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Breaker Types & Ratings Explained

The main families of circuit breakers and protectors, and what each rating on the label means. It explains how to match a replacement without guessing; sizing comes from the NEC and the equipment label.

Ref EL-0002NEC 240UL 489UL 1077UL 943UL 1699

Safety-critical sheet, not yet signed off by an expert reviewer. Use it to understand the topic, then take every value and decision from the governing standard, the manufacturer’s instructions and a qualified person.

What it is

A circuit breaker is a resettable overcurrent device. In North America, breakers used for branch-circuit and feeder protection are listed to UL 489. UL 1077 supplementary protectors look similar, especially on DIN rail, but they are not branch-circuit protection. A supplementary protector can only be used downstream of a properly rated branch-circuit device.

How to identify

Mounting: plug-on (residential and light commercial load centers), bolt-on (panelboards), DIN-rail (control panels) or a molded-case frame with lugs.
Label: the ampere rating is shown on the handle. The voltage rating, interrupting rating (kA), standard (UL 489 or UL 1077), wire range and temperature rating are printed on the side or face.
Function buttons: a TEST button means GFCI (UL 943), AFCI (UL 1699), dual-function or equipment ground-fault protection.

Families and ratings

Item	What it means
Miniature / plug-on breaker	Panel-brand-specific branch breaker; listed for use only in the panels named on the panel label
Molded-case circuit breaker (MCCB)	Larger frames with thermal-magnetic or electronic trip units
Insulated-case / power circuit breaker	Large-frame breakers in switchboards and switchgear
GFCI breaker	Trips on ground-fault current to protect people
AFCI / CAFCI breaker	Trips on arcing signatures in branch wiring
Dual-function breaker	Combined AFCI and GFCI
Supplementary protector (UL 1077)	Equipment-internal protection only, not a branch-circuit device
Ampere rating	Continuous current the device carries before tripping on overload
Voltage rating, straight (e.g. 240 V) vs slash (e.g. 120/240 V)	A slash-rated breaker may only be used on a solidly grounded system that fits both numbers
Interrupting rating (AIC, kA)	The largest fault current it can safely interrupt; must equal or exceed the available fault current
Poles	Number of ungrounded conductors it opens
HACR, SWD, HID markings	Suitable for HVAC equipment, switching duty, or HID lighting switching
80% vs 100% rated	Whether the breaker can carry its full rating continuously in its enclosure
Terminal temperature / CU-AL	Conductor materials and temperature column allowed at the lugs

Key points

Breaker size depends on the conductor, the load and the equipment nameplate (for example MOP on HVAC equipment). Use NEC 240.4, 240.6 and the article for that equipment. Do not guess from the old breaker; it may have been wrong.
Replacement breakers must be the type listed on the panel label, or a UL Classified replacement that is evaluated for that panel.
A low interrupting rating is a serious hazard. A breaker that cannot interrupt the available fault current can fail violently.

Common mistakes

Using a UL 1077 DIN-rail protector as a branch breaker.
Putting a slash-rated (e.g. 480Y/277 V) breaker on a corner-grounded or ungrounded delta system.
Fitting a breaker because it "snaps in" to a panel it is not listed or classified for.
Selling unauthorized-channel or used breakers. This is a known counterfeit risk.

Governing standards: NEC (NFPA 70) Article 240 and related equipment articles (210.8, 210.12, 440); UL 489; UL 1077; UL 943; UL 1699; UL 1066 for low-voltage power circuit breakers.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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