Find how to size nema molded case circuit breaker

A very specific question!

NEMA (National Electrical Manufacturers Association) molded case circuit breakers (MCCBs) are designed to provide overcurrent protection in electrical distribution systems. To size a NEMA MCCB, you need to consider several factors, including the maximum current rating, voltage rating, and fault current rating of the circuit. Here's a step-by-step guide to help you size a NEMA MCCB:

  1. Determine the maximum current rating: Calculate the maximum current that the circuit is expected to carry. This is usually specified by the electrical load or the National Electric Code (NEC).
  2. Choose the voltage rating: Select the voltage rating of the MCCB based on the voltage of the circuit. Common voltage ratings for MCCBs are 120/240V, 208V, 277V, and 480V.
  3. Determine the fault current rating: Calculate the maximum fault current that the circuit is expected to experience. This is usually specified by the electrical load or the NEC.
  4. Select the MCCB type: NEMA MCCBs come in different types, including:
    • Frame sizes: 1, 2, 3, 4, 5, and 6 (e.g., 1-pole, 2-pole, 3-pole, etc.)
    • Trip units: thermal-magnetic, magnetic-only, or thermal-only
    • Interrupting ratings: 15 kA, 20 kA, 30 kA, or 40 kA
  5. Consult the NEMA standards: Refer to the NEMA standards for molded case circuit breakers, such as NEMA AB 3-2018 or NEMA AB 4-2018. These standards provide guidance on the selection and application of MCCBs.
  6. Use a sizing calculator or consult with a professional: You can use online calculators or consult with a licensed electrician or electrical engineer to ensure that you select the correct MCCB size.

Here's a simple example to illustrate the process:

Suppose you need to size a NEMA MCCB for a 120/240V, 3-phase circuit with a maximum current rating of 100A and a fault current rating of 20 kA.

  1. Choose a frame size: Based on the maximum current rating, you might select a frame size 3 or 4.
  2. Select a trip unit: A thermal-magnetic trip unit would be suitable for this application.
  3. Determine the interrupting rating: A 20 kA interrupting rating would be sufficient for this fault current rating.
  4. Consult the NEMA standards: Refer to NEMA AB 3-2018 or NEMA AB 4-2018 to ensure that the selected MCCB meets the required standards.
  5. Use a sizing calculator or consult with a professional: Verify that the selected MCCB size meets the requirements using an online calculator or consulting with a licensed electrician or electrical engineer.

Remember to always follow the relevant electrical codes and standards, such as the NEC, when selecting and installing a NEMA MCCB.