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Grounding and Bonding (NEC 250) — NEC 2023 Edition

Free NEC 2023 practice test on grounding and bonding. Covers grounding purpose, grounded conductor identification, grounding electrode conductor sizing, electrode types, and bonding requirements. Simple English, full explanations.

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The National Electrical Code (NFPA 70) is the benchmark standard for safe electrical installation in the US. It is updated every 3 years by the NFPA.

Study the edition adopted by your state. As of 2025, most states use the 2020 or 2023 NEC. These tests are written to the 2023 edition.

The most tested articles are 100 (Definitions), 200–250 (Wiring & Grounding), 300–314 (Wiring Methods), 400–411 (Conductors), 430 (Motors), and 700–701 (Emergency Systems).

How to Use This Practice Test

This Grounding and Bonding (NEC 250) — NEC 2023 Edition has 30 questions with detailed answers and NEC references. Take the full test, review every wrong answer, and look up the NEC article it references before moving on.

Tip: Use the NEC index (back of the book) — find answers by keyword, not by article number.

Topics covered: NEC articles 100, 210, 220, 240, 250, 300, 310, 430, and special occupancies.
Passing score: Aim for 80%+ on NEC code tests before exam day.
Questions: 30 multiple-choice, one correct answer each

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Student Reviews 4.0 / 5 · 1 review

Rhonda F.

Grounding and bonding is honestly the hardest section of the NEC for most people and this quiz does a good job covering it. I had to look up a few things in the code book after but that's kind of the point. Good questions.

Grounding and Bonding (NEC 250) — NEC 2023 Edition — Question List

Question 1: Why are electrical systems connected to earth (grounded), according to 250.4(A)(1)?

To limit voltage from lightning, line surges or contact with higher-voltage lines
To remove the need for overcurrent devices
To carry the normal load current back to the source
To raise the voltage to ground

NEC Reference: NEC 250.4(A)(1)

Question 2: The grounding electrode conductor runs only to ground rods, and the service conductors are 3/0 copper. What is the largest GEC size the NEC requires for that connection?

2 AWG copper
4 AWG copper
6 AWG copper
8 AWG copper

NEC Reference: NEC 250.66(A)

Question 3: The transformer supplying a building service is located outside the building. Where must an additional grounding connection of the grounded service conductor be made?

No additional connection is required
At every subpanel in the building
Only inside at the service panel
At the transformer or elsewhere outside the building

NEC Reference: NEC 250.24(A)(2)

Question 4: A 3-phase, 3-wire delta service has a grounded (corner-grounded) conductor. How must the grounded service conductor be sized?

12.5% of the ungrounded conductor area
Not smaller than the GEC from Table 250.66
One size smaller than the ungrounded conductors
Ampacity not less than that of the ungrounded conductors

NEC Reference: NEC 250.24

Question 5: In a single-raceway service, the grounded (neutral) service conductor must be at least what size?

The size required by Table 250.102(C)(1)
The EGC size from Table 250.122
The size of the largest ungrounded conductor
Half the size of the ungrounded conductors

NEC Reference: NEC 250.24

Question 6: In a single-phase, 3-wire electrical system, what is the middle conductor generally required to be?

Grounded
Ungrounded
A hot conductor
Not connected at all

NEC Reference: NEC 250.26

Question 7: Grounded (neutral) conductors 6 AWG and smaller are most commonly identified by which color?

White or natural gray
Red
Green
Black

NEC Reference: NEC 200.6(A)

Question 8: When may an overcurrent device be connected in series with an intentionally grounded conductor?

Whenever a single-pole fuse is used
When it opens all conductors of the circuit at the same time
Whenever the circuit is 120 volts
Only in dwelling units

NEC Reference: NEC 240.22

Question 9: A single-phase service uses 1/0 AWG aluminum as its largest ungrounded conductor. What size aluminum grounding electrode conductor is required?

Aluminum 6 AWG
Aluminum 4 AWG
Copper 2 AWG
Copper 6 AWG

NEC Reference: NEC Table 250.66

Question 10: Lightning protection system ground terminals must generally be bonded to which grounding system?

The building or structure grounding electrode system
A distribution panelboard only
The service entrance only
Nothing -- bonding is not required

NEC Reference: NEC 250.106

Question 11: An insulated equipment grounding conductor 6 AWG or smaller is usually identified by what color?

