Find how to size nemamolded case circuit breaker

A very specific question!

NEMA (National Electrical Manufacturers Association) molded case circuit breakers (MCCBs) are designed to protect electrical circuits from overcurrents and short circuits. To size a NEMA molded case circuit breaker, you need to consider several factors, including the maximum current rating of the circuit, the voltage rating, and the type of load being protected.

Here's a step-by-step guide to help you size a NEMA molded case circuit breaker:

  1. Determine the maximum current rating of the circuit: This is the maximum amount of current that the circuit is designed to carry. You can find this information on the circuit diagram or by consulting with the electrical engineer or designer who designed the circuit.
  2. Determine the voltage rating of the circuit: This is the maximum voltage that the circuit is designed to operate at. NEMA MCCBs are available in various voltage ratings, including 120/240V, 277V, and 480V.
  3. Determine the type of load being protected: NEMA MCCBs are designed to protect various types of loads, including resistive, inductive, and capacitive loads. You need to determine the type of load being protected to select the correct breaker.
  4. Consult the NEMA breaker catalog: NEMA publishes a catalog of molded case circuit breakers that lists the various ratings and characteristics of each breaker. You can consult the catalog to find a breaker that matches your circuit's requirements.
  5. Choose the correct breaker rating: Based on the maximum current rating of the circuit, voltage rating, and type of load being protected, choose a NEMA MCCB that meets the following criteria:
    • Current rating: The breaker's current rating should be equal to or greater than the maximum current rating of the circuit.
    • Voltage rating: The breaker's voltage rating should match the voltage rating of the circuit.
    • Type of load: The breaker should be designed to protect the type of load being protected (resistive, inductive, or capacitive).
  6. Consider additional factors: When selecting a NEMA MCCB, you should also consider additional factors such as:
    • Interrupting rating: The breaker's interrupting rating should be sufficient to handle the maximum fault current of the circuit.
    • Arc energy: The breaker's arc energy rating should be sufficient to handle the maximum arc energy of the circuit.
    • Operating temperature: The breaker should be designed to operate within the temperature range of the circuit.

Here's an example of how to size a NEMA molded case circuit breaker:

Suppose you have a 120V, 20A circuit that is designed to protect a resistive load. You need to select a NEMA MCCB that meets the following criteria:

Based on the NEMA breaker catalog, you find a breaker that meets the following specifications:

In this example, the NEMA MCCB would be suitable for the 120V, 20A circuit that is designed to protect a resistive load.

Remember to always consult with a licensed electrician or electrical engineer if you are unsure about sizing a NEMA molded case circuit breaker for a specific circuit.