Our Brands

My Documents 0

Welcome to the ASCO Website

Welcome to our website.

Nomenclature for Low-Voltage Circuit Breakers

“Aren’t they all power breakers?”

To the uninitiated, the term “power breaker” would seem to apply to any circuit breaker. To those more familiar, the term focuses on a range of low voltage circuit breakers. Because breaker nomenclature can seem complex, this article reviews the terms that describe commonly used in low-voltage, backup power systems.

Molded Case Circuit Breakers

Some breaker types are named for the manner in which they are constructed. The term molded case circuit breaker identifies a breaker that is assembled as an integral unit within a supportive enclosure of insulating material … typically, a molded plastic case. Within this category, breakers are differentiated by their trip mechanisms. There are two principle types - thermal-magnetic and solid-state.

Thermal-Magnetic Trip Mechanism

Thermal-magnetic circuit breakers can be tripped open by either of two actuators, a thermal device that responds to prolonged overcurrents, and a magnetic device that responds to sudden short circuit currents.

The thermal device usually consists of a bimetallic element alongside a conductor. When overcurrents occur, resistive heating causes the metals in the element to heat at differing rates, resulting in differential expansion that causes the component to bend. When the bend is sufficient, it displaces a trip bar that opens the breaker contacts, stopping the flow of current through the circuit. The concept is shown in Figure 1.
Magnetic trip mechanisms protect against sudden current from shorts and faults. High current flow through a coil magnetizes the trip device, which moves to open the breaker contacts. The effect is conceptually shown in Figure 2.
Some molded case breakers use solid state components to sense current parameters and initiate the opening of contacts to protect circuits. Figure 3 shows a molded case breaker with a solid-state trip unit.
Listings and Applications for Molded Case Circuit Breakers

Molded case breakers are tested according to requirements specified in UL 489 - Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures. Those that test successfully are listed by UL. Notably, breakers are identified as “80%-rated” and “100%-rated” models. Those rated at 100% undergo thermal testing beyond those listed at the 80% threshold. Consequently, 80%-rated molded case breakers are sometimes called “standard breakers”.

In Chapter 2 – Wiring and Protection of the 2020 National Electrical Code® (NEC®), Article 240.20(B) states:

210.20(A) Continuous and Noncontinuous Loads.

Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

In Article 100, the NEC defines continuous loads as those “… where the maximum current is expected to continue for 3 hours or more.

To understand the application of the code, consider these examples:
  1. A 100 Amp load that will persist for less than 3 hours can use an 80%-rated breaker with a handle rating of 100 Amps.
  2. A 100 Amp load that may persist beyond 3 hours can rely on a 100%-rated breaker with a handle rating of 100 Amps or an 80%-rated breaker with a handle rating of 125 Amps (125 Amps x 0.8 = 100 Amps).
Regardless of the application strategy, molded case circuit breakers are most often used in power distribution circuits downstream of transfer switches and paralleling switchgear. They are not typically used for utility or paralleling applications.

Insulated Case

Sometimes called a “power” breaker, an Insulated Case Circuit Breaker is similar to a molded case breaker, but Is built on a frame inside an “insulated” molded plastic case. They are tested to the same UL489 standard as molded case circuit breakers. Insulated Case Circuit Breakers can be fixed-mounted or draw-out types. Because of their molded case design, they offer limited serviceability.

ANSI-Rated Breakers

The term “power” breaker more often refers to circuit breakers listed to UL 1066 - Standard for Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures, which in turn references additional standards promulgated by the American National Standards Institute (ANSI). Also known as Metal Frame or Air Frame breakers, these are built as an assembly of parts in a welded metal frame, which is mounted on a draw-out mechanism that enables it to slide out of its enclosure for inspection and service. This enables greater serviceability when compared to other types.

Common Power Breaker Design Elements

Insulated case and ANSI-rated Breakers consist of three principle components: (1) Element, (2) Cell, and (3) Trip Unit. These are explained below.


The element is the current-carrying member of the circuit breaker. All of the electrical and protective functions of the breaker are provided by this unit. These units have internal motor-drive operators that can provide higher reliability than external motor drives available for some molded case breakers. An element is shown in Figure 4.

The cell, sometimes called a cradle, is the framework that contains and supports the element. It includes the draw-out mechanism that makes this type of power breaker accessible for inspection and service. The cell, in turn, is installed in switchgear or a dedicated enclosure. A cell assembly is shown in Figure 5.
Trip Unit

The trip unit is the component that monitors electrical conditions and initiates breaker operation. It causes the breaker to open when an overload or short circuit occurs. These solid-state units feature controls for selecting operating modes and setting operating parameters. Trip units may also feature detailed display, and some present information about electrical parameters, metering, and harmonics, as well as trip and event data. They mount on the circuit breaker element. A solid-state trip unit is shown in Figure 6.
Listings and Applications

UL Standards

Two standards are used to test and evaluate circuit breakers: (1) UL 489 – Molded Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures, and (2) UL 1066 – Standard for Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures. A comparison of their key distinctives is shown in the following table:
Breakers with more than one listing

In the past, power breakers were rated to either UL 489 or UL 1066. Major manufacturers have since developed some models that have been tested and listed to both standards. These dual-rated breakers offer application flexibility.

UL listings are used in North America and some additional regions that rely on North American codes or have enacted similar standards. In a large portion of the world, electrical devices are listed according to standards promulgated by the International Electrical Commission (IEC). Some manufacturers have developed power breaker models that are listed to both UL and IEC standards. Because these can be used in most global regions, they are known as universal breakers.


Power circuit breakers are used in a wide range of applications and equipment. These include service entrances, mains, tie, generator, utility, and distribution equipment. It also includes mission-critical applications where motors are used to operate breakers, such as remote-control scenarios. In backup power systems, UL489-listed units are suitable for use in UL 891-listed switchgear, while UL 1066-listed models can be installed in UL-1558 gear as well.


Common types of circuit breakers include molded case units. Contained in insulated plastic housings, they are operated by thermal-magnetic trip mechanisms as well as solid-state trip devices. Molded case circuit breakers are listed to UL 489 and are typically used downstream of transfer switches and paralleling switchgear. Because they are available as “standard” or “80%-rated” models as well as “100%-rated” units, it is important to understand both the terminology and underlying codes that govern their proper application.

The term “power breakers” can include insulated case units listed to UL 489 and ANSI-rated breakers of draw-out design listed to UL 1066. The latter offer the highest level of serviceability. Breakers listed to both UL 489 and UL 1066 are dual-rated, and those also rated to IEC standards are termed universal breakers. These can be used in the broadest range of jurisdictions and backup power equipment.

Get the latest white papers, product news and insights straight to your inbox

Subscribe Now