Green
Orange
Brown
Pink

NEC Reference: NEC 250.119

Question 12: A concrete-encased grounding electrode (Ufer ground) generally must have at least what length of qualifying conductor or rebar?

20 feet
10 feet
30 feet
50 feet

NEC Reference: NEC 250.52(A)(3)

Question 13: At least how much of a rod or pipe electrode must be in contact with the soil?

6 ft
7 ft
8 ft
10 ft

NEC Reference: NEC 250.53(A)(4)

Question 14: Where two ground rods are used, they should be spaced at least how far apart?

6 feet apart
1 foot apart
50 feet apart
Touching each other

NEC Reference: NEC 250.53(A)(3)

Question 15: Metal water piping in a building served by a service must be bonded to which of the following?

The nearest receptacle EGC only
The gas piping only
The structural steel only
The service equipment enclosure, grounded conductor at the service, GEC or electrode

NEC Reference: NEC 250.104(A)(1)

Question 16: Which materials are permitted for a grounding electrode conductor?

Copper, aluminum or copper-clad aluminum
Copper only
Stainless steel only
Galvanized steel only

NEC Reference: NEC 250.62

Question 17: Where is the main bonding jumper installed to connect the equipment grounding conductors to the grounded conductor?

At each separate building feeder panel
At the service
At every subpanel
At the last panelboard on the feeder

NEC Reference: NEC 250.28

Question 18: A service uses 2 AWG copper as its largest ungrounded conductor. What size copper grounding electrode conductor is required?

8 AWG
10 AWG
6 AWG
4 AWG

NEC Reference: NEC Table 250.66

Question 19: A circuit is protected by a 60-ampere overcurrent device. What is the minimum size copper equipment grounding conductor required?

10 AWG
12 AWG
8 AWG
14 AWG

NEC Reference: NEC Table 250.122

Question 20: Which method is NOT permitted for connecting grounding and bonding conductors?

A listed clamp
A listed pressure connector
A connection that depends solely on solder
Exothermic welding

NEC Reference: NEC 250.8

Question 21: The bonding jumper size for the water piping system is based on what?

The largest ungrounded service conductor
The pipe color
The homeowner's preference
Nothing -- any size works

NEC Reference: NEC 250.104(A)(1) / Table 250.102(C)(1)

Question 22: Isolated ground receptacles are installed mainly for what purpose?

To provide GFCI protection
To reduce electrical noise on sensitive electronic equipment
To increase the available fault current
To eliminate the need for an EGC

NEC Reference: NEC 250.146(D)

Question 23: An intersystem bonding termination at the service is provided for bonding which systems?

Lightning protection systems only
Building structural steel
Metal gas piping
Communications systems such as telephone and CATV

NEC Reference: NEC 250.94

Question 24: A metal underground water pipe is used as a grounding electrode. What else does the NEC require?

It must be encased in concrete
It must be bonded to the gas piping
It must be supplemented by an additional electrode
It must be at least 30 ft long

NEC Reference: NEC 250.53(D)(2)

Question 25: Metal gas piping that is likely to become energized must be treated how?

Bonded to the equipment grounding conductor of the circuit likely to energize it
Connected to the grounded conductor at each appliance
Used as the grounding electrode
Isolated with dielectric unions only

NEC Reference: NEC 250.104(B)

Question 26: A portable generator supplies only cord-and-plug equipment through its own receptacles. What is it NOT required to have?

A connection to a grounding electrode
Overcurrent protection
Bonding of its frame to the receptacle grounding terminals
Equipment grounding terminals on its receptacles

NEC Reference: NEC 250.34(A)

Question 27: A grounding electrode conductor is exposed to physical damage. How must it be protected?

Stapled tight to the surface
Coated with insulating varnish
Wrapped with electrical tape
In RMC, IMC, Schedule 80 PVC, RTRC-XW, EMT or cable armor

NEC Reference: NEC 250.64(B)

Question 28: A separately derived system, such as the secondary of a step-down transformer, needs what bonding connection at the source or first disconnect?

A main bonding jumper at the service only
A system bonding jumper
An equipment bonding jumper on the load side only
No bonding connection

NEC Reference: NEC 250.30(A)(1)

Question 29: Where multiple grounding electrodes are present at a building (such as a water pipe, ground rod, and concrete-encased electrode), what does NEC 250.50 require?

