The load calculation should account for demand factors where applicable and for loads that run simultaneously. Emergency systems must have adequate capacity and rating for the loads they serve per the NEC, so the connected load drives both the ATS rating and the generator sizing.
Also consider future growth. Selecting an ATS with a modest margin above the current calculated load makes the system easier to extend without a full replacement.
Calculate the Required Current Rating
The ATS current rating must be equal to or greater than the calculated load current. Industry practice is to add a margin, commonly 20 to 30 percent, above the calculated continuous load to cover load growth, startup, and continuous-duty requirements. The rating should also align with the main breaker or panel rating it feeds.
Compute the design current from the calculated volt-ampere load and the system voltage using the appropriate single-phase or three-phase formula. Then select a standard ATS rating that is at least as large as this value and compatible with the connected equipment.
For motor-heavy loads, the current rating must also account for the highest expected starting demand, not just the steady running current, because a starting motor can draw several times its running current.
Select the Number of Poles
The number of poles depends on the phase configuration and on whether the neutral conductor must be switched. A 2-pole ATS is used for single-phase systems, a 3-pole ATS for three-phase systems with a solid continuous neutral, and a 4-pole ATS when the neutral must be switched with the phase conductors.
The neutral switching decision usually hinges on the grounding and separately derived source arrangement. A 3-pole switch with a solid neutral is common when both sources share a grounded neutral. A 4-pole switch is used in some separately derived generator or utility configurations, and it isolates the neutral, which affects ground fault response. The table below compares pole configurations.
Pole type | Typical system | Neutral handling |
|---|---|---|
2-pole | Single-phase 120/240 V | Switches phase(s), common neutral |
3-pole | Three-phase with solid neutral | Switches phases, solid neutral pass-through |
4-pole | Separately derived or switched-neutral systems | Switches phases and neutral together |
For emergency systems, verify that the pole configuration is compatible with the source grounding per the NEC and any applicable standards, because an incorrect neutral arrangement can create circulating currents or ground fault masking.
Check System Voltage and Frequency
The ATS voltage rating must match the nominal system voltage and withstand the phase-to-phase potential of the distribution system. Frequencies must also match, typically 60 hertz in the U.S. and 50 hertz in many other regions, and the ATS must be rated for the correct frequency.
Selecting an ATS with a higher voltage rating than necessary is acceptable, but the rating must at least meet the system voltage. The same applies to interrupting and withstand ratings, which are expressed relative to the system voltage.
Verify the voltage selection and any internal taps match the actual system before energizing. A mismatch between the ATS voltage sensing and the real system can cause incorrect transfer decisions or failure to detect a source condition.
Consider Motor Starting and Inrush Current
Motor loads raise the peak current demand well above the running current because a locked-rotor or starting inrush can be several times the full-load current. The ATS must be able to handle this surge without nuisance issues or overheating.
Size the ATS current rating and, where relevant, its thermal capability to cover the highest starting demand. For a system with large motors, evaluate the aggregate starting current that could occur, especially in sequential motor starting, and ensure the ATS is not undersized.
Motor inrush also matters during transfer. When the load is transferred to the generator, the starting current of motors can stress the generator and the ATS contacts. The design should account for the transient inrush and ensure the ATS can close onto the load without damage.
Check Short-Circuit and Withstand Ratings
The short-circuit withstand rating, often called the WCR, is one of the most important ATS ratings. The ATS must have a WCR at least equal to the available fault current at its terminals, so it can carry a short-circuit current long enough for the upstream protective device to clear the fault without damaging the switch.
The withstand rating depends on both the magnitude and the duration of the fault the ATS will see. Obtain the available fault current from the fault study at the ATS location and select a switch whose WCR exceeds it. If the ATS is intended to interrupt fault current, its interrupting rating must also be sufficient.
For emergency systems, the ATS must also be coordinated with the upstream overcurrent protective devices so that a fault clears properly and does not unnecessarily trip higher-priority circuits.
Determine the Required Transfer Time
Transfer time is how long the load is without power during the changeover. For an open transition ATS, the load is disconnected from one source and then connected to the other, so there is a brief interruption. For closed transition, the two sources momentarily parallel and the interruption is near zero.
