LEW common mistakes
LEW · Understand the traps and how to avoid them.
Computing voltage drop or conductor sizing from memory using a single rule-of-thumb (e.g., 2% drop, 3% total) instead of applying the actual load current, one-way versus round-trip length, and conductor resistance/reactance values from the tables.
Candidates memorize simplified heuristics from field work and skip the step of identifying whether the length is one-way or the full circuit, and whether the formula uses resistance only or impedance. Small arithmetic slips then compound because the wrong reference value was chosen.
What to do instead
Write the governing equation first, define each variable with units, and substitute values only after stating the source table or given data. Track one-way length explicitly, double it for round-trip drop, and sanity-check the result against a rough estimate before selecting the answer.
Confusing series and parallel behavior when a question asks about current, voltage, or equivalent resistance in a combination circuit, especially when a component is added, removed, or shorted.
Under time pressure candidates default to 'current is the same everywhere' or 'voltage is the same everywhere' without checking the topology. Questions that change one element mid-circuit exploit this by making the intuitive answer wrong.
What to do instead
Redraw the circuit before answering, label nodes, and reduce it stepwise: parallel branches first, then series. For 'what happens if' items, recompute the two or three affected quantities rather than guessing direction of change.
Selecting a wiring method or raceway fill without verifying the specific article and table that governs the installation type, ambient condition, and conductor insulation.
The exam mixes installation scenarios (wet location, underground, hazardous area, cable tray) and candidates apply the rule they remember best rather than the one tied to the location and wiring method named in the stem.
What to do instead
Parse the stem for location classification, wiring method, and conductor type before choosing. Then confirm the applicable table or article applies to that combination, and check fill, derating, and support requirements as separate steps.
Treating conductor ampacity adjustment and correction as a single step, or applying derating before applying the overcurrent device rules, leading to a conductor that is undersized or a device that is oversized relative to the adjusted ampacity.
Candidates apply one factor and stop, or apply factors in the wrong order. The interaction between ambient temperature correction, conduit fill adjustment, and the next-standard-size overcurrent device is the point of many items.
What to do instead
Build a small worksheet: base ampacity from the table, apply temperature correction, apply fill adjustment, compare to load, then check the overcurrent device against the adjusted value and the permitted rounding rule. Do each step explicitly even when the answer seems obvious.
Sizing overcurrent protection by load current alone without checking the device's interrupting rating, the conductor's ampacity, and the motor or equipment-specific allowances for branch-circuit and short-circuit protection.
Candidates conflate overload protection with short-circuit and ground-fault protection, and forget that motors, transformers, and air-conditioning equipment have separate rules for each. The stem often gives a nameplate value that must be converted before sizing.
What to do instead
Classify the protection type first (overload versus short-circuit/ground-fault), then identify the article for that equipment. Convert nameplate data to the basis the rule requires, select the device, and verify it does not exceed the conductor or equipment limit.
Treating grounding and bonding as interchangeable: connecting equipment to a grounding electrode when the question is about an equipment grounding conductor, or omitting the main bonding jumper in a service configuration.
The terms overlap in everyday speech, and diagrams in the exam show both systems. Candidates answer 'ground it' without identifying which conductor or connection the question actually asks about.
What to do instead
For every grounding item, name the system involved: service neutral, main bonding jumper, equipment grounding conductor, grounding electrode conductor, or bonding jumper. Then state its purpose (fault-current path versus earth reference) before selecting an answer.
Calculating motor branch-circuit conductor ampacity and overload protection from the nameplate full-load current instead of the table value required for the conductor and protection sizing.
Nameplate current is the number printed on the equipment and feels authoritative, but conductor and protection sizing rules use tabulated values for the motor type and voltage. Candidates also forget the percentage multipliers for overload devices and the allowance for short-circuit protection.
What to do instead
Identify motor type, voltage, and phase from the stem, look up the tabulated full-load current, then apply the specific multiplier for the device being sized. Keep nameplate current separate and use it only where the rule explicitly calls for it.
Sizing a transformer's primary or secondary conductor and protection without accounting for the transformer's turns ratio, inrush allowance, and the difference between primary and secondary current.
Candidates compute one current and apply it to both sides, or forget that protection sizing for transformers uses a percentage of rated current rather than the load current. Voltage ratio and phase relationship are often the hidden step.
What to do instead
Compute rated primary and secondary currents separately from kVA and voltage, note the ratio, then apply the transformer protection percentages to the correct side. Check whether the question asks about conductor ampacity, overcurrent device rating, or both.
Answering safety and standards questions from habit or site practice rather than from the specific requirement in the code or standard, especially for lockout/tagout, approach boundaries, and PPE selection.
Experienced candidates have seen installations that passed inspection despite noncompliant details, so their intuition conflicts with the written requirement. The exam tests the requirement, not common practice.
What to do instead
When a safety item appears, recall the governing principle (de-energize, verify, protect, document) and then the specific numeric or procedural requirement. If two answers both sound safe, choose the one that matches the defined procedure rather than the more cautious-sounding option.
Jumping to a diagnosis from a symptom without following a test sequence, or misreading meter readings because of range, lead placement, or expected versus measured values.
Troubleshooting items reward a systematic path: verify supply, isolate the section, test continuity or voltage, then confirm. Candidates skip steps and pick the first plausible failed component, or they confuse a voltage reading taken across an open switch with one taken across a load.
What to do instead
State the expected reading at each test point before interpreting the meter. Work from source to load or load to source consistently, and eliminate sections rather than guessing components. For meter questions, confirm function, range, and lead position first.