When purchasing an MCCB, standard sizes offer a range of ready options. Below, we explain how to size the device. Let’s first define MCCB size before delving into their standard choices. We will also match typical sizes with suitable applications.
What is MCCB Size?
MCCB size refers to its rated capacity for voltage, current, and short circuit. Size can also refer to the product’s dimensions, usually expressed in mm. Each size represents an application limit.
As you can see, the product is available in many sizes, from small for lighter circuits to large for heavy-duty use. Electricians choose the device to use based on these specifications.
Why MCCB Size Matters
Size is critical when choosing molded case circuit breakers. By selecting a suitable size (rating or physical), you ensure the following: electrical system safety, efficiency, and reliable operation.
In terms of physical size, you must select a type that fits the available installation space. The size is usually small for lower ratings and large for heavier currents and higher voltages.
MCCB Standard Sizes
MCCB standard sizes range from small choices for low amp circuits to large options for heavy currents above 1000A. They indicate the device’s capacity in amperes and include the following, grouped by size:
- Low: 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A, 160A, 200A, 250A
- Medium: 320A, 400A, 500A, 630A
- High: 800A, 1000A, 125A, 1600A+
The above ratings are the ones you will usually choose when selecting molded case circuit breakers. They represent one of the many MCCB specifications. The following section explains details to consider when sizing the device.
Key MCCB Sizing Parameters
MCCB sizing is based on its physical measurements, but more importantly, on its rated voltage, current, and short-circuit current capacity. Here is more about the parameters and what they mean when selecting the device.
Frame Size
Standard MCCB frame sizes, or physical dimensions, vary widely. While there’s no sizing convention, we can primarily divide the frames into small, medium, and large.
Small sizes are MCCBs with ratings between 16A and 20A. They have smaller widths below 140 mm. Medium sizes have ratings ranging from 250A to 630A and widths below 240 mm.
Large MCCB sizes are heavy-duty units rated above 630A. Their widths may reach 300 mm, depending on the specific brand.
Device Ratings
The size of MCCB breakers defines their application limits, providing essential information. They include system voltage, current, and anticipated fault or short circuit current.
The breaker will also have a temperature limit. This rating provides a sizing parameter based on the usage environment and installation requirements.
Most often, manufacturers use 40 degrees Celsius as the standard against which to rate the device for current. Derating factors apply for temperatures outside of this limit.
415V, 36kA, and 100A MCCB
How to Choose MCCB Size
You need these system details to choose a suitable device from the many standard MCCB sizes: voltage, load current, and the protective short-circuit interruption capacity. Let’s examine each requirement in more detail.
System Voltage
This rating indicates the breaker’s operating voltage. A 240V MCCB is designed for a 240V electrical system, while a 400V type protects circuits in a 400V installation.
You choose the rating based on your system’s voltage. The rating must be equal to or higher than, and never lower than. A value below the system voltage would make the protected circuits unsafe.
Load Current
Rated current is the maximum amps the breaker can continuously carry without tripping. For instance, a 32A MCCB means a breaker that can safely endure 32 amps of continuous current.
The value is usually specified against an ambient temperature. In most cases, the temperature is 40°C, which represents the operating range of most applications.
Short Circuit Interrupt Needs
Short-circuit rating indicates the maximum fault current the breaker can withstand before disconnecting circuits. It makes the breaker sensitive to a defined fault range or limit.
MCCB manufacturers usually specify short-circuit ratings in kiloamperes (kA). Options range from values as low as 25kA to as high as 100kA.
Protection devices in this range can withstand very high short momentary currents before tripping. This capacity is essential to prevent nuisance trips and unnecessary power disruptions.
Installation Space
It’s usually essential to select MCCB frame sizes that match the available space. That’s because units with similar voltage, current, and breaking capacity ratings differ in their physical measurements.
MCCB installation
Examples of MCCB Sizes and Applications
Different breaker sizes suit varying application requirements. Below is a breakdown of the available choices matched with their ratings and suitable usage situations.
Small Size
Small sizes are suitable for the distribution circuits of buildings, including homes. Other users include motor protection and industrial applications, such as protecting electrical systems in manufacturing.
Small MCCBs are usually rated below 250A for load current and below 50kA for fault current. Their physical dimensions typically range from 70 to 140 mm.
Medium Size
Medium sizes protect feeder and sub-mains circuits in commercial buildings and light industries. Their ratings range from 250A to 630A for load current and from 35 kA to 85 kA for fault current interruption.
The breakers in this range measure between 140 mm and 240 mm. It depends on the number of poles and other construction factors.
Large Size
These are heavy-duty molded case circuit breakers for higher voltages and currents. They typically protect incomers and switchgear installations. Their application is mainly industrial, with ratings ranging from 630A to 1600A.
The fault current protection of large sizes is usually in the range of 65 kA to 150 kA. They are also physically massive, with widths of up to 300 mm.
Conclusion
MCCBs protect entire circuits, making their proper selection crucial. Their availability in many standard sizes makes choosing a suitable breaker less challenging. Whether looking for a small breaker for a residential building or an industrial system, ensure you pick the correct size or rating.
