A circuit breaker is an automatic switch that protects an electrical circuit from damage caused by overcurrent, short circuits, and, in some designs, ground faults or arc faults. When the current rises above a safe level, the breaker opens and interrupts the flow of electricity. That's what people mean when they say a breaker has "tripped." A tripped circuit breaker has done its job. It has interrupted the circuit before the conductors or connected equipment could overheat.

The function of a circuit breaker is protection. Its purpose is to keep wiring and equipment within their rated limits and to give people a safe, resettable way to open a circuit. A fuse does similar work but destroys itself in the process, while the breaker resets and continues working. Breakers are used for almost every modern branch circuit, feeder, and service.

Circuit breakers live inside a circuit breaker panel, also called a load center, breaker box, or panelboard. The panel distributes incoming power to individual circuits, and each circuit gets its own breaker sized to the conductor it protects. So, the main goal of a circuit breaker is to protect a specific circuit and everything connected to it, and it serves as the disconnect for that circuit.

The article covers every common breaker type, from residential to utility-grade. Each entry explains what the breaker is, where to use it, whether code requires it, or it is simply good practice, and what it is best for.

Residential and Panel Configuration Breakers

These are the everyday breakers that fill a home panel, defined mostly by how many poles they use and how they fit the panel rather than by any special sensing technology.

Standard / Single-Pole Circuit Breaker

A single-pole breaker occupies one slot in the panel and connects to one hot leg, protecting a single 120V circuit. Ratings are usually 15A or 20A. Use it for general lighting, receptacle circuits, and small appliances. A 15A single-pole breaker typically feeds bedroom and hallway lighting; a 20A single-pole breaker feeds general-purpose receptacles; and a dedicated 20A single-pole breaker feeds a single appliance, such as a microwave or a garbage disposal.

No code section mandates a single-pole breaker by name. It is the default building block of a 120V branch circuit, and its ampere rating must match the conductor it protects under NEC 240.4.

Best for: everyday 120V lighting and receptacle circuits in homes and light commercial spaces.

Double-Pole Circuit Breaker

A double-pole breaker, also called a two-pole breaker, spans two slots and connects to both hot legs, delivering 240V or 120/240V with a common trip. Use it for 240V loads such as a 30A breaker for an electric dryer, a 50A breaker for a range, a 30A breaker for a central air-conditioning condenser, a 40A to 60A breaker for an EV charger, and a two-pole breaker for a well pump or water heater. A 240V appliance requires a two-pole device, and the 2023 NEC now pushes many 240V receptacles in listed locations toward two-pole GFCI breakers under 210.8.

Best for: 240V appliances and any 120/240V circuit that needs both hot legs.

Tandem / Duplex / Twin Circuit Breaker

A tandem breaker places two independent 120V circuits in one panel space. It has two handles and two trip mechanisms, so either circuit can trip without shutting off the other.

Tandems are used in compatible residential load centers, including certain Square D Homeline and QO, Eaton BR, and Siemens panels. The exact breaker model and permitted slot positions must appear on the panel label. A panel rated 20 spaces/40 circuits, for example, can accept enough tandems to provide up to 40 circuits.

Because both circuits in a standard tandem connect to the same bus leg, the breaker cannot supply 240V and should not be used for the two hot conductors of a standard multiwire branch circuit with a shared neutral.

Standard tandems provide only overload and short-circuit protection. AFCI tandem models are available for some panel lines, but AFCI and GFCI options vary by manufacturer.

Do not install more circuits than the panel label permits. Tandem use must follow the panel and breaker listings under NEC 110.3(B).

Best for: adding two 120V circuits in one space when the panel is specifically rated for the tandem breaker.

Quad Circuit Breaker

A quad breaker packs four circuits into two slot spaces, usually two outer 120V poles with an inner two-pole 240V breaker. Use it where the panel is out of full-size spaces but still needs a mix of circuits, for example, adding a 240V circuit for a new air-conditioning unit and two 120V circuits in a panel with no open slots left. Eaton BR and Square D both make quad breakers listed for their panels. As with tandems, the panel has to be listed for quad or high-density breakers and marked for the position.

Best for: dense panels that need both 120V and 240V circuits in minimal space.

Main Circuit Breaker

The main breaker feeds the panel's bus bars and powers every branch breaker downstream. It is rated to the service size, such as a 100A main in a small home, a 200A main in a typical modern home, or a 400A main in a large home or small commercial building. It protects the panel and service conductors from overload and serves as the service disconnect. NEC 230.70 requires a means to disconnect the service, and the main breaker is commonly used for that purpose.

Best for: whole-panel disconnect and service-level overcurrent protection.