The required transfer time is set by the sensitivity of the loads and the code classification. Emergency lighting must transfer within 10 seconds, and hospital life-safety and critical branches also have strict timing. Sensitive electronic loads may need a closed-transition or UPS-assisted arrangement.
Match the ATS transfer performance and any time-delay settings to the load requirements. The ATS must bring the load onto the emergency source within the allowed window after the source is ready. The table below compares transition types.
Transition type | Behavior | Best fit |
|---|---|---|
Open transition | Break-before-make, brief interruption | Most emergency systems; short outage acceptable |
Closed transition | Make-before-break, momentary parallel | Sensitive loads that avoid even brief interruption |
Delayed transition | Pause in neutral/open position | Loads with motor or inductive decay needs |
Consider Service Entrance Requirements
If the ATS will be used at the service entrance, it must satisfy all service-entrance requirements for the installation, including acting as the main disconnect, providing acceptable fault ratings, and including the required overcurrent protection. Service-entrance ATSs must also be approved for the application.
Verify the interrupting and withstand ratings against the available fault current at the service point, which is usually higher than at downstream locations. Confirm that the ATS provides the disconnecting means and that all grounding and bonding requirements are met.
Using the ATS as service-entrance equipment integrates the disconnect with the transfer function, which reduces equipment but concentrates the protection in one device. The design must confirm that all applicable code and utility requirements are satisfied.
ATS Solutions for Different Emergency Applications
ATS for Hospitals and Healthcare Facilities
Hospitals use a highly regulated essential electrical system split into life safety, critical, and equipment branches per NEC Article 517 and NFPA 99. Each branch has its own transfer requirements, and the ATS layout is designed to keep these branches separated so a fault on one does not bring down the others.
Life safety and critical branches transfer within 10 seconds, while the equipment branch can delay. Hospital generators can supply all branches, and the ATSs are arranged so that life-safety loads restore first. This separation is critical for patient safety.
Hospital ATSs are subject to stringent testing and maintenance under NFPA 110, including monthly exercising and annual load testing. Bypass-isolation ATSs are often used so the switch can be serviced without interrupting critical loads.
ATS for Data Centers
Data centers depend on continuous power for servers and networking gear. Their approach typically layers a UPS with a generator and an ATS. The UPS rides through brief interruptions, the ATS transfers the facility to the generator for extended outages, and the generator supplies the UPS and cooling.
Because even a momentary break can affect critical IT loads, the transfer time and sequence between the UPS and the generator must be coordinated. Closed-transition or synchronized transfer can be used to minimize interruption, and the ATS must bring the load onto the generator without disturbing the UPS.
Redundancy is a priority in data centers. Multiple generators, multiple ATSs, or utility plus generator configurations are used to achieve the required availability, and maintenance continuity is supported by bypass-isolation switch designs.
ATS for Commercial Buildings
Commercial buildings such as offices and retail centers use emergency systems primarily for life safety, emergency lighting, fire alarm, and elevator loads. The ATS configuration is usually a utility plus generator arrangement, sometimes with an ATS at the service entrance or at a dedicated emergency panel.
The choice of full-building backup versus selective load backup depends on the building’s needs. A service-entrance ATS can back up the whole building, while an emergency panel ATS backs up only the loads that must transfer.
Transfer timing in commercial buildings is driven by the 10-second requirement for egress and emergency lighting. The ATS and generator must work together so these loads restore quickly after a utility failure.
ATS for Industrial Facilities
Industrial facilities have significant motor loads, process equipment, and stringent reliability needs. The ATS must be sized for motor starting inrush as well as running current, and the short-circuit withstand rating must match the often high available fault current at industrial load centers.
Industrial ATSs may serve process-critical equipment alongside life-safety loads, and the configuration can involve multiple generators or multiple ATSs feeding different process areas. Sequencing transfers loads in priority so critical processes restore first.
Harsh environments may require higher enclosure ratings, and the design must account for the possibility of regenerative loads, large starting currents, and the need for selective coordination with protective devices throughout the plant.
