Guide to Sizing Overcurrent Protection for Branch-Circuits and Feeders

Overcurrent protection for branch-circuits and feeders is a critical component of electrical systems that protects conductors and equipment from harmful overcurrents. These questions frequently appear on electrical licensing exams because proper sizing of overcurrent for branch-circuits and feeders is essential for safety and code compliance. Understanding how to size these components is crucial for passing your exam. You can also use our circuit calculator to instantly determine breaker size, conductor size, and other branch circuit and feeder requirements. Because sizing overcurrent protection for both branch-circuits and feeders is very similar, we are going to look at these components together.

‍

Example Branch-Circuit and Feeder Overcurrent Sizing Questions on NEC Electrical Exams

‍

What is the minimum size circuit breaker permitted to protect a branch-circuit that supplies 12 amps of continuous load and 5 amps of non-continuous load?

What is the minimum size circuit breaker required to protect a feeder circuit that supplies 150 amps of continuous load and 45 amps of non-continuous load?

What are the minimum size fuses permitted to protect the feeder conductors to a panelboard that supplies 100 amps of continuous load and 225 amps of non-continuous load?

‍

How to Identify a Branch-Circuit or Feeder Overcurrent Sizing Question on NEC Electrical Exams

‍

Key phrases to look out for:

• “Minimum size breaker”
• "Minimum size fuse(s)"
• "Branch-Circuit” or “feeder”
• References to Section 210.20
• References to Section 215.3

When you spot these elements:

1. Determine all of the parameters involved
2. Determine whether it is a branch-circuit or feeder calculation.
3. Determine the amount of continuous and/or non-continuous load.
4. Use the requirements found in 210.20 for branch-circuits
5. Use the requirements found in 215.3 for feeders

‍

Branch-Circuit and Feeder Overcurrent Protection Articles: NEC 210.20 and 215.3

‍

For Branch-Circuits:

1. 210.20(A) covers standard sizing where continuous and non-continuous loads are known:
   
   • Determine continuous and non-continuous loads
   • Overcurrent protective device (OCPD)  must not be less than (greater-than or equal-to) the sum of the non-continuous load plus (+) 125% of the continuous load
   • OCPD ≥ NCL + (125% x CL)
2. Using 210.20(B)-(D) covers branch-circuit overcurrent protection for specific conditions:
   
   • Section 210.20(B) requires overcurrent protection to comply with 240.4 and 240.5 for protection of conductors, flexible cords and fixture wires
   • 210.20(C) references requirements for overcurrent protection for specific types of loads
   • 210.20(D) covers overcurrent protection for outlet device

For Feeders 

3. 215.3 covers standard sizing where continuous and non-continuous loads are known:
   
   • Determine continuous and non-continuous loads
   • Overcurrent protective device (OCPD)  must not be less than (greater-than or equal-to) the sum of the non-continuous load plus (+) 125% of the continuous load
   • OCPD ≥ NCL + (125% x CL)

‍

Walkthrough for NEC Electrical Exam Branch-Circuit Overcurrent Protection Questions

‍

Standard Sizing Question

What is the minimum size OCPD permitted to be used for a branch-circuit that supplies 12 amps of continuous load and 5 amps of non-continuous load?

‍

Step 1: Identify Key Information

• Continuous load = 12 amps
• Non-continuous load = 5 amps

‍

Step 2: Calculate Minimum OCPD Using Section 210.20(A) 

• OCPD ≥ NCL+ (125% x CL)
• OCPD ≥ 5A + (125% x 12A)
• OCPD ≥ 20A

‍

Step 3: Select OCPD Size Using Table 240.6(A)

• Table 240.6(A) shows 20 amps as a standard OCPD 

Minimum size branch-circuit OCPD = 20 amps

‍

Standard Feeder OCPD Sizing Question

Question: What is the minimum size circuit breaker required to protect a feeder circuit that supplies 150 amps of continuous load and 45 amps of non-continuous load?

‍

Step 1: Identify Key Information

• Continuous load = 150 amps
• Non-continuous load = 45 amps

‍

Step 2: Calculate Minimum OCPD Using Section 210.20(A) 

• OCPD ≥ NCL+ (125% x CL)
• OCPD ≥ 45A + (125% x 150A)
• OCPD ≥ 232.5A

‍

Step 3: Select OCPD Size Using Table 240.6(A)

• Table 240.6(A): Next higher standard OCPD rating = 250 amps 

Minimum size OCPD for feeders = 250 amp breaker

‍

‍

‍