Proper sizing of grounding electrode conductors (GEC) is crucial for ensuring safe and compliant electrical installations. These questions frequently appear on electrical licensing exams because they involve critical calculations, especially when dealing with parallel conductors and different conductor materials. Understanding how to apply Table 250.66 and calculate equivalent conductor sizes is essential for passing your exam.

Example Grounding Electrode Conductor Sizing Questions on NEC Electrical Exams

  1. A data center upgrade involves replacing the main copper feeder with four parallel sets of 250 kcmil copper conductors per phase. The minimum size copper grounding electrode conductor for this installation should be ________.
  2. A commercial building has a service entrance with a largest ungrounded copper conductor of 350 kcmil. The minimum size aluminum bonding jumper for connecting the grounding electrodes in this installation must be ________.
  3. A three-phase industrial service uses four parallel sets of conductors per phase. Each set consists of 500 kcmil copper conductors. The minimum size copper grounding electrode conductor for this service must be ________.

How to Identify a Grounding Electrode Conductor Sizing Question on NEC Electrical Exams

Key phrases to look out for:

  • "Grounding electrode conductor"
  • "GEC size"
  • "Bonding jumper size"
  • "Parallel conductors"
  • "Service entrance grounding"
  • "Main bonding jumper"

When you spot these elements:

  1. Check if parallel conductors are involved
  2. Note the conductor material (copper or aluminum)
  3. Identify if it's for GEC or bonding jumper
  4. Refer to NEC Article 250.66
  5. Grounding electrodes / bonding jumpers are not the same as equipment grounding conductors. Be clear when identifying these questions.

Grounding Electrode Conductor Articles: NEC 250.66

To correctly apply NEC Article 250.66, focus on these main elements:

  1. Basic Sizing Rules:
    • Use Table 250.66 based on largest ungrounded conductor
    • Different columns for copper and aluminum/copper-clad aluminum
  2. Parallel Conductor Calculations:
    • Add circular mil areas of all parallel conductors
    • Use total area to determine GEC size
  3. Maximum Size Requirements:
    • 3/0 AWG copper for copper GEC
    • 250 kcmil aluminum for aluminum GEC
  4. Special Conditions:
    • Concrete-encased electrodes
    • Ground rings
    • Rod, pipe, and plate electrodes

Walkthrough for NEC Electrical Exam GEC Questions

Parallel Conductor Question

Question: A data center upgrade involves replacing the main copper feeder with four parallel sets of 250 kcmil copper conductors per phase. The minimum size copper grounding electrode conductor for this installation should be ________.

Step 1: Calculate Total Circular Mil Area

  • Individual conductor size: 250 kcmil
  • Number of parallel sets: 4
  • Total circular mil area = 4 × 250 kcmil = 1000 kcmil

Step 2: Apply Table 250.66

  • Find the row for 1000 kcmil (between 600-1100 kcmil)
  • Look in copper column
  • For this range, minimum GEC size is 2/0 AWG copper

Step 3: Verify Requirements

  • Meets minimum size requirements
  • Complies with parallel conductor rules
  • Doesn't exceed maximum size limitations

The minimum size copper grounding electrode conductor required is 2/0 AWG.

Service Entrance Question

Question: A commercial building has a service entrance with a largest ungrounded copper conductor of 350 kcmil. The minimum size aluminum bonding jumper for connecting the grounding electrodes in this installation must be ________.

Step 1: Identify Key Information

  • Largest ungrounded conductor: 350 kcmil copper
  • Need aluminum bonding jumper size
  • Standard service entrance installation

Step 2: Use Table 250.66

  • Find row for 350 kcmil copper
  • Look in aluminum/copper-clad aluminum column
  • Note: For copper conductors over 3/0 AWG through 350 kcmil
  • Minimum aluminum bonding jumper size is 1/0 AWG

Step 3: Verify Code Compliance

  • Meets minimum size requirements
  • Appropriate for service entrance application
  • Correct material conversion from copper to aluminum

The minimum size aluminum bonding jumper required is 1/0 AWG.

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