ATS for Telecommunications Systems
Telecommunications systems require high availability for communication equipment that keeps networks and emergency communications operating. The ATS ensures that the communication loads reach the backup source when normal power fails.
Telecom facilities often combine batteries or a DC power system with a generator and ATS. The ATS transfers the facility to the generator, and the DC or UPS system covers the transfer interval. The transfer time must be compatible with the ride-through capability of the DC plant.
Because communication networks are critical during emergencies, the ATS design prioritizes reliability and maintenance continuity. Bypass-isolation arrangements and redundant configurations help keep telecom power available.
ATS for Airports and Transportation Facilities
Airports and transportation hubs rely on power for lighting, communications, security, and operational equipment such as baggage handling and airfield lighting. Emergency and standby power systems keep these facilities operational during outages and maintain safety.
The ATS configuration may involve multiple sources and multiple load groups, transferring life-safety and critical loads first. Airfield lighting and air traffic control support equipment have strict requirements that shape the transfer design.
Reliability and redundancy are priorities in transportation. Multiple generators and ATSs may be used so that critical operations continue without interruption, and maintenance is supported by designs that allow servicing without taking loads off power.
ATS for Fire Protection Systems
Fire protection systems, including fire alarm panels and fire pumps, must stay powered during an outage because a fire may be detected during a utility failure. These are life-safety loads that belong on the emergency or standby power system.
Fire pumps draw large starting currents and require the ATS and generator to handle the inrush. The wiring and control must be reliable and code-compliant, and the ATS must transfer the fire protection loads within the required time.
Because fire protection is so critical, the ATS serving these loads is confirmed to be listed for emergency use, and the system is tested regularly to ensure the transfer works. In many configurations, fire protection loads are given priority in the transfer sequence.
Conclusion: Choose the Right ATS for Your Emergency Power System
Choosing the right Automatic Transfer Switch for Emergency Systems is about more than selecting a suitable current rating. You also need to consider your normal and emergency power sources, connected loads, system voltage, number of poles, transfer time, transition type, grounding arrangement, applicable standards, and long-term maintenance requirements. A properly selected and configured ATS helps ensure that critical loads can be transferred to the emergency power source when the normal supply fails.
LSP specializes in manufacturing Automatic Transfer Switch (ATS) solutions for a wide range of emergency and backup power applications. Whether you need an ATS for a hospital, data center, commercial building, industrial facility, telecommunications system, or fire protection system, LSP can provide configurations based on your project’s electrical and application requirements.
Contact LSP today and provide your system voltage, rated current, number of poles, normal and emergency power sources, load type, and required transfer time. Our team can help you select a suitable ATS solution and provide a professional quotation for your project.
Frequently Asked Questions About Automatic Transfer Switches for Emergency Systems
Should I Use a 3-Pole or 4-Pole ATS for an Emergency System?
Whether you should use a 3-pole or 4-pole ATS depends on your system grounding and neutral-switching requirements. A 3-pole ATS switches the three phase conductors while keeping the neutral continuous. A 4-pole ATS switches the three phases and neutral, which may be required when the emergency source is separately derived.
Can an ATS Work With Two Utility Power Sources?
Yes, an ATS can work with two utility power sources when it is specifically designed and configured for dual-utility applications. It monitors both sources and automatically transfers the connected load from the preferred source to the alternate source when the primary source fails or falls outside acceptable limits. You should confirm the ATS controller, voltage ratings, switching configuration, and applicable standards meet your system requirements.
Can an ATS Work With a Generator and UPS?
Yes, an ATS can work with a generator and UPS when the system is properly designed. Typically, the UPS provides uninterrupted power during the short period before the generator starts and stabilizes, while the ATS transfers the load to the generator. You should ensure compatibility between the ATS, UPS, and generator, including voltage, frequency, transfer timing, load capacity, and control signals.
What Standards Apply to an ATS Used in Emergency Systems?
The applicable standards depend on your country, system type, and installation requirements. In the United States, UL 1008, NFPA 70 (NEC), and NFPA 110 are commonly relevant to emergency and standby power systems. In IEC-based markets, IEC 60947-6-1 may apply to transfer switching equipment. You should verify local codes and project-specific certification requirements before selecting an ATS.
