NEC Article 250 Practice Questions: Grounding and Bonding

The two tables every examinee must own — 250.66 and 250.122 — plus electrodes, bonding jumpers, and separately derived systems. Grounding is the most feared article; make it a strength.

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49 Article 250 questions in the bankWritten in the PSI computer-based exam formatEvery answer cites the NEC section

What Article 250 Covers on the Exam

Article 250 is the longest article in the code and the one candidates fear most. The exam concentrates on a few high-value skills: sizing grounding electrode conductors from Table 250.66, sizing equipment grounding conductors from Table 250.122, identifying grounding electrodes (250.52), and bonding at services and separately derived systems. Master the two tables and the electrode list and Article 250 becomes free points.

Tested subtopics from Article 250

Grounding electrode conductor sizing — Table 250.66
Equipment grounding conductor sizing — Table 250.122
Grounding electrode system: which electrodes qualify — 250.52 and 250.53
Main bonding jumpers and system bonding jumpers — 250.24 and 250.30
Bonding of metal water piping and structural steel — 250.104

Drill a different article

49 questions in our bank cite Article 250. Practice the other high-yield articles too:

Art. 110 — General Requirements Art. 210 — Branch Circuits Art. 220 — Load Calculations Art. 230 — Services Art. 240 — Overcurrent Protection Art. 300 — Wiring Methods Art. 310 — Conductors & Ampacity Art. 314 — Boxes & Box Fill Art. 430 — Motors Art. 450 — Transformers Art. 680 — Swimming Pools & Spas

How to Study Article 250

Tab Table 250.66 and Table 250.122 and drill telling them apart: 250.66 keys off the service-entrance conductor size (for grounding electrode conductors), while 250.122 keys off the overcurrent device rating (for equipment grounding conductors). Half of all Article 250 exam mistakes are using the wrong table. Also memorize the 250.66 shortcuts: a GEC to a rod electrode never needs to be larger than 6 AWG copper, and to a concrete-encased electrode never larger than 4 AWG.

Then put it under exam conditions: the free diagnostic scores you across every category and shows whether grounding and bonding is actually a weak area — or whether your study time belongs somewhere else.

Sample Questions (From Our Real Question Bank)

Here are 4 real questions from our question bank. The free diagnostic quiz has 10 more.

NEC NEC Table 250.66Services & Separately Derived Systems

1. Per NEC 250.66, what is the minimum size copper grounding electrode conductor for a service with 2 AWG copper service-entrance conductors?

A10 AWG
B8 AWG
C6 AWG
D4 AWG

Show explanation

NEC Table 250.66 specifies minimum 8 AWG copper grounding electrode conductor for 2 AWG service-entrance conductors.

NEC NEC 250.52(A)(1)Services & Separately Derived Systems

2. Per NEC 250.52(A)(1), a metal underground water pipe in direct contact with the earth for how many feet or more shall be used as a grounding electrode?

A5 feet
B8 feet
C10 feet
D20 feet

Show explanation

NEC 250.52(A)(1) permits metal underground water pipe as a grounding electrode when 10 feet or more is in direct earth contact.

NEC NEC 250.30(A)Services & Separately Derived Systems

3. Per NEC 250.30(A), a separately derived system that is a grounded system shall have which of the following?

AA system bonding jumper
BAn isolated ground
CA separate grounding electrode system disconnected from the building ground
DNo connection to the grounding electrode system

Show explanation

NEC 250.30(A) requires a grounded separately derived system to have a system bonding jumper connecting the grounded conductor to the equipment grounding conductor.

NEC NEC 250.52(A)(2)Grounding & Bonding

4. Per NEC 250.52(A)(2), a metal in-ground support structure shall qualify as a grounding electrode when which condition is met?

AThe structure is over three stories tall
BThe structure is in direct contact with the earth vertically for 10 ft (3.0 m) or more, with or without concrete encasement
CThe structure supports electrical equipment
DThe building has a lightning protection system

Show explanation

Per NEC 250.52(A)(2), one or more metal in-ground support structures (such as pilings, casings, or other structural metal) qualify as grounding electrodes when they are in direct contact with the earth vertically for 3.0 m (10 ft) or more, with or without concrete encasement. The phrase 'effectively grounded' does not appear in this section and is not the specified condition.

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Frequently Asked Questions

What is the difference between Table 250.66 and Table 250.122?

Table 250.66 sizes the grounding electrode conductor based on the size of the service-entrance conductors. Table 250.122 sizes equipment grounding conductors based on the rating of the overcurrent device ahead of the circuit. Picking the wrong table is the most common Article 250 exam mistake.

What size grounding electrode conductor do I need for a ground rod?

Per 250.66(A), the portion of a grounding electrode conductor that connects only to a rod, pipe, or plate electrode is never required to be larger than 6 AWG copper (or 4 AWG aluminum), regardless of service size.

Which electrodes make up the grounding electrode system?

Per 250.52(A): metal underground water pipe (10 ft or more in contact with earth), metal in-ground support structure, concrete-encased electrode (Ufer), ground ring, rod and pipe electrodes, plate electrodes, and other listed electrodes. All that are present at a building must be bonded together into one system per 250.50.