Low-Voltage Distribution Breakers

These breakers protect larger low-voltage circuits, from IEC-standard branch circuits up to the high-current mains of a commercial switchboard.

Miniature Circuit Breaker (MCB)

The MCB is the international term for a compact, DIN-rail modular breaker that protects low-voltage circuits from overloads and short circuits, typically up to about 125A under IEC standards. Use it for residential and light commercial panels and control cabinets in IEC-standard markets, for example, a lighting circuit in a European-standard consumer unit or the circuit protecting a PLC and its contactors inside a machine control panel. Do not use it for high-fault-current feeders or high continuous current; it is intended for moulded-case breakers. In North America, the same job is handled by standard thermal-magnetic breakers, so "MCB" appears more often on international and industrial control drawings than on a typical U.S. residential panel.

Best for: branch-circuit protection in IEC-standard residential, light commercial, and control panels.

Molded Case Circuit Breaker (MCCB)

The MCCB carries far more current than an MCB, up to roughly 2500A, inside a molded insulating case rated at 600V or below. On larger MCCBs, the trip unit is adjustable. That means a dial lets you set the trip current within a range instead of at a single fixed value. A 250A-frame breaker, for example, can be set to trip at 200A on one job and 150A on another. That flexibility lets one breaker serve several loads, and it lets an engineer coordinate it with the breakers above and below it so that only the breaker closest to a fault trips, rather than the whole building going dark.

Use it for commercial and industrial feeders, sub-distribution, and equipment circuits, for example, a 400A feeder to a subpanel, the supply to a rooftop HVAC unit, or the feeder to a large production machine. It is oversized and costly for small residential branch circuits.

MCCBs are ordered by frame size, which is the largest current the housing can carry. Several trip ratings share one frame. This is worth understanding if you ever wonder about the frame size of a 30A circuit breaker: the 30A is the trip setting, and the frame is the housing it lives in. In industrial gear, a 30A trip is often the low end of a small frame, such as a 100A frame, while a plug-on 30A residential breaker simply belongs to its own series frame.

Best for: feeders and equipment circuits above typical miniature-breaker ratings.

Air Circuit Breaker (ACB)

The ACB interrupts faults in air at atmospheric pressure and is built for low-voltage, high-current service, commonly 630A through 6300A, often in a draw-out design that pulls out for maintenance. Use it in main low-voltage switchgear and large service entrances, for example the main incoming breaker in a 4000A commercial switchboard or a tie breaker between two main sections in a data center distribution board.

Best for: main low-voltage switchgear carrying high continuous current.

Medium- and High-Voltage Breakers

These breakers switch and protect utility and heavy-industrial systems above 1kV, where the arc must be quenched in vacuum, gas, or oil rather than in open air.

Vacuum Circuit Breaker (VCB)

The VCB extinguishes the arc within a sealed vacuum interrupter and is the standard modern choice for medium-voltage systems, ranging from roughly 1kV to 38kV. Use it in medium-voltage industrial distribution and utility feeders, for example, a 15kV feeder breaker in a factory substation or a distribution feeder in indoor metal-clad switchgear serving a hospital or university campus. It sits between low-voltage panels and extra-high-voltage transmission, where other technologies fit better. Its low maintenance and long service life make it the default for new MV installations.

Best for: medium-voltage switching in industrial and utility distribution.

Sulfur Hexafluoride Circuit Breaker (SF6CB)

The SF6 breaker uses sulfur hexafluoride gas to quench high-voltage arcs and is common in substations and gas-insulated switchgear. Use it in high-voltage substations and transmission systems, for example, a 230kV transmission-line breaker or a substation bay in a gas-insulated switchgear lineup where space is tight. One current caveat: SF6 is a potent greenhouse gas, and the industry is actively moving toward lower-impact alternatives, so newer specifications may steer away from it.

Best for: high-voltage substation and transmission switching.

Shock, Ground-Fault, and Arc-Fault Protection

These breakers add life-safety sensing on top of overcurrent protection, watching for current leaking to ground or for dangerous arcing that a standard breaker would never catch.

Residual Current Circuit Breaker / Device (RCCB / RCD)

The RCCB, also called an RCD, detects an imbalance between line and neutral, the residual current, and trips to protect people from electric shock. Personnel devices typically trip at about 30mA. It does not provide overload or short-circuit protection on its own, so it is paired with an overcurrent device. Use it for shock protection in IEC-standard systems alongside MCBs, for example, a 30mA RCD protecting a group of lighting and socket circuits in a European consumer unit. Do not rely on it as the sole protection for a circuit, since it does nothing to protect against overload or short circuits on its own.

Best for: ground leakage and shock protection in IEC-standard installations, used with separate overcurrent protection.

