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. Because sizing overcurrent protection for both branch-circuits and feeders is very similar, we are going to look at these components together.

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

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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?

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

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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

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

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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)

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Walkthrough for NEC Electrical Exam Branch-Circuit Overcurrent Protection Questions

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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?

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Step 1: Identify Key Information

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

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

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

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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

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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?

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Step 1: Identify Key Information

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

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

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

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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

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