AR Electrician common mistakes
AR Electrician · Understand the traps and how to avoid them.
Computes voltage drop with the conductor size entered in kcmil (e.g., 250) instead of circular mils (250,000), producing an answer off by a factor of 1,000.
The formula 2KID/CM is memorized, but under time pressure the unit conversion from kcmil to circular mils is skipped and the calculator gets the table value directly.
What to do instead
Before computing, write the CM value in full (4/0 = 211,600; 250 kcmil = 250,000) on scratch paper, then do a sanity check: a 100 A load at 240 V through 100 ft should drop a few volts, never thousands.
Selects NM-B cable for an exposed run in a commercial ceiling, a wet location, or a dropped ceiling plenum because 'it is the same wire' used on residential jobs.
Familiarity with dwelling wiring and the fact that NM looks identical to MC leads to choosing by habit instead of by location classification and occupancy type.
What to do instead
Before picking a wiring method, write down two things: the location class (dry, damp, wet; exposed or concealed) and the occupancy (dwelling vs. non-dwelling). Then open 334.10 and 334.12 and confirm the method is permitted and not in the prohibited list.
Applies the 83% reduced service-conductor ampacity (310.12) to a feeder that does not carry the entire dwelling load, or to a non-dwelling service.
The rule was formerly 310.15(B)(7) and is heavily tested, so candidates recognize the table but forget the limiting conditions attached to it.
What to do instead
Before using the table, verbally confirm all conditions: dwelling unit, conductors carry the entire load of the dwelling, and service or feeder rating between 100 A and 400 A. If any condition fails, size at 100% of the calculated load.
Sizes a grounding electrode conductor with Table 250.122 or sizes an equipment grounding conductor with Table 250.66.
The two tables look similar and both are titled 'grounding conductor,' so the selection is made from memory of the wrong article.
What to do instead
Write the rule on scratch paper first: GEC is sized from the largest ungrounded service conductor using Table 250.66; EGC is sized from the overcurrent device rating using Table 250.122. Match the table to its reference number before looking anything up.
Applies a 25-ft feeder tap rule to a tap that passes through a wall, is outdoors, or does not terminate in a single overcurrent device or a single set of fuses.
Candidates memorize only the length limit (10 ft vs. 25 ft) and treat the tap rules in 240.21(B) as interchangeable.
What to do instead
For every tap question, list all conditions before answering: length, ampacity not less than one-third of the device protecting the tap, termination in a single breaker or single set of fuses, no penetration of walls or floors (10-ft rule), and physical protection where required.
Undercounts box fill by omitting the equipment grounding conductors, cable clamps, or the device yoke before multiplying by the Table 314.16(B) volume per conductor.
The counting rules in 314.16(B)(1) through (B)(5) are scattered and read quickly, and the EGC reduction to a single count is misapplied as 'don't count it.'
What to do instead
Use a written checklist every time: all circuit conductors, plus EGCs counted as one, plus one for cable clamps, plus one per support fitting, plus two for each device yoke. Take the largest conductor size present, get its volume from Table 314.16(B), multiply, and compare to the box's marked volume.
Adds the 3 VA per square foot general lighting load at 100% with no demand factor, or applies the lighting demand factors to receptacle and appliance loads.
Table 220.42 looks like a general-purpose multiplier, and the calculation is often done as one lump sum instead of by load category.
What to do instead
Lay the calculation out in labeled columns (general lighting, small appliance, laundry, range, dryer, water heater, etc.) and apply the demand factor only to the column it governs. The first 3,000 VA of lighting and small appliance load is always at 100%.
Sizes motor branch-circuit conductors and the disconnecting means from the nameplate FLA instead of the NEC tables, and forgets the 125% multiplier on the branch conductors.
The nameplate value feels more precise and more 'real world' than a table, and 430.6(A)(1) is a short sentence that gets skimmed.
What to do instead
Make step one of every motor problem: look up the FLA in Table 430.248 (single-phase) or 430.250 (three-phase) using horsepower and voltage, then multiply by 125% for branch-circuit conductors. Use the nameplate only where the Code explicitly says to, such as overload sizing in 430.32.
Applies only the bathroom GFCI rule and misses required GFCI or AFCI protection in garages, outdoors, kitchen countertops, within 6 ft of a sink, laundry areas, unfinished basements, and similar locations.
The list of locations in 210.8 and 210.12 is long and reads as a series of separate rules, so one location gets memorized as representative of all.
What to do instead
Study the location list as grouped categories (wet/damp areas, kitchens, garages and accessory buildings, laundry, basements, outdoors) and check the Arkansas amendments sheet for any added locations before answering.
Answers working-clearance questions with a space that is 30 in. deep and 36 in. wide, or fails to measure the depth from the live parts rather than the enclosure front.
The two numbers are close and easy to transpose, and the depth is measured from the live parts, which is not the surface the eye focuses on in the question.
What to do instead
Memorize the triplet for 0-150 V to ground: 30 in. wide, 36 in. deep, 6 ft 6 in. high, with width measured edge to edge of the equipment and depth measured from the live parts. Check 110.26(E) for the dedicated equipment space above the equipment.