Ground-Fault Circuit Interrupter Breaker (GFCI)

A GFCI breaker is the North American device for shock protection. It is a breaker that monitors current leakage to ground and cuts power before it can harm someone. Use it in the wet locations listed in NEC 210.8, for example, a bathroom receptacle circuit, an outdoor patio receptacle, a garage circuit, a kitchen countertop circuit, or a two-pole GFCI breaker for a 240V spa or pool pump. The 2023 NEC expanded the scope to 125V through 250V receptacles supplied by single-phase circuits rated 150V or less to ground. Do not treat it as arc-fault protection, which addresses a different hazard. Protection can come from a breaker or a receptacle, and in the listed locations, it is required, not optional.

Best for: personnel shock protection in wet and damp locations required by NEC 210.8.

Arc-Fault Circuit Interrupter Breaker (AFCI)

An AFCI breaker detects the dangerous series and parallel arcing that can ignite a fire inside walls, then de-energizes the circuit. Use it on the 120V living-area circuits listed in NEC 210.12: kitchens, family rooms, dining rooms, living rooms, bedrooms, sunrooms, recreation rooms, closets, hallways, and laundry areas, as well as similar rooms. In practice that reaches nearly every 120V circuit in a dwelling's living space. The 2023 NEC applies the requirement to 10A, 15A, and 20A single-phase circuits, and the combination-type AFCI is the standard method. Do not treat an AFCI as a substitute for GFCI shock protection, since arc detection and ground-fault detection are different jobs.

Best for: fire protection on 120V living-area circuits required by NEC 210.12.

Dual-Function AFCI/GFCI Breaker

A dual-function breaker combines arc-fault detection and Class A ground-fault protection in a single device. Use it where both NEC 210.8 and NEC 210.12 apply, for example, a laundry-area circuit, a kitchen receptacle circuit, or a finished-basement circuit, so one breaker satisfies both requirements. Do not confuse it with a "combination-type AFCI," which refers to arc detection alone and does not add ground-fault protection.

Best for: circuits that need both arc-fault and ground-fault protection from one slot.

Residual Current Breaker with Overcurrent Protection (RCBO)

The RCBO combines the leakage protection of an RCD with the overload and short-circuit protection of an MCB in one module. Think of it as an RCCB plus an MCB. Use it in IEC-standard installations where each circuit should have its own leakage and overcurrent protection, for example, a dedicated circuit feeding a freezer where you do not want a shared RCD trip on another circuit to knock out the freezer too. Do not specify it for North American panels, where GFCI and dual-function breakers fill the same role.

Best for: single-circuit combined leakage and overcurrent protection in IEC-standard panels.

Motor and Specialized Breakers

These breakers do not fall into previous categories and are defined by a specific job or trip technology.

Motor Protection Circuit Breaker (MPCB)

The MPCB protects motors from overloads and phase-loss, also called single-phasing, with an adjustable overload range matched to the motor's full-load current. Use it in motor branch circuits, starters, and motor control centres, for example, the starter for a 10 HP pump motor, a conveyor drive in a motor control centre, or the supply to an HVAC compressor. NEC Article 430 governs motor protection, and the MPCB is a common means of providing the required overload and short-circuit protection in a single device.

Best for: protecting motors from overload, short circuit, and single-phasing.

Thermal-Magnetic Circuit Breaker

The thermal-magnetic breaker is defined by its trip mechanism rather than its size. A bimetal strip provides time-delayed protection against overloads, and an electromagnet provides instantaneous tripping on short circuits. This mechanism is found in most residential breakers and many commercial ones, making it the most common breaker mechanism in use. Use it for general branch and feeder protection, for example, the standard breakers on a residential panel or the lighting and receptacle circuits in a small office. Do not choose it where precise, adjustable coordination is required, because an electronic trip unit does that better.

Best for: general-purpose overcurrent protection across residential and commercial circuits.

Electronic Trip Circuit Breaker

An electronic trip breaker replaces the bimetal-and-electromagnet mechanism with a solid-state trip unit, allowing adjustable long-time, short-time, instantaneous, and ground-fault settings, often abbreviated LSIG. These units appear in larger MCCBs and ACBs. Use them in large commercial and industrial systems that need selective coordination and, in many cases, built-in metering, for example, a 2000A main in a hospital switchboard with built-in ground-fault protection, or coordinated feeders in a data centre where only the faulted circuit should drop. The added cost buys nothing useful on small residential circuits.

Best for: large systems that need precise, adjustable trip settings and coordination.

