Electrical Engineer exam mistakes to avoid

Updated 17 September 2026 · 3 min read

Engineering candidates rarely fail on theory. They fail on the small procedural errors that turn a correct method into a wrong number, and on answering the question they expected instead of the one in front of them. These are the mistakes that recur, and the habits that prevent them.

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Mixing line and phase quantities

In a three-phase system, line voltage is higher than phase voltage by the square root of three, and the relationship between line and phase current depends on whether the load is star or delta. Candidates who substitute one for the other produce answers that are wrong by that factor and still look plausible. Write down which quantity you are using at every step.

Treating kVA and kW as the same

Cables, transformers and generators are rated in kVA because they carry current whatever the power factor. Loads are usually described in kW. Converting without the power factor undersizes equipment. Whenever a question mentions motors, welding or discharge lighting, expect power factor to matter.

Forgetting to derate

A cable's tabulated rating assumes conditions. Grouping with other cables, high ambient temperature, passage through insulation and the installation method all reduce it. Questions that mention any of those expect the factor to be applied. Candidates who size on the base rating choose a cable that would overheat.

Sizing for current and ignoring voltage drop

A conductor can satisfy its current rating and still deliver too little voltage at the end of a long run. Both checks are required, and the larger conductor wins. Questions often set the run length precisely so that the two checks disagree.

Assuming discrimination from ratings

Two devices with different ratings do not necessarily discriminate. Their characteristics must be graded across the range of fault currents that can occur. A question describing an upstream device tripping with a downstream fault is testing exactly this.

Reading past the constraints

Design questions carry conditions in ordinary words: maintenance without shutdown, limited space, a maximum voltage drop, equipment that must be locally available. An answer that ignores one of those is wrong however sound the engineering. List the constraints before choosing.

Designing for today only

A supply sized exactly to the present load is a design that fails the first time the building changes. Questions often mention a planned extension, a second shift or equipment to be added later, and expect the answer to leave room for it in the transformer, the switchboard and the containment. Spare capacity is cheap at the design stage and expensive afterwards.

Questions where these mistakes cost marks

An operator reports that a 3-phase conveyor motor is "humming loudly and getting extremely hot" but not turning. Voltage measurements at the motor peckerhead show 480V between L1-L2, 0V between L2-L3, and 0V between L1-L3. What is happening?
1. The motor bearings have seized completely
2. The VFD frequency is set to zero
3. The motor is single-phasing due to a blown fuse or broken wire on L3
4. The stator windings are shorted to ground
Show the answer
C. The motor is single-phasing due to a blown fuse or broken wire on L3
With 0V across two phase pairs, one of the incoming lines (L3) is dead. The motor is experiencing a single-phasing condition, causing high current, heat, and humming without rotation.
Original practice question, written against published exam objectives. Open this question
A technician is sizing the branch circuit conductors for a single 25 HP, 460V, 3-phase motor with a table FLC of 34A. What is the minimum ampacity required for these conductors?
1. 34A
2. 42.5A
3. 40A
4. 85A
Show the answer
B. 42.5A
NEC requires branch circuit conductors for a single continuous duty motor to be sized at 125% of the motor full-load current rating (FLC). 34A * 1.25 = 42.5A.
Original practice question, written against published exam objectives. Open this question
A certification question asks about verification of zero energy. Which answer is correct?
1. confirms hazardous electrical, mechanical, pressure, thermal, and stored energy is controlled before work
2. is performed only after maintenance is finished
3. means trusting the position of a switch
4. eliminates the need for lockout and tagging
Show the answer
A. confirms hazardous electrical, mechanical, pressure, thermal, and stored energy is controlled before work
verification of zero energy confirms hazardous electrical, mechanical, pressure, thermal, and stored energy is controlled before work.
Original practice question, written against published exam objectives. Open this question

Common questions

How do I stop making unit errors under time pressure?

Write units at every line and convert at the start, not the end. Most unit errors happen when someone carries a bare number through three steps and then decides what it meant.

If two answers both satisfy the calculation, how do I choose?

Return to the constraints in the stem. The tie-breaker is almost always a stated condition, such as cost, space, maintainability or a limit on drop.

How much working should I show?

Enough that a marker can follow the method even if the arithmetic slips. Formula, substitution and result, with units, earns partial credit where a bare number earns nothing.