Circuit Breaker Symbol
Definition: The Circuit Breaker symbol represents an automatically operated electrical switch designed to interrupt fault current and protect a circuit from damage caused by overcurrent or short circuit, shown in wiring diagrams as a two-terminal device (Line, Load) per IEC 60617-07 and ANSI Y32.2 / IEEE 315 switch and protective-device symbol conventions.
Also known as: MCB, miniature circuit breaker, circuit breaker, thermal-magnetic breaker, OCPD, overcurrent protective device, branch-circuit breaker.
What the Circuit Breaker symbol means
The Circuit Breaker symbol denotes a resettable overcurrent protective device that automatically opens its contacts when current exceeds a rated threshold (the trip rating in amperes), interrupting the circuit and preventing conductor insulation damage, fire, or equipment failure. Unlike a fuse, a circuit breaker can be manually reset after the fault condition is cleared and the breaker has tripped. In wiring diagrams the symbol appears between the panel bus bar (Line terminal) and the branch circuit conductor (Load terminal).
Circuit breakers operate via two tripping mechanisms: a thermal element (bimetal strip) that bends under sustained overload current to open the contacts after a time delay proportional to overload magnitude, and a magnetic element (solenoid) that trips instantaneously on high fault currents such as short circuits. This combination is called a thermal-magnetic circuit breaker.
How to identify the Circuit Breaker symbol
The Circuit Breaker symbol in IEC 60617 is drawn as two circles (representing the normally-closed contacts) with a line through one circle indicating the trip mechanism, or alternatively as a switch symbol with a diagonal 'trip' arrow. In ANSI Y32.2 the circuit breaker is drawn as a square with diagonal lines or a switch symbol with a special qualifier. In residential wiring diagrams it commonly appears as a rectangle with Line (top) and Load (bottom) terminals and the ampere rating labelled inside or adjacent to the block.
Function in a circuit
A circuit breaker carries load current through its closed contacts under normal conditions. When load current exceeds the trip rating, the bimetal thermal element heats up and deflects, releasing the latch mechanism and allowing a spring to snap the contacts open. For a hard short circuit, the large magnetic field from the high fault current pulls the solenoid plunger instantly, opening contacts within milliseconds. Once tripped, the breaker handle moves to a centre ('tripped') position; the user must push it to OFF and then ON to reset.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the circuit breaker symbol as a switch contact symbol with a thermal or magnetic trip indicator. IEC 60898-1 specifies miniature circuit breaker (MCB) performance requirements for household and similar installations. IEC 60947-2 covers industrial moulded-case circuit breakers (MCCB). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 defines the circuit breaker symbol as a square with qualifying lines or as a switch with a trip indicator. UL 489 (North America) governs MCB performance. ANSI C37 series covers power circuit breakers for distribution and transmission. |
| Key difference | IEC 60617 draws a circuit breaker with contact and trip-mechanism notation using standardised qualifier symbols; ANSI Y32.2 uses a square block with diagonal lines. In residential plan-view diagrams both conventions are often simplified to a labelled rectangle. Functionally the symbols represent identical devices. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
| load | Load |
Typical values
Standard residential trip ratings: 15 A, 20 A, 30 A, 40 A, 50 A, 60 A (120/240 V AC). Standard voltage ratings: 120 V, 240 V (residential), 277/480 V (commercial). Interrupting capacity: 10 kAIC (standard residential), 22 kAIC or 65 kAIC (commercial/industrial). Thermal trip time at 100% overload: varies from seconds to minutes depending on rating.
Where the Circuit Breaker symbol is used
- Residential electrical panel boards protecting branch circuits for lighting, receptacles, appliances, and HVAC equipment
- Commercial and industrial distribution panels providing overcurrent protection for motor branch circuits and feeder circuits
- Sub-panels in detached garages, workshops, and accessory dwelling units receiving feeder power from a main panel
- Motor control centres (MCCs) protecting individual motor branch circuits per NEC 430.52 requirements
- Photovoltaic (solar) disconnect and protection in residential and commercial solar installations (DC-rated circuit breakers)
- EV charger circuits requiring a dedicated 40 A or 50 A double-pole breaker on a 240 V NEMA 14-50 or 6-50 circuit
Example
In a residential panel wiring diagram, the Circuit Breaker symbol sits between the panel bus bar (Line pin) and the 12 AWG branch circuit conductor feeding a 20 A kitchen receptacle circuit (Load pin); the breaker is rated 20 A and interrupts the circuit automatically if a fault causes current to exceed 20 A, with a short-circuit interrupting capacity of 10,000 A at 120/240 V per UL 489.
