Grounding electrode conductor (GEC) sizing questions are high-frequency on Journeyman exams because they test whether you can correctly use NEC Table 250.66—especially when the problem includes parallel service conductors, copper vs aluminum, and “bonding jumper vs GEC” wording.
This guide is exam-focused: short, direct, and step-by-step.
What a GEC sizing question is testing
On most exams, they want to see if you can:
- Identify the largest ungrounded service-entrance conductor (or equivalent area if paralleled)
- Apply NEC Table 250.66 correctly
- Know the common traps:
- Parallel conductor rule (Note 1 / equivalent area)
- GEC vs EGC vs bonding jumper (they are not the same)
- Maximum GEC sizes for some electrode types (often misunderstood)
How to identify a GEC sizing question (keywords)
If you see these phrases, it’s almost always Table 250.66:
- “grounding electrode conductor” / “GEC”
- “size the GEC” / “minimum size GEC”
- “bonding jumper for the grounding electrode system”
- “main bonding jumper” (MBJ) or “system bonding jumper”
- “parallel service conductors”
- “largest ungrounded conductor”
Exam tip: If the question says “grounding electrode conductor,” don’t size an equipment grounding conductor (EGC). Different table.
The fast exam method (works for most questions)
Step 1 — Confirm WHAT you’re sizing
- GEC → Table 250.66
- Main bonding jumper / system bonding jumper → often Table 250.102(C)(1) (many exam writers mix terms)
- Equipment grounding conductor (EGC) → Table 250.122
If the question explicitly says GEC, stay with 250.66.
Step 2 — Identify the “largest ungrounded conductor”
Look for:
- “largest ungrounded service-entrance conductor”
- “largest ungrounded feeder conductor (at service)”
- “service conductors are ____ kcmil copper”
That value drives Table 250.66 sizing.
Step 3 — If conductors are in parallel: use total equivalent area
Most exam questions do this:
Total equivalent kcmil = (number of parallel sets) × (kcmil per set)
Example: 4 sets of 250 kcmil → 1000 kcmil total.
Then use the row range in Table 250.66 for that total.
Step 4 — Use Table 250.66 to pick the minimum GEC size
- Match your conductor size (or total equivalent size) to the correct table row.
- Choose the copper or aluminum/copper-clad aluminum GEC size per the table.
Common exam rules you should remember
Parallel conductors
- For parallel sets, you use the sum of areas to find the correct Table 250.66 row (exam-style approach).
“Maximum size” confusion
Many electricians memorize “3/0 Cu max” or “250 kcmil Al max,” but that limitation applies in specific electrode situations (exam questions may or may not include those electrode types).
So your safest exam process is:
- Size from Table 250.66
- Then check if the question mentions a specific electrode type (rod, pipe, plate, ground ring, etc.) that might cap the requirement.
Example 1 (Parallel copper service conductors)
Question: Four parallel sets of 250 kcmil copper conductors per phase are installed. What is the minimum size copper GEC?
Step 1: Total equivalent area
= 4 × 250 kcmil = 1000 kcmil
Step 2: Use Table 250.66
Find the row range that includes 1000 kcmil and read the copper GEC size.
✅ Answer (exam method): The copper GEC is the size listed in Table 250.66 for the row covering ~1000 kcmil total.
(You would pick the exact AWG size based on that table row in your codebook.)
Example 2 (Standard copper service, single run)
Question: Service has largest ungrounded conductor = 350 kcmil copper. What is the minimum copper bonding jumper for the grounding electrode system?
Step 1: Identify “largest ungrounded conductor” = 350 kcmil Cu
Step 2: Use Table 250.66 row for over 3/0 through 350 kcmil (copper column).
✅ Answer: The copper bonding jumper size is the value in Table 250.66 for that row.
Example 3 (Parallel large conductors)
Question: A 3-phase service uses four parallel sets per phase, each set is 500 kcmil copper. Minimum copper GEC?
Step 1: Total equivalent area
= 4 × 500 = 2000 kcmil
Step 2: Use Table 250.66 copper column
Find the row that contains 2000 kcmil.
✅ Answer: The copper GEC is the size listed in Table 250.66 for the row covering 2000 kcmil total.
Quick “Exam Checklist”
Before answering:
- ✅ Is it GEC (250.66), not EGC (250.122)?
- ✅ Are conductors copper or aluminum?
- ✅ Any parallel sets? If yes, sum kcmil.
- ✅ Use correct row range in Table 250.66
- ✅ Check if the problem mentions a specific electrode type that changes/caps sizing.
Common mistakes that cost points
- Using Table 250.122 (EGC) instead of 250.66 (GEC)
- Forgetting to sum parallel conductors
- Sizing off the neutral instead of the largest ungrounded conductor
- Mixing up main bonding jumper vs grounding electrode conductor
- Ignoring electrode-type exceptions when the question explicitly includes them
