What Type of Circuit Breaker Do I Need? How to Match Rating and Trip Unit

A 400 A feeder breaker and a 16 A lighting breaker sit on the same catalog page, look almost identical in a photograph, and cannot be swapped. The type you need is decided by six things: system voltage, continuous load current, trip characteristic, pole count, short-circuit breaking capacity, and what the circuit must do beyond carrying current, such as tripping remotely, reporting status or being pulled out for service. Get those six right and the catalog narrows itself. Get one wrong and the result is either a breaker that never trips or one that trips every time a compressor starts.

What follows is the order in which experienced panel builders check them, working backwards from the load to the part number.

Check the Load First, Then the Breaker

Breaker selection begins with a measurement, not a catalog. Add up the continuous current of everything the circuit will carry, then apply the 1.25 factor that wiring codes use for continuous duty. That margin exists because terminals, cable and the breaker itself all run warmer at full load than a paper calculation suggests. Ambient temperature matters just as much: a breaker inside a hot enclosure carries less than its nameplate rating, and manufacturers publish derating tables for exactly that reason.

Once the current, the voltage and the prospective fault current at the point of installation are known, the frame size and the trip unit mostly choose themselves.

Frame Size Sets the Ceiling, the Trip Unit Sets the Behaviour

Frame size is the moulded case and the current path inside it: contacts, terminals and arc chute. It defines the highest current the breaker is built to carry. A 250 AF breaker does not have to be ordered at 250 A. The same case accepts trip units from roughly 100 A upwards, and a 400 AF case is often specified where the load is smaller, simply because the fault level or the cable cross-section demands the larger body.

Two practical points catch people out. First, a breaker with a 250 A trip unit inside a 400 AF case is not a 400 A breaker, and that distinction matters during inspection. Second, if the breaker feeds a cable or busbar rated below the frame rating, the conductor becomes the limiting component in the circuit, which is a design decision rather than a breaker selection.

B, C or D: Choosing the Trip Curve

Every thermal-magnetic breaker has two sensing elements. The thermal element handles overloads with an inverse-time response, and the magnetic element handles short circuits almost instantly. The instantaneous part is what separates the curve types. Under IEC 60898-1 conventions, Type B trips magnetically between 3 and 5 times rated current, Type C between 5 and 10, and Type D between 10 and 20.

The rule is to pick the lowest magnetic threshold that still rides through normal inrush. Motors draw several times full-load current for a fraction of a second during start, which is why motor circuits usually land on Type D or an electronic trip unit with an adjustable short-time delay. Transformers, capacitor banks and welding equipment have similar profiles. Choose too low and the breaker trips on every start. Choose too high and a genuine short circuit may be cleared slowly, or lose selectivity against the device upstream.

MCB, MCCB or ACB: Where Your Numbers Land

Once current and fault level are known, the family usually follows. Miniature circuit breakers cover roughly 0.5 A to 125 A with breaking capacities around 6 to 10 kA, fixed thermal-magnetic trips and a modest accessory range. Moulded case circuit breakers span roughly 16 A to 1600 A with breaking capacities from 16 kA to over 100 kA, adjustable trip units, and a wide accessory set. Air circuit breakers take over from about 630 A to 6300 A, use air-insulated arc chutes and are normally draw-out, which is why they dominate main incoming sections where maintenance without a full shutdown is expected.

A simple way to read the chart: if the load is 250 A on a 400 V switchboard with a 36 kA fault level, an MCCB answers the question. If it is a 2500 A incoming section that must be racked out for maintenance, the answer is an ACB.

When Accessories Change the Type You Order

Two breakers with the same frame and the same rating can carry different product codes because of what is fitted inside and around them. A shunt trip opens the breaker on an external signal, which is how fire alarm interfaces and emergency stop circuits work. An auxiliary contact reports open or closed status back to a control system. An alarm contact indicates that the breaker tripped on a fault rather than being switched off by hand. An undervoltage release drops the breaker out when supply voltage collapses. Motor operators, plug-in devices and withdrawable units handle the mechanical side of the same problem.

Specify these at the ordering stage. Retro-fitting a trip unit or a contact block into an energised switchboard is slower and riskier than choosing the right configuration up front. If you are unsure which accessory belongs in which control scheme, our complete guide to MCCB accessories explains each one against the circuit it serves.

MCCB Shunt Trip Unit ManufacturerA shunt trip unit specifically designed for (1600A-6300A) low-voltage frame circuit breakers, used to provide the circuit breaker with a fast tripping function.View Product →

Inside the Case: What Produces the kA Rating

The number that matters most for safety, the rated short-circuit breaking capacity, is not a property of the plastic case. It comes from the contact and arc-quenching system inside: the moving and stationary contact assemblies, the arc chute that splits and cools the arc, and the insulated base frame that holds everything in alignment. A breaker rated 50 kA and one rated 25 kA can share the same external dimensions while using completely different internal parts.

Contact assemblies are consumables in a practical sense. Every interruption erodes a little contact material, and after heavy fault clearing the contact resistance and contact force are no longer what they were at the factory. Breaker manufacturers buy these parts in matched sets, and the same principle applies to repair work: moving contact, stationary contact and arc chute should be replaced together rather than one at a time.

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

Above 1600 A: The Current Path Matters as Much as the Breaker

On large frames the breaker is only part of the current path. Busbars, terminal blocks and plug-in contacts carry the same current and are subject to the same temperature-rise limits. A joint bolted to the wrong torque, or a copper bar with the wrong plating, becomes the hottest point in the switchboard long before the breaker itself complains. Matching the copper parts to the frame's continuous and short-time ratings is therefore part of breaker selection, not an afterthought. This is the reason ACB copper busbars, moving contact systems and array contactors are specified alongside the breaker rather than sourced separately afterwards.

Copper Bars and Copper BarsStamped Copper Busbar P09130-004-001View Product →

Frequently Asked Questions

From Selection to Sourcing

Working out what type of circuit breaker you need is mostly a matter of discipline: current first, then voltage and poles, then trip curve, then breaking capacity, then the accessories that make the circuit do what the process requires. When the design is settled, the parts behind the rating matter just as much, from auxiliary and alarm contacts through shunt and undervoltage releases to complete moving contact assemblies, arc chutes and copper conductors for frames up to 6300 A.

If you are specifying components rather than complete breakers, contact our engineering team with your frame size, current rating and drawings, and we will confirm what fits.