Smart / Wi-Fi Circuit Breaker

A smart breaker adds communication, remote monitoring, control, and energy metering, usually as part of a smart panel controlled through an app. Use it for energy management and remote control, for example, a smart panel that monitors solar production and EV charging, sheds selected loads during peak demand, or reports which circuit is drawing the most power. Do not treat a smart breaker as a replacement for required GFCI or AFCI protection unless the device is also listed for that function. Code does not require smart breakers. They are a convenience and energy-management feature.

Best for: remote monitoring, control, and energy management in modern smart panels.

Other Breaker Terms You Will Encounter

A few terms come up constantly and describe features, ratings, or brands rather than a distinct breaker family.

A shunt trip circuit breaker is a standard breaker fitted with a shunt trip accessory that lets an external control signal trip it remotely. Fire alarm systems, emergency-stop circuits, and elevator shutdown schemes use shunt trips to open a breaker instantly on command.

An HACR circuit breaker is rated for Heating, Air Conditioning, and Refrigeration equipment, making it suitable for the group motor and compressor loads found in HVAC gear. Most modern breakers are HACR-rated, so the marking mainly confirms suitability for those loads.

A Stab-Lok circuit breaker is a Federal Pacific Electric product line with well-documented failure-to-trip problems. Many inspectors and electricians recommend replacing FPE Stab-Lok panels for that reason, so the term is usually used in a safety or replacement context rather than for new installations.

A Square D circuit breaker comes from one of the largest North American breaker manufacturers. Square D's QO line is a widely used residential and light-commercial series known for fast trip response, while its Homeline line is the value-tier option. So a QO circuit breaker is simply Square D's QO-series breaker, sized like any other single-pole, double-pole, tandem, GFCI, AFCI, or dual-function unit.

Frame size describes the maximum current a breaker's housing is built to carry. Several trip ratings share a single frame, and the trip is set at or below that frame's rating.

Matching Breakers to the Right Conductor

The conductor's ampacity under NEC 310.16 must match the breaker's rating under NEC 240.4, and the breaker's interrupting rating must be capable of handling the available fault current at that point in the system. Correct breaker selection starts with correct conductor selection.

Nassau National Cable stocks the wire and cable that sit behind these breakers across residential, commercial, industrial, and utility work:

Matching Breakers to the Right Conductor

A breaker protects the wire, not the other way around. The conductor's ampacity under NEC 310.16 must match the breaker's rating under NEC 240.4, and the breaker's interrupting rating must handle the available fault current at that point in the system. An oversized breaker on an undersized conductor is a fire waiting to happen, because the wire can overheat before the breaker ever trips. Correct breaker selection starts with correct conductor selection.

Nassau National Cable stocks the wire and cable that sit behind these breakers across residential, commercial, industrial, and utility work:

• NM-B (Romex) for residential branch circuits behind single-pole, double-pole, GFCI, AFCI, and dual-function breakers.

• UF-B for direct-burial residential circuits, such as a run to a detached garage, a well pump, or landscape lighting behind a GFCI breaker.

• THHN/THWN-2 for conduit and raceway runs on commercial and industrial breaker-protected circuits.

• XHHW-2 for feeders and services on larger MCCB and ACB circuits.

• MC cable for commercial branch and feeder circuits, including the metal-clad home-run methods that some AFCI compliance options call for.

• SER for interior service-entrance and subpanel feeders behind main breakers.

• SEU for the service drop into the meter and main disconnect.

• USE-2 and RHW-2 for underground service and feeder runs into the main breaker, and for feeder and equipment circuits that call for a rugged thermoset insulation.

• URD for underground residential distribution feeding the service and main.

• Medium-voltage (MV) cable for the 5kV through 46kV circuits switched by vacuum and SF6 breakers in industrial and utility systems.

• Tray cable (TC-ER) for industrial power and control runs in a cable tray, feeding motors and equipment.

• Control cable for shunt-trip, interlock, and electronic-trip control wiring.

• Instrumentation cable for the signal and sensor wiring that ties into MPCBs and electronic trip units.

• Portable cord (SOOW, SJOOW) for equipment connections and flexible drops on machine and motor circuits.

• Welding cable for high-current flexible feeds to welders and similar equipment behind their dedicated breakers.

• Marine and shipboard cable for vessel and offshore panels where breakers and conductors must meet marine standards.

• Bare copper for grounding and bonding across every panel and service.

Choose the breaker for the load and the hazard, then choose the conductor for the breaker. Browse Nassau National Cable's full selection to match the right cable to every circuit, or reach out to the sales team for help sizing a run.

Choose the breaker for the load and the hazard, then choose the conductor for the breaker. Browse Nassau National Cable's full selection to match the right cable to every circuit, or reach out to the sales team for help sizing a run.