Journeyman Electrician common mistakes
Journeyman Electrician · Understand the traps and how to avoid them.
On combination series-parallel circuits, the candidate divides the full source voltage by each individual parallel branch resistance to get branch current, ignoring voltage drops that occur across series elements ahead of the parallel group.
Memorized 'voltage is the same in parallel' and applies the source voltage to branches even though a series resistor ahead of the split has already dropped part of it.
What to do instead
Reduce each parallel group to one equivalent resistance, add the series elements to find total resistance, compute total current from the source voltage, then work backwards: subtract each series voltage drop from the source to find the actual voltage across the parallel group before dividing by individual branch resistances.
Sizes conduit for a two-conductor pull using the 40% fill column, and for a single conductor also using 40%.
The '40%' number gets memorized as the universal raceway fill limit because most pulls involve three or more conductors.
What to do instead
Use Chapter 9, Table 1: 53% for one conductor, 31% for two conductors, 40% only for over two conductors; then read the actual square-inch area from Chapter 9, Table 4 or 5 for the specific raceway type and size rather than estimating from trade size.
Uses Table 250.122 (equipment grounding conductor sized by overcurrent device rating) to size the grounding electrode conductor to a rod or water pipe.
Both tables contain a column that looks like an overcurrent rating or conductor size, and both are titled with 'grounding,' so they get swapped under time pressure.
What to do instead
Anchor the distinction: grounding electrode conductors come from 250.66 and are sized by the largest service-phase conductor; equipment grounding conductors come from 250.122 and are sized by the rating of the overcurrent device protecting the circuit. Sketch which conductor lands where before opening the table.
Rounds a calculated overcurrent device up to the next standard size in every situation, including small-conductor branch circuits and cases where the next size exceeds 800 A.
The 'next size up' rule from 240.4(B) gets memorized as an unconditional allowance without its three conditions.
What to do instead
Before rounding up, verify all of 240.4(B): the conductors are not supplying multioutlet receptacle circuits for cord-and-plug loads, the ampacity does not already match a standard rating, and the next higher standard size does not exceed 800 A. Also check 240.4(D) small-conductor limits and any 240.4(G) specific-circuit rules, which override the general allowance.
Applies the 35% demand factor to the entire calculated general lighting load for a dwelling instead of only to the portion above 3,000 VA.
Table 220.42 is read as a single '35%' factor and the tiered structure of the table is skipped.
What to do instead
Split the general lighting load into tiers before applying anything: first 3,000 VA at 100%, 3,001 through 120,000 VA at 35%, remainder at 25%; keep the 1,500 VA small-appliance and laundry circuits separate at 100% per 220.52, then add the largest of the heating or cooling load.
Reads conductor ampacity straight from the 90 degrees C column for a THHN conductor terminated on a 75 degrees C lug, or defaults everything to the 60 degrees C column.
The insulation rating on the conductor is confused with the termination temperature limit, and the equipment label is not checked.
What to do instead
Ampacity is the lesser of the conductor's table value and the termination limit: check the equipment marking under 110.14(C) first, then read the corresponding column of the ampacity table. Keep separate scratch columns for 'insulation ampacity' (used for derating and adjustment) and 'final ampacity' (after applying termination limits and adjustment/correction factors).
Counts box fill by counting only the current-carrying conductors and the device, leaving out cable clamps, equipment grounding conductors, or counting a single-gang device as one conductor.
The mental image of 'wires in a box' excludes hardware, and 314.16(B) item-by-item counting is slow under exam time pressure.
What to do instead
Build a fill tally line by line: each unbroken conductor counts once, each conductor cut or spliced counts once, all equipment grounding conductors together count as one based on the largest, each cable clamp or connector counts as one, and each single-gang yoke or strap counts as two. Multiply the count by the volume per conductor from Table 314.16(B) and compare to the box's marked volume.
Sizes the motor branch-circuit short-circuit and ground-fault protective device from the conductor ampacity or the nameplate current instead of from the table full-load current, and skips the 125% multiplier on branch-circuit conductors.
The motor is treated like a standard appliance branch circuit, so 210.20 and conductor ampacity are used where 430.22 and 430.52 govern.
What to do instead
Use the sequence: Table 430.250 full-load current (not nameplate) at 125% for branch-circuit conductors; overload protection from 430.32 based on nameplate or service factor; short-circuit and ground-fault protection sized as a percentage of Table 430.250 FLC per Table 430.52, verifying the conductor is still protected. Keep the three protections physically distinct on scratch paper.
Specifies metallic liquidtight flexible conduit with standard fittings, or nonmetallic conduit, for the wiring method in a Class I, Division 1 location.
The location is treated like an ordinary wet or industrial area, and the division classification is not connected to the wiring-method table.
What to do instead
First confirm the class, division or zone, and group, then open the article 500-series wiring method table for that exact combination (501.10(A) for Class I, Division 1, for example, which requires threaded rigid metal conduit or threaded IMC, MI cable, or specific listed cable types). Division 2 relaxes the method, so never downgrade Division 1 requirements to Division 2 allowances.
Verifies a circuit is de-energized for troubleshooting using only a non-contact voltage tester, or uses a multimeter that has not been checked on a known live source.
Speed and habit on the job; the tester is treated as an indicator rather than as a test instrument requiring validation.
What to do instead
Follow the test-before-and-after sequence: confirm the meter and leads on a known energized source, test the conductors being worked, then re-test the meter. Use only a meter rated for the available voltage and category, apply the shock and arc-flash PPE the task requires, and treat non-contact testers as screening only, never as the sole verification.