These NEC grounding and bonding practice questions are drawn from Article 250, and each one names the section it rests on. Grounding and bonding is the single biggest section on most journeyman and master exams, and it is where I see the most people lose points — not because the code is hard, but because the words get mixed up. Grounded, grounding, bonded, bonding, equipment grounding conductor, grounding electrode conductor: four different things, and the exam will hand you all four in the same question to see whether you know which is which.
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Written by Kevin, a licensed
Michigan master electrician — 26 years in the trade and 17 of them as an electrical
inspector. Every answer below names the NEC section it comes from, so you learn where to
look it up rather than memorising it.
Exam tipIf a question mentions a rod, a pipe, or the building steel, you are in Part III of Article 250 — electrodes. If it mentions a raceway, a box, or the metal case of a piece of equipment, you are in Part VI — equipment grounding. Sorting that out first turns most of these into one table lookup.
Tap an answer to see whether you got it right, the code section it comes
from, and the reasoning worked out in full. Nothing to sign up for.
Question 1Grounding & Bonding
What is the minimum size copper bonding jumper for a service with 300 kcmil ungrounded conductors?
Correct answer: A. #2
NEC 250.66
Rule — supply-side bonding jumper (250.102(C)(1))
A supply-side bonding jumper is sized like a GEC — from Table 250.66, keyed to the largest ungrounded service conductor (250.102(C)(1)).
Step 1 — largest ungrounded service conductor
300 kcmil copper
Step 2 — Table 250.66, copper column
over 3/0 through 350 kcmil → 2 AWG
#2 copper
How to do it
Supply-side bonding jumpers and the GEC both come out of Table 250.66, sized by the service-conductor size (250.102(C)(1)). Find the ungrounded-conductor row, read the copper column.
Common mistakes
Using Table 250.122 — that is for load-side equipment grounds.
Sizing from the overcurrent-device rating instead of the conductor size.
Confusing the supply-side jumper (250.102(C), Table 250.66) with a load-side jumper (250.102(D), Table 250.122).
Question 2Grounding & Bonding
What is the minimum size copper grounding electrode conductor to be run to the concrete-encased electrode on a 200A service?
Correct answer: A. 4 awg
NEC 250.66(B)
Rule — GEC to a concrete-encased electrode (250.66(B))
Size the GEC from Table 250.66 as usual — but 250.66(B) caps the conductor that runs only to a concrete-encased electrode at 4 AWG copper, no matter how big the service is.
Step 1 — Table 250.66 for the service
a 200 A service would point to 4 AWG or larger
Step 2 — apply the 250.66(B) cap
to a concrete-encased electrode: need not exceed 4 AWG copper
4 AWG copper
How to do it
A GEC run only to a concrete-encased (Ufer) electrode never has to be larger than #4 copper (250.66(B)) — the sole-connection caps in 250.66 override the table. The rod cap is #6 (250.66(A)); the ground-ring cap is #2 (250.66(C)).
Common mistakes
Sizing the full Table 250.66 value and ignoring the #4 concrete-encased cap.
Mixing up the caps — rod is #6, concrete-encased is #4, ring is #2.
Applying the cap when the same GEC also serves a non-capped electrode (then it must be full size to that one).
Question 3Grounding & Bonding
What is the minimum size copper equipment grounding conductor for a circuit protected by a 60-ampere overcurrent device?
The EGC is sized from the branch-circuit overcurrent device.
Step 1 — overcurrent device
60 A
Step 2 — Table 250.122, copper column
60 A → 10 AWG
10 AWG copper
How to do it
The equipment grounding conductor is sized by the rating of the overcurrent device protecting the circuit — not the load, not the phase-conductor size. Go to Table 250.122, find the row for your breaker/fuse rating (if your exact size is not listed, use the next row up), and read the copper column.
Common mistakes
Sizing the EGC from the phase-conductor size instead of the overcurrent device.
Reaching for Table 250.66 — that table is for grounding-electrode conductors, not equipment grounds.
Forgetting to upsize the EGC when the phase conductors are upsized for voltage drop (250.122(B)).
Question 4Grounding & Bonding
What is the minimum size copper equipment grounding conductor for a 100-ampere feeder?
The EGC is sized from the feeder overcurrent device.
