Engineering Tutorials

Generally, molded case circuit breakers can be mounted in any position. Mounting them up, down, horizontal or vertical does not affect the tripping or interrupting characteristics of the breaker. However, mounting them in a vertical position with the ON position as anything other than up, is in violation of National Electric Code.
In some cases because of the physical arrangement of a panelboard or switchboard, it is necessary to reverse feed a circuit breaker. The circuit breaker must be tested and listed accordingly for this type of application. Only breakers that have fixed trips can be used, and they usually have sealed covers. They often do not have "Line" and "Load" marked on the cover, so the power source can be connected to either the line or the load terminations.
In addition to being mounted in motor control centers, switchboards and panelboards, molded case circuit breakers are mounted individually in separate enclosures. The National Electric Code and local electric codes determine the proper selection of an enclosure type for a particular application. The National Electrical Manufacturers Association (NEMA) and the International Electro-technical Commission (IEC) have set standards for the protection of devices in various environmental situations. Enclosure types are rated to withstand water, dust, oil, and other environmental conditions. NEMA assigns Type classifications to enclosures. When an enclosure is rated a particular type, it means it is made of the specified materials and has passed specific tests. IEC also has tests and standards that enclosures must conform to. They assign an IP classification.

Mounting Circuit Breakers

The most common types of enclosures are:
NEMA Type 1 (Conforms to IP40) - These enclosures are designed for indoor applications. They are suitable for installations where unusual conditions do not exist, but where a measure of protection from accidental contact is required. They are commonly used in commercial buildings and apartment buildings. They are often referred to as general purpose enclosures.
NEMA Type 3R (Conforms to IP52) - These enclosures are designed for outdoor use where falling rain, sleet or external ice might form. They have a gasket on the cover to keep out water. Some versions have a top hinged front cover which must be opened to gain access to the circuit breaker handle. Other versions have an external side operated handle mechanism. These enclosures are often referred to as raintight enclosures.
NEMA Type 4 (Conforms to IP65) - These enclosures are designed for use either indoors or outdoors. They provide protection from splashing water, wind blown dust or rain. They even protect the circuit breaker from hose directed water. They are well suited for application in dairies, breweries, paper mills, food processing plants and other process industries. These enclosures are often referred to as watertight enclosures.
NEMA Type 4X (Conforms to IP65) - These are much the same as the Type 4 except that they are made of gasketed, stainless steel. In some designs, they are made of a nonmetallic material. They provide better resistance to corrosion than the Type 4. Industries that have a high amount of corrosive liquids, require a high measure of hose-down cleaning, or are in a salt-water environment use these enclosures. They are often referred to as corrosion-proof enclosures.
NEMA Type 12 (Conforms to IP62) - These enclosures are designed for indoor use in dirty and dusty applications. They are constructed of sheet metal and provide protection from dripping liquids (non-corrosive), falling dirt and dust. A special NEMA 12K version provides Knockouts for conduit. These enclosures are often referred to as dust-tight enclosures.
There are other ratings of enclosure types, but these are the most commonly used.

The selection of molded case circuit breakers is generally determined in two ways:
• For protection of non-motor circuits.
• For protection of motor circuits.
We will discuss protection of non-motor circuits first.
Protecting Non-Motor Circuits
Non-motor circuit applications usually center around cable protection and require molded case circuit breakers with both overload and short circuit capabilities. They should be able to distinguish between harmless and destructive conditions, and function appropriately for its application. It is very important that the MCCB selected be adequately rated and equipped for all the electrical and physical conditions that are likely to exist when the system is energized.
The standard selection factors for molded case circuit breakers include:
• Voltage Rating
• Frequency
• Continuous Current Rating
• Interrupting Rating
• Number of Poles
• Fixed or Interchangeable Trip Unit
• Trip Unit Functions
• Accessories
Protecting Motor Circuits
The selection of an HMCP is based on the full load current of the motor it is to protect. Data shown in the National Electric Code (tables 430-148 and 430-150) list the full load currents of induction motors running at speeds normal for belted motors and with normal torque characteristics. However, actual motor nameplate ratings should be used for selecting the motor running overload protection.
Other considerations in the selection of HMCPs include:
• The ambient temperature outside the enclosure should not exceed 40°C (104°F).
• Infrequent starting, stopping and reversing of the motor.
• Motor accelerating time of 10 seconds or less.
• Locked rotor rating is a maximum of six times the motor FLA rating.