Equipment practice set

Three weeks before her exam date, Dana set aside a Saturday morning for a timed practice session on Chapter 4. Marcus came over with coffee and his own code book, Ray left a stack of old exam-style problems on the table, and Tyler sat in to watch. The rule was simple: one problem at a time, a timer running, the code book open, and the answer written with the section that supports it. Equipment questions make up a large share of most master exams because they combine table lookups with arithmetic, and because the same few traps catch candidates every time. This lesson walks through the session's problems, organized by the earlier lessons, then gives a strategy for working equipment questions under time pressure.

Each problem below uses this course's illustrative motor currents and percentages. The method is what matters; on the real exam you will read the values from your own adopted edition, which may differ.

A plan for each problem

Before starting the timer, Dana wrote her five-step plan at the top of the page, and she used it on every problem. It is the habit that keeps equipment questions from turning into guesswork.

• Identify the equipment and the article: a standalone motor is Article 430, a hermetic compressor Article 440, a transformer Article 450, a generator Article 445, a capacitor Article 460, a converter Article 455, a machine Article 670.
• Find the right current: the table full-load current for motor circuits, the nameplate for overloads, the marked rated-load or selection current for compressors, the kVA calculation for transformers and generators.
• Apply the right factor: 125%, 115%, 135%, 175%, 250% or another value, from the right section, to the right current.
• Round in the right direction: up to the next standard size where the code permits it, down where the result is a maximum.
• Check the answer choices for the common distractors: the unrounded value, the value from the wrong current, or the value with the factor applied to every load.

Problems on motor currents and conductors

Marcus answered the first part with 31.25 amperes, using the nameplate for both. That one mistake costs a point on almost every exam.

Problems on motor protection

Dana nearly wrote 150 amperes by reflex, rounding up as she had in Problem 3. Writing "feeder: round down" in the margin of her plan saved her.

Problems on air conditioning

Tyler asked why the unit's own nameplate would not just give the answer. It does, and on the job you use it; but exam writers like to make you calculate it, and they almost always include both a rated-load and a selection current to see which you choose.

Problems on transformers

Problems on generators, capacitors and machinery

Reviewing the session

Dana finished the twelve problems with eight minutes to spare and got two wrong on her first pass, both from rounding: she rounded the feeder breaker in Problem 4 up before catching it, and in Problem 9 she first rounded the 250% primary value up to 350 amperes. Marcus got four wrong, three of them from using the nameplate current for conductors or applying 125% to every motor. They agreed on a plan for the last three weeks: one timed set of twenty mixed equipment questions every second day, with every wrong answer recorded in a log by cause, not by topic. A log of causes, such as "wrong current", "wrong rounding" or "wrong article", shows patterns that a log of topics hides.

Time management

Equipment problems take longer than lookup questions, but they are worth the same single point. A good strategy on a timed exam is two passes. On the first pass, answer every question you can finish in about two minutes and mark the rest. On the second pass, return to the marked calculations. Within a calculation, write each intermediate value in the margin; when you check your work, you can find a single slip without redoing the whole problem. Finally, never leave a question blank, because a guess among four choices has a one-in-four chance and a blank has none.

import math
s3 = math.sqrt(3)
problems = {
    "1 conductors (table 27 A)": 1.25 * 27,
    "1 overloads (nameplate 25 A)": 1.25 * 25,
    "2 feeder": 1.25 * 40 + 27 + 14 + 1.25 * 12,
    "3 branch breaker before rounding": 2.5 * 27,
    "4 feeder breaker before rounding": 100 + 27 + 14,
    "6 disconnect": 1.15 * 52,
    "7 MCA": 1.25 * 19.0 + 1.4,
    "8 primary current": 112500 / (s3 * 480),
    "9 secondary current": 112500 / (s3 * 208),
    "10 generator conductors": 1.15 * 150000 / 0.8 / (s3 * 480),
    "11 capacitor conductors": 1.35 * 15000 / (s3 * 480),
    "12 machine supply": 64 + 0.25 * 28 + 0.25 * 16,
}
for name, value in problems.items():
    print(f"Problem {name:34} {value:7.2f}")

Problem 1 conductors (table 27 A)            33.75
Problem 1 overloads (nameplate 25 A)         31.25
Problem 2 feeder                            106.00
Problem 3 branch breaker before rounding     67.50
Problem 4 feeder breaker before rounding    141.00
Problem 6 disconnect                         59.80
Problem 7 MCA                                25.15
Problem 8 primary current                   135.32
Problem 9 secondary current                 312.27
Problem 10 generator conductors             259.36
Problem 11 capacitor conductors              24.36
Problem 12 machine supply                    75.00

Two more problems

Marcus got Problem 14 right and Problem 13 wrong, sizing the drive's supply from the motor's table current instead of the drive's input current. Dana pointed out that it was the same kind of error as using the nameplate for conductors: the right current depends on the equipment, and the equipment is named in the first words of the question.