IA Electrician common mistakes
IA Electrician · Understand the traps and how to avoid them.
Bonding the neutral bar to the enclosure in a subpanel and landing equipment grounding conductors on the neutral bar.
In a dwelling the main service panel often has a factory bonding screw, and that habit is carried to downstream panels. The neutral is a current-carrying conductor; bonding it downstream creates parallel return paths and objectionable current.
What to do instead
At each panel ask whether it is the service disconnect or a separately derived system. Bond neutral to ground only at that point. In subpanels keep the neutral bar isolated, install a separate grounding bar, and verify with a meter before energizing. See NEC 250.24(A)(5) and 250.32(B).
Selecting conductor ampacity from the 90°C column for a 75°C termination and skipping ambient-temperature and raceway-fill adjustments.
Table 310.16 has several temperature columns, and the highest column looks best. Terminals are typically rated 75°C, and conduit fill or high ambient reduces the allowable ampacity.
What to do instead
Use the 90°C column only as the starting point for adjustment and correction under 310.15, then cap the final ampacity at the termination temperature per 110.14(C). Apply ambient correction and conductor-fill adjustment, and confirm the OCPD does not exceed the final ampacity.
Going to the next standard OCPD size whenever conductor ampacity does not match a standard rating.
240.4(B) permits next-size-up protection only for specific conditions, but candidates apply it universally, including multioutlet receptacle branch circuits and feeders above 800 A.
What to do instead
Verify every condition before rounding up: the circuit is not a multioutlet branch circuit supplying receptacles, the conductor ampacity does not exceed 800 A, and the next standard OCPD does not exceed 800 A. Otherwise select an OCPD that does not exceed conductor ampacity.
Counting only current-carrying conductors for box fill and ignoring cable clamps, device yokes, support fittings, and equipment grounding conductors.
Box fill is a volume calculation, not a wire count. Candidates count the wires visible at rough-in and forget the volume consumed by clamps, devices, and grounds.
What to do instead
Use 314.16(B): count each conductor, one for all cable clamps, one for all EGCs, one for each support fitting, and two for each yoke or device. Multiply by the volume per conductor from Table 314.16(B) and compare with the marked box volume.
Computing a dwelling service load by adding general lighting VA and then applying demand factors to the small-appliance and laundry branch circuits.
The standard method has separate line items, and small-appliance and laundry circuits are 1,500 VA each and are not subject to general-lighting demand.
What to do instead
Use a worksheet: general lighting at 3 VA/ft² subject to Table 220.42 demand; two or more small-appliance circuits at 1,500 VA each; laundry at 1,500 VA; then add fixed appliances, range, dryer, and other loads. Apply demand only where the code permits it.
Installing GFCI receptacles only in bathrooms and kitchens and missing garages, outdoors, laundry areas, crawlspaces, unfinished basements, and receptacles within 6 ft of sinks.
GFCI rules have expanded over many code cycles, and candidates rely on older residential experience or a memorized short list.
What to do instead
Walk the current 210.8(A) list location by location. Remember that GFCI protection may be at the breaker or at the first receptacle, and test and label it. Also confirm AFCI protection under 210.12 for dwelling branch circuits.
Running NM cable in a wet location or using EMT outdoors without addressing corrosion, and supporting cable at arbitrary spacing.
NM cable is common and inexpensive, and wet/damp rules and support distances are easy to overlook when reading a plan.
What to do instead
Check 334.10 and 334.12 for uses permitted and not permitted. Do not use NM in wet locations. Support NM per 334.30 (every 4.5 ft and within 12 in. of boxes). For EMT outdoors, verify corrosion protection and support per 358.30.
Sizing motor overload protection from the branch-circuit conductor ampacity or breaker size instead of the motor nameplate.
The two protections look like one requirement. Overload protection protects the motor from overheating, while branch-circuit protection protects conductors from short circuits and ground faults.
What to do instead
Use motor nameplate full-load current for overload protection per 430.32. Use Tables 430.248 and 430.250 to size branch-circuit conductors and short-circuit/ground-fault protection per 430.52. Keep the two calculations separate.
Measuring 110.26 working space only as a 36-in. depth from the equipment cover and forgetting the required width and height.
Candidates focus on the familiar 36-in. depth and overlook that depth is measured from live parts and that width and height also apply.
What to do instead
Measure depth from the live parts, not the door. Provide width of 30 in. or the equipment width, whichever is greater, and height of 6½ ft. Verify dedicated space and no foreign objects. Check 110.26(A)(1) through (A)(3).
Using P = V × I for an inductive motor load and treating kVA as kW.
Theory questions involve AC circuits, and power factor and the √3 factor for three-phase systems are easy to omit under time pressure.
What to do instead
For single-phase use P = V × I × PF; for three-phase use P = √3 × V × I × PF. Convert VA to amperes by dividing by V (single-phase) or by √3 × V (three-phase). Check units before choosing an answer.