Grounding and bonding practice set

Two weeks before her exam date, Dana Whitfield sat a fresh set of twenty grounding questions under exam conditions: code book open, tabs in place, a basic calculator, and forty minutes on the clock. She finished with six minutes to spare and scored 18 out of 20, up from 13 at the start of this course. Her two misses were both reading errors, not knowledge errors, which Ray Ochoa told her was exactly where she wanted to be. This lesson gives you a set like hers, worked step by step, with a routine for each type of question. Work each problem yourself before reading the solution, time yourself at about two minutes per problem, and check every table value against your own copy of the code, because the values below are this course's illustrative examples, not reproductions of the NEC.

Every grounding question on a master exam falls into one of five families: purpose and definitions, sizing, electrodes, bonding, and special systems such as separately derived systems, separate buildings and ungrounded systems. The routine is the same for each: decide the family, decide the section, decide the table if there is one, then read the question again for the facts that change the answer.

The three-table decision

Most sizing questions turn on choosing the right table, so start with the decision that Lessons 2, 5, 6 and 7 built. If the conductor connects a service or separately derived system to an electrode, it is a grounding electrode conductor, sized from Table 250.66 by the largest ungrounded service or derived conductor, with caps for rods, concrete-encased electrodes and ground rings. If it connects the grounded conductor to the equipment grounding system at a service or derived system, or bonds metal parts on the supply side, it is a main, system or supply-side bonding jumper, sized from Table 250.102(C)(1) by the largest ungrounded conductor, with the 12.5% rule above the last row. If it is an equipment grounding conductor or a load-side bonding jumper, it is sized from Table 250.122 by the overcurrent device rating.

The examples below use these illustrative rows, stated as this course's examples. For Tables 250.66 and 250.102(C)(1), with copper conductors: 2 AWG or smaller gives 8 AWG; 1 or 1/0 gives 6 AWG; 2/0 or 3/0 gives 4 AWG; over 3/0 through 350 kcmil gives 2 AWG; over 350 through 600 kcmil gives 1/0; over 600 through 1,100 kcmil gives 2/0; over 1,100 kcmil gives 3/0 in Table 250.66 and 12.5% of the area in Table 250.102(C)(1). For Table 250.122 with copper: 20 A gives 12 AWG, 60 A gives 10 AWG, 100 A gives 8 AWG, 200 A gives 6 AWG, 300 A gives 4 AWG, 400 A gives 3 AWG, 600 A gives 1 AWG, 800 A gives 1/0 and 1,200 A gives 3/0.

Problems 1 to 5: sizing

Problems 6 to 9: electrodes and bonding

Problems 10 to 14: special systems

Problem 15: a purpose question

Timing and review habits

Dana's timing on this set settled at about ninety seconds per sizing problem and sixty seconds per rule question, leaving time for a full review pass. Her review routine had three checks. First, for each sizing answer, did she use the table that matches the conductor's job? Second, did she add parallel conductors, apply the rod or concrete-encased cap, or apply the 12.5% rule where the question's facts called for it? Third, did the answer make physical sense: a grounding electrode conductor for a dwelling is rarely larger than 4 AWG, and an equipment grounding conductor is never larger than the circuit conductors.

Marcus Lee's weak point was reading. He would see a detail such as rods only, existing installation, or no line-to-neutral loads, and skip past it. On his review pass he now underlines every qualifier in the question stem before checking his answer. Those qualifiers are where exam writers put the difference between the right answer and the attractive wrong one.

One more habit is worth adopting from Gloria Haynes. When a question describes an installation, picture it as an inspector would: trace the neutral current home, trace the fault current home, and look at every electrode and bond. If neutral current can reach metal, there is an extra bond. If fault current must cross the earth, a conductor is missing. If an electrode present at the building is not connected, the grounding electrode system is incomplete. Those three traces answer most scenario questions without a single table lookup.

A drill you can edit

The listing below drills the three-table decision. It describes a series of conductors, decides which table sizes each one, and prints the reasoning. Add your own descriptions from practice questions and see whether your answer matches.

def which_table(job):
    job = job.lower()
    if "electrode" in job and "bond" not in job:
        return "Table 250.66 (largest ungrounded service or derived conductor; check caps)"
    if any(word in job for word in ("main bonding", "system bonding", "supply-side", "water piping", "structural")):
        return "Table 250.102(C)(1) (largest ungrounded conductor; 12.5% above last row)"
    return "Table 250.122 (overcurrent device rating)"

jobs = [
    "conductor from service to electrode at building steel",
    "main bonding jumper in a 400 A service",
    "equipment grounding conductor in a 225 A feeder",
    "supply-side bonding jumper from transformer to panel",
    "bond to interior water piping",
    "load-side jumper around flexible conduit",
]
for j in jobs:
    print(f"{j}: {which_table(j)}")

conductor from service to electrode at building steel: Table 250.66 (largest ungrounded service or derived conductor; check caps)
main bonding jumper in a 400 A service: Table 250.102(C)(1) (largest ungrounded conductor; 12.5% above last row)
equipment grounding conductor in a 225 A feeder: Table 250.122 (overcurrent device rating)
supply-side bonding jumper from transformer to panel: Table 250.102(C)(1) (largest ungrounded conductor; 12.5% above last row)
bond to interior water piping: Table 250.102(C)(1) (largest ungrounded conductor; 12.5% above last row)
load-side jumper around flexible conduit: Table 250.122 (overcurrent device rating)