Step 1 — overcurrent device
100 A feeder
Step 2 — Table 250.122, copper column
100 A → 8 AWG
8 AWG copper
How to do it
The equipment grounding conductor is sized by the rating of the overcurrent device protecting the circuit — not the load, not the phase-conductor size. Go to Table 250.122, find the row for your breaker/fuse rating (if your exact size is not listed, use the next row up), and read the copper column.
Common mistakes
Sizing the EGC from the phase-conductor size instead of the overcurrent device.
Reaching for Table 250.66 — that table is for grounding-electrode conductors, not equipment grounds.
Forgetting to upsize the EGC when the phase conductors are upsized for voltage drop (250.122(B)).
Question 5Grounding & Bonding
What is the minimum size copper equipment grounding conductor for a 200-ampere feeder?
The EGC is sized from the feeder overcurrent device.
Step 1 — overcurrent device
200 A feeder
Step 2 — Table 250.122, copper column
200 A → 6 AWG
6 AWG copper
How to do it
The equipment grounding conductor is sized by the rating of the overcurrent device protecting the circuit — not the load, not the phase-conductor size. Go to Table 250.122, find the row for your breaker/fuse rating (if your exact size is not listed, use the next row up), and read the copper column.
Common mistakes
Sizing the EGC from the phase-conductor size instead of the overcurrent device.
Reaching for Table 250.66 — that table is for grounding-electrode conductors, not equipment grounds.
Forgetting to upsize the EGC when the phase conductors are upsized for voltage drop (250.122(B)).
Question 6Grounding & Bonding
A service has 2/0 AWG copper service-entrance conductors. What is the minimum size copper grounding electrode conductor (GEC)?
The GEC is sized from the largest ungrounded service conductor.
Step 1 — largest ungrounded service conductor
2/0 AWG copper
Step 2 — Table 250.66, copper column
2/0 through 3/0 copper → 4 AWG
4 AWG copper
How to do it
The grounding electrode conductor is sized by the largest ungrounded service (or derived) conductor, in Table 250.66 — not by the overcurrent device. Find that conductor's size, read across to the copper GEC column, then check the caps in 250.66(A)/(B)/(C).
Common mistakes
Using the overcurrent-device rating — that is the EGC table (250.122).
Ignoring the caps: a GEC to a ground rod need not exceed #6, to a concrete-encased electrode #4, to a ground ring #2.
Reading the wrong service-conductor-size row.
Question 7Grounding & Bonding
A service has 250 kcmil copper service-entrance conductors. What is the minimum size copper grounding electrode conductor?
The GEC is sized from the largest ungrounded service conductor.
Step 1 — largest ungrounded service conductor
250 kcmil copper
Step 2 — Table 250.66, copper column
over 3/0 through 350 kcmil → 2 AWG
2 AWG copper
How to do it
The grounding electrode conductor is sized by the largest ungrounded service (or derived) conductor, in Table 250.66 — not by the overcurrent device. Find that conductor's size, read across to the copper GEC column, then check the caps in 250.66(A)/(B)/(C).
Common mistakes
Using the overcurrent-device rating — that is the EGC table (250.122).
Ignoring the caps: a GEC to a ground rod need not exceed #6, to a concrete-encased electrode #4, to a ground ring #2.
Reading the wrong service-conductor-size row.
Question 8Grounding & Bonding
A bare aluminum grounding conductor must be how many inches above ground?
Correct answer: B. 18”
NEC 250.64(A)(3)
18” — NEC 250.64(A)(3)
Question 9Grounding & Bonding
An AC system of 50 to 1000V shall be grounded where the voltage to ground does not exceed ____.
Correct answer: B. 150V
NEC 250.20(B)
150V — NEC 250.20(B)
Question 10Grounding & Bonding
High-impedance grounded systems are limited to systems operating at _____.
Correct answer: C. 480
NEC 250.36
480 — NEC 250.36
These are 10 of 803 questions
The full course carries 654 code and calculation questions plus 149 photo questions taken on real job sites — my own photographs from 17 years of electrical inspections. Every one of them is worked out like the ones above.
NEC 2023 and NEC 2026 both included, switch between them any time
Every question names the code section, so you learn to find it in the book
The whole course reads itself out loud, with the words highlighted as they are spoken
Timed 80-question practice test built like the real exam
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