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Specifying ATS for Selectively Coordinated Power Distribution Systems

In a prior Technical Brief, ASCO defined the practice of selective coordination, where overcurrent protection device timings are set to open circuit breakers nearest to faults. This article provides further guidance on selecting Automatic Transfer Switches (ATS) for selectively coordinated systems.

Background Information

Selective Coordination is the practice of setting overcurrent protection device trip times to minimize the amount of equipment deenergized when faults are cleared. Figure 1 compares an uncoordinated system (where breakers at all levels of a power distribution system provide the same trip times) is compared to a selectively coordinated system (which assigns shorter trip times close to loads and longer trip times to breakers farther from loads). The times shown are breaker trip settings.
Where the same trip times are applied to multiple circuit levels, the opening of a single breaker to clear a fault current could depower an unnecessarily large amount of load equipment, as shown at left in Figure 1. The diagram at right shows a selectively coordinated system that tiers trip times, causing only the breaker closest to a fault to open. This clears the fault while depowering a relatively small amount of load equipment, reducing the impact to facility operations. Additional information is available in the ASCO Power Technologies Technical Brief entitled Selective Coordination Basics.

The 2020 National Electrical Code® (NEC®) requires selective coordination in power distribution circuits serving life-safety, legally required, and Critical Operations Powers System (COPS) loads. For instance, Article 700.32 states:

Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.

Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.

Similar language is stated for legally required and COPS loads in Articles 701.32 and 708.24. For more info on NEC codes for backup power systems, review the ASCO Power Technologies Technical Brief entitled Standards for Backup Power.

Choices for Specifiers

A primary function of transfer switches is to withstand fault currents until they are cleared by over-current protection devices. Consequently, each ATS must be appropriately rated to withstand fault and short circuit currents until overcurrent protection devices, usually circuit breakers, clear over-currents that occur. Primary factors that affect selection of appropriate ATS are discussed as follows.

Selective Coordination Approach

Figure 2 below shows two common approaches to selective coordination. At left is a “slow trip” arrangement with trip times ranging from 0.5 seconds at the primary breaker to 0.05 seconds on the tertiary breaker. At right is a “quick trip” scheme with trip times ranging from 0.2 to 0.05 seconds. Both will comply with code, but what are the ramifications of choosing one scheme over the other?
Fault Currents

The amount of energy transmitted during a fault or short circuit event increases with time. As a result, longer time ratings require larger transfer switches (and potentially larger distribution devices and conductors) that can handle the larger currents until they are cleared by a breaker or fuse. For a specific ATS, the amount of fault current it can carry decreases with longer times. Whereas an ATS may be able to withstand a certain fault current for 0.2 seconds, its rating at 0.5 secs, if available, will be reduced. As a result, a larger or different, and likely more costly, ATS may be required to achieve the short circuit ratings needed for that location. Figure 3 illustrates the relationship by showing the larger and smaller ATS needed for each approach.
ATS Location

Importantly, ATS are rated for nominal capacity as well as the amount of fault current they can carry. The amount of fault current required rises with the ampacity of the load. Consequently, the position of the ATS in a power distribution system has a significant effect on the ratings required to provide adequate short circuit performance.

ASCO Power Technologies typically suggests locating transfer switches close to the loads they serve. One reason is that the location of an ATS in a power distribution system affects the ratings required to serve downstream loads. Transfer switches located close to loads can use switches with lower ampacity and short current ratings.

Figure 4 shows two configurations. The configuration at left offers the advantage of simple installation and operation of a single switch. However, an excess capacity rating is needed to achieve an adequate short circuit rating. At right, transfer switches are located closer to the loads they serve. To handle the amount of load and the amount of fault current on each tertiary circuit, 600 Amp switches may provide adequate nominal capacity and short circuit rating.

Incident Energy and Arc Flash Safety

Longer trip times increase the amount of energy available within equipment. Any work inside equipment must comply with requirements of NFPA 70E - Standard for Electrical Safety in the Workplace. Incident energy values in a slow trip approach could increase required levels of protection, or in worst cases, preclude servicing without deenergizing the switch. Consequently, using a selective coordination scheme with shorter trip times can reduce electrical safety risks to workers and promote serviceability. See the ASCO Power Technologies white paper entitled Arc Flash Studies and Transfer Switch Serviceability for more information.
Rules of Thumb

To comply with NEC requirements, designers must determine the instantaneous clearing times of breakers from their published trip curves to confirm that these times are less than or equal to the time-based ratings of transfer switches. If the instantaneous trip time of a breaker exceeds 0.05 seconds or if the breaker has a “short time response”, a short-time rating of the transfer switch must be applied, which is typically a lower current level. If the transfer switch has no short-time rating, the Short Time response cannot be enabled on the Circuit Breaker ahead of the switch, which can hinder the ability to selectively coordinate the system.

The challenge for specifiers is to produce a specification that will require fewest changes. As a rule of thumb, an approach can be selected based on the type of breaker specified ahead of the ATS. Molded case breakers generally offer relatively short time ratings and limited adjustability. Conversely, power breakers typically offer longer trip times and larger ranges of adjustment.

Because of these generalities, providing 0.05-second rated ATS for locations served by molded case breakers and 0.3 second switches for locations served by power breakers can often provide workable solutions. This approach makes changes to specification submittals less likely and can reduce change orders and conflicts between vendors, specifiers, and contractors.


Because each power distribution system is unique, the adequacy of selectively coordinated systems and their ATS must be verified by a qualified expert following the performance of a selective coordination study. The guidance herein is provided for conceptual purposes only. A qualified engineer, an overcurrent protection device provider, or a transfer switch manufacturer can provide additional guidance for specific applications.


ATS must be appropriately rated to withstand fault and short circuit currents until overcurrent protection devices clear over-currents that occur, including longer over-currents that could occur on selectively coordinated power distributions systems. “Slow trip” selective coordination schemes can offer simpler transfer switching. They can also require more costly ATS with higher short circuit ratings. Conversely, “quick trip” schemes locate smaller ATS closer to loads. Fault current, ATS location, and arc flash safety characteristics are all impacted by the specific attributes used under each approach.

To minimize changes to submittals, specifiers can select transfer switches based on the type of breaker identified ahead of the ATS. Providing 0.05-second rated ATS for locations served by molded case breakers and 0.3 second switches for locations served by power breakers can often provide workable solution that can reduce change orders and conflicts between vendors, specifiers, and contractors.

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