Key facts
- The circuit breaker symbol represents a resettable overcurrent protective device; it automatically opens on overload or short circuit and can be manually reset after the fault is cleared.
- Terminals are Line (supply / panel bus bar side) and Load (circuit conductor side); reversing these reduces interrupting capacity in some breaker designs.
- IEC 60898-1 governs miniature circuit breakers (MCBs) for household use; IEC 60947-2 covers moulded-case circuit breakers (MCCBs) for industrial applications. ANSI/UL 489 is the North American MCB standard.
- Thermal-magnetic breakers combine a bimetal thermal element (overload protection with inverse time delay) and a magnetic solenoid (instantaneous short-circuit protection).
- Trip curves: Type B (3–5× rated current instantaneous), Type C (5–10×), Type D (10–20×) per IEC 60898-1; selected based on load inrush current characteristics.
- Interrupting capacity (AIC — Ampere Interrupting Capacity) must equal or exceed the available fault current at the installation point; standard residential is 10 kAIC.
- A tripped circuit breaker handle moves to a centre position between ON and OFF; it must be pushed fully to OFF before resetting to ON to re-latch the mechanism.
- The IEC designator for a circuit breaker in a wiring diagram is CB or Q (for protective switching device); the ANSI designator is CB.
Diagrams that use this symbol
- circuit diagram
- and circuit diagram
- wiring diagram
- 3 way switch wiring
- wiring a three way switch
- 3 way switch wiring diagram
- 3 way switch wiring schematic
- three way switch connection diagram
Frequently asked questions
What does the circuit breaker symbol look like in a wiring diagram?
In residential wiring plans the circuit breaker symbol is a rectangle labelled with its ampere rating (e.g. '20A') with a Line terminal at the top and a Load terminal at the bottom. In IEC 60617 logic the symbol shows two contact circles with a trip-mechanism indicator. In ANSI Y32.2 it appears as a switch symbol with a square qualifier or a diagonal-line block.
What does the circuit breaker symbol mean?
The circuit breaker symbol marks an automatically operated overcurrent protective device in the circuit. It indicates that the circuit is protected against overloads and short circuits, that the device will trip (open) when current exceeds its rated threshold, and that it can be reset manually after a fault is corrected — unlike a fuse which must be replaced after operating.
What is the IEC standard for circuit breaker symbols?
IEC 60617-07 defines the graphical symbols for circuit breakers in electrical diagrams. IEC 60898-1 specifies performance requirements for miniature circuit breakers (MCBs) in household installations. IEC 60947-2 covers moulded-case circuit breakers (MCCBs) for industrial use. The IEC designator for a circuit breaker is CB or Q.
What is the difference between a fuse symbol and a circuit breaker symbol?
A fuse symbol (IEC: a rectangle with a line through it; ANSI: a diagonal slash in a rectangle) represents a one-time overcurrent protection device that must be replaced after it operates. A circuit breaker symbol represents a resettable device that can be manually reset. Functionally both protect against overcurrent, but circuit breakers offer greater convenience and are required by NEC in most residential panels.
What do the trip curve letters B, C, D mean on a circuit breaker?
Trip curve letters classify the instantaneous magnetic trip threshold relative to the rated current (In). Type B trips instantaneously at 3–5× In (used for resistive loads and cables), Type C at 5–10× In (general-purpose, motors with moderate inrush), and Type D at 10–20× In (high-inrush loads such as transformers and large motors). These curves are defined in IEC 60898-1 and determine how the breaker behaves during starting currents.
What does AIC mean on a circuit breaker?
AIC stands for Ampere Interrupting Capacity (also called short-circuit interrupting capacity or SCCR — Short Circuit Current Rating). It is the maximum fault current the breaker can safely interrupt without being destroyed. Standard residential MCBs are rated 10,000 AIC; commercial MCCBs may be rated 22,000–65,000 AIC. NEC 110.9 requires the breaker AIC to equal or exceed the available fault current at the installation point.
What is the designator letter for a circuit breaker on an electrical schematic?
In North American and ANSI/IEEE schematics the circuit breaker designator is CB followed by a number (e.g. CB1, CB2). In IEC and European schematics the designator is Q (for protective switching device) or CB, following IEC 81346-2 reference designation conventions. In residential wiring diagrams breakers are often labelled by circuit number and ampere rating (e.g. 'CKT 1 – 20A') without a formal designator letter.
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