AT and AF in Circuit Breakers: Ampere Trip Setting vs Ampere Frame Explained

A purchase order that reads "250AF frame, 160AT trip, 3-pole, with shunt trip release" is not a contradiction. It describes a breaker whose frame can carry 250 amps but whose trip unit opens the circuit above 160 amps. Suppliers who treat those two numbers as one rating quote the wrong product, and the error usually only shows up during panel commissioning.

The short answer: AF (ampere frame) is the maximum continuous current the breaker's frame, terminals and internal current path are built to carry, fixed at the factory. AT (ampere trip) is the current at which the trip unit is set to open, and on many molded case circuit breakers (MCCBs) it is adjustable inside that frame. AF is a physical ceiling; AT is an operating point inside it.

Everything that follows on an order form - lugs, contact assemblies, arc chutes, operating mechanisms and control accessories - hangs off that distinction. Get it right and the breaker, its accessories and its cable protection line up. Get it wrong and you pay for the rework.

What AF, the ampere frame, actually fixes

AF is a manufacturing characteristic, not a setting. You cannot dial it up on site, and you cannot buy a "160 A frame" and later turn it into a 250 A one.

Frame ratings are catalogued in standard steps. In North American practice, molded case breakers are commonly offered in 100, 125, 150, 225, 250, 400, 600, 800, 1200, 1600, 2000, 2500 and 3200 AF frames. IEC-oriented catalogs use a similar ladder, typically 125, 250, 400, 630, 800, 1250 and 1600 A frames. Component suppliers follow the same ladder, because the parts they make have to match it.

What the frame physically determines:

• Terminal and lug capacity, and the busbar interface the breaker bolts onto.
• The cross-section of the current path - moving contacts, stationary contacts, contact fingers and plug-in contacts.
• Arc chute volume and the number of splitter plates, which drive interrupting performance.
• Mechanism energy and, on larger frames, the closing electromagnet.
• The mounting interface for accessories: auxiliary contacts, alarm contacts, shunt trips, undervoltage releases, motor operators, plug-in and withdrawable bases.

That is why a 630AF breaker is not simply a scaled-up 250AF unit. Its contacts, its arc chute and its accessory geometry are different parts.

Internal conductive assemblies are built to the frame, not to the trip setting. A moving contact assembly family such as KYM125-250-400-630-800 covers one range of molded case frames, while frame-class parts for 1600 A to 4000 A breakers use a completely different geometry. Quoting by AT alone is how wrong contact sets get shipped.

KYM-125/250/400/630/800 Molded Case Moving Contact AssemblyKYM-125/250/400/630/800 Molded Case Moving Contact AssemblyView Product →

What AT, the ampere trip, controls

AT is the rating at which the breaker is designed to open. On a thermal-magnetic MCCB, the thermal element (a bimetal) defines the continuous rating, backed by a fixed magnetic element for short-circuit currents. On an electronic trip unit, AT becomes a set of settable parameters: long-time pickup (often written as Ir), short-time pickup and delay, and instantaneous pickup.

Two rules follow directly from the hardware:

• AT can never exceed AF. A 250AF frame physically cannot be fitted with a 320 A trip unit.
• AT is usually well below AF, and that gap is deliberate.

Standardizing on one frame with several trip ratings lets a panel builder carry a single frame, one set of lugs and one accessory kit across a row of outgoing feeders; only the trip unit changes. For manufacturers, that means one tooling set covering 100 A through 250 A devices. For distributors, it means fewer spare parts on the shelf, because a frame family covers many circuit ratings.

Continuous loads add a second constraint that has nothing to do with the frame. A 160 AT breaker should not carry more than roughly 128 A continuously, even if the frame underneath it could physically handle far more current.

AF versus AT at a glance

Where AF matters more than AT: accessories and internal assemblies

If you are buying anything other than the trip unit, the frame is the number that matters. Auxiliary contacts, alarm contacts, combined auxiliary-alarm contacts, shunt trips, undervoltage releases, motor operators, plug-in devices and withdrawable devices all attach to the frame's moulding, mechanism or terminal area.

A practical consequence: a 250AF breaker ordered with a 160 AT trip and the same frame ordered with a 250 AT trip accept the same shunt trip, the same auxiliary contact block and the same plug-in base. Catalog tables list these parts by frame and current range - one auxiliary contact series typically spans a family of frames, for example 100 A through 800 A - not by trip setting.

The same logic applies inside the breaker. Moving and stationary contact assemblies, arc chute assemblies and insulated base frames are dimensioned for the frame current, because they must carry the full frame rating and interrupt the associated fault current.

If the accessory map is unfamiliar, an overview of what actually goes into an MCCB, from contacts to release units, is a useful starting point: what are MCCB accessories.

FC38-63/100/250/400/630/800 MCCB Auxiliary ContactsFC34-63/100/250/400/630/800 MCCB Auxiliary ContactsView Product →

How to specify AF and AT without a rework

Both numbers belong on the order, written separately, with the frame first. A specification that says only "160 A breaker" tells a supplier nothing about which frame, which lug or which accessory kit to supply.

1. Write the frame and trip ratings separately: 250AF / 160AT, three-pole. Add the frame's interrupting rating, for example 36 kA at 415 V, because that also belongs to the frame.
2. List frame-dependent hardware explicitly: terminals and lugs, plug-in or withdrawable base, operating mechanism, auxiliary and alarm contacts, shunt trip or undervoltage release.
3. State the trip unit type: thermal-magnetic with a fixed rating, or electronic with an adjustable setting range, and whether the setting must be field-adjustable.
4. Check derating before fixing AT. Ambient temperature inside the enclosure and the 80% continuous-load rule both reduce the usable current.
5. Confirm accessory compatibility against the frame's catalog table rather than the trip rating, especially when mixing frames from different product families.

Common mistakes

• Assuming a larger AT is always safer. An oversized trip setting on a small feeder may no longer protect the cable at its ampacity.
• Assuming accessories follow the trip rating. They follow the frame.
• Assuming the frame's instantaneous magnetic element is adjustable. On thermal-magnetic devices it usually is not.
• Mixing IEC and NEMA frame designations in one bill of materials without converting the current levels.

Control accessories follow the same rule. A remote-tripping device such as the FT-S-1600-6300 shunt trip unit is catalogued against a family of large frames and mounts the same way whether the breaker is set at 1250 AT or 1600 AT.

FT(S)-1600~6300 Shunt Trip UnitFT(S)-630 Shunt ReleaseView Product →

FAQ: AF and AT in circuit breakers