All present qualifying electrodes must be bonded together
Only one electrode may ever be used
They must be kept completely separate
Nothing -- bonding is optional

NEC Reference: NEC 250.50

Question 30: Which of the following is permitted as an equipment grounding conductor under 250.118?

The jacket of a flexible cord
The outer sheath of NM cable
Rigid metal conduit
PVC conduit

NEC Reference: NEC 250.118

Key NEC References for This Test

Article	What It Covers
Ch. 1–4	General rules — apply to all installations
Ch. 5–7	Special rules — override Ch. 1–4 where they conflict
Ch. 8	Communications — completely independent of Ch. 1–7
Ch. 9	Tables — referenced by articles, not standalone enforceable rules
Art. 210.12	AFCI — major expansion in NEC 2026
Art. 230.67	Surge protection at service equipment — new NEC 2026

Study Tip: Learn the NEC index — use keywords, not article numbers. Practice finding 5 different topics in under 2 minutes each.

Grounding and Bonding (NEC 250) — NEC 2023 Edition

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Grounding and Bonding (NEC 250) — NEC 2023 Edition

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Grounding and Bonding (NEC 250) — NEC 2023 Edition — Question List

Question 1: Why are electrical systems connected to earth (grounded), according to 250.4(A)(1)?

Question 2: The grounding electrode conductor runs only to ground rods, and the service conductors are 3/0 copper. What is the largest GEC size the NEC requires for that connection?

Question 3: The transformer supplying a building service is located outside the building. Where must an additional grounding connection of the grounded service conductor be made?

Question 4: A 3-phase, 3-wire delta service has a grounded (corner-grounded) conductor. How must the grounded service conductor be sized?

Question 5: In a single-raceway service, the grounded (neutral) service conductor must be at least what size?

Question 6: In a single-phase, 3-wire electrical system, what is the middle conductor generally required to be?

Question 7: Grounded (neutral) conductors 6 AWG and smaller are most commonly identified by which color?

Question 8: When may an overcurrent device be connected in series with an intentionally grounded conductor?

Question 9: A single-phase service uses 1/0 AWG aluminum as its largest ungrounded conductor. What size aluminum grounding electrode conductor is required?

Question 10: Lightning protection system ground terminals must generally be bonded to which grounding system?

Question 11: An insulated equipment grounding conductor 6 AWG or smaller is usually identified by what color?

Question 12: A concrete-encased grounding electrode (Ufer ground) generally must have at least what length of qualifying conductor or rebar?

Question 13: At least how much of a rod or pipe electrode must be in contact with the soil?

Question 14: Where two ground rods are used, they should be spaced at least how far apart?

Question 15: Metal water piping in a building served by a service must be bonded to which of the following?

Question 16: Which materials are permitted for a grounding electrode conductor?

Question 17: Where is the main bonding jumper installed to connect the equipment grounding conductors to the grounded conductor?

Question 18: A service uses 2 AWG copper as its largest ungrounded conductor. What size copper grounding electrode conductor is required?

Question 19: A circuit is protected by a 60-ampere overcurrent device. What is the minimum size copper equipment grounding conductor required?

Question 20: Which method is NOT permitted for connecting grounding and bonding conductors?

Question 21: The bonding jumper size for the water piping system is based on what?

Question 22: Isolated ground receptacles are installed mainly for what purpose?

Question 23: An intersystem bonding termination at the service is provided for bonding which systems?

Question 24: A metal underground water pipe is used as a grounding electrode. What else does the NEC require?

Question 25: Metal gas piping that is likely to become energized must be treated how?

Question 26: A portable generator supplies only cord-and-plug equipment through its own receptacles. What is it NOT required to have?

Question 27: A grounding electrode conductor is exposed to physical damage. How must it be protected?

Question 28: A separately derived system, such as the secondary of a step-down transformer, needs what bonding connection at the source or first disconnect?

Question 29: Where multiple grounding electrodes are present at a building (such as a water pipe, ground rod, and concrete-encased electrode), what does NEC 250.50 require?

Question 30: Which of the following is permitted as an equipment grounding conductor under 250.118?

Key NEC References for This Test

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