The 20 NEC articles you must know for the C-10

Key numbers: 4 years / 8,000 hours journey-level experience required. Must pass 2 exams: Law & Business + Electrical Trade. Passing score: 70%. Contractor bond: $25,000. License renewal: every 2 years.
TRAP: 4,000 hours (2 years) is a common wrong answer. It's 4 YEARS = 8,000 hours.

Contracts over $500 must be written and signed. Max down payment: 10% or $1,000 (whichever is LESS). Subcontractor payment: within 10 days of GC receiving payment. Preliminary 20-day notice: within 20 days to preserve lien rights.
TRAP: 25% down payment is a common wrong answer. Exceeding 10%/$1,000 is a misdemeanor.

NEC covers buildings, structures, and premises wiring. Excludes: utility supply systems (generation, transmission, distribution). Utilities follow NESC, not NEC.
TRAP: The NEC does NOT apply to utility company distribution systems.

Ampacity = current a conductor can carry continuously without exceeding temp rating. Branch Circuit = from final OCPD to outlets. Feeder = from service equipment to final branch-circuit OCPD. Grounded conductor = neutral (intentionally grounded). Grounding conductor (EGC) = green/bare wire to ground system. Dwelling Unit = must have living, sleeping, cooking, AND sanitation.
TRAP: Mixing up "grounded" (neutral) and "grounding" (EGC) is the #1 definitions trap.

GFCI required for all 125V, 15A/20A receptacles in: bathrooms (ALL receptacles, not just near sink), kitchens, outdoors (all heights), unfinished basements, garages, crawl spaces, laundry areas, boathouses, bathtub/shower areas.
TRAP: "Within 6 feet of sink" is the OLD rule. Current NEC requires GFCI for ALL bathroom receptacles.

AFCI required for ALL 120V, 15A/20A branch circuits throughout the dwelling unit.
TRAP: "Bedrooms only" is the OLD rule. Current NEC requires AFCI for all 120V circuits in dwellings.

Wall receptacles: no point more than 6 feet from a receptacle (12 ft max between receptacles). Kitchen countertop: no point more than 24 inches from a receptacle. Min 2 small appliance circuits (20A) required for kitchen. Garage: at least 1 receptacle (attached or detached).
TRAP: Countertop spacing (24") is different from wall spacing (6 ft). Don't mix them up.

Conductors for continuous loads must be sized at 125% of the continuous load.
Conductor ampacity >= 1.25 x continuous load + 1.00 x non-continuous load TRAP: The 125% applies to conductor sizing AND OCPD sizing (unless 100%-rated device).

Dwellings: 3 VA/sq ft. Each small appliance/laundry circuit: 1,500 VA.
Lighting load = square footage x 3 VA + (# SA circuits x 1,500) + (1 laundry x 1,500) Demand factors (Table 220.42): First 3,000 VA at 100%, remainder at 35%.

Single range (8.75-12 kW): demand = 8 kW. Do NOT use nameplate rating. For multiple ranges, use table column C values directly.
1 range (<=12 kW) = 8 kW demand | 3 ranges (<=12 kW) = 14 kW demand TRAP: Using the actual 12 kW nameplate instead of the 8 kW table demand is the #1 calculation error.

240V range neutral: 70% of the demand load from Table 220.55.
Range neutral = demand load x 0.70 TRAP: The 70% factor applies to the TABLE demand (8 kW), not the nameplate rating.

The MBJ connects grounded conductor (neutral) to EGC at the service. This is the only place neutral and ground are bonded. Never re-bond in a sub-panel.
TRAP: Bonding neutral to ground in a sub-panel creates parallel paths and is a code violation.

All present electrodes must be bonded together. Ufer ground: 20 ft of #4 AWG bare copper in concrete. Ground rod: if one rod exceeds 25 ohms, add a second at least 6 ft away. Two rods = no further testing required.
TRAP: Two rods satisfy the requirement regardless of final resistance. No third rod needed.

GEC size is based on service conductor size. Common break points:
2 AWG Cu service = 8 AWG Cu GEC
1/0 AWG Cu service = 6 AWG Cu GEC
2/0 AWG Cu service = 4 AWG Cu GEC
TRAP: GEC is sized from Table 250.66, NOT from the ampacity table. Different tables.

EGC size is based on the OCPD rating, not conductor ampacity.
20A OCPD = 12 AWG Cu EGC | 60A = 10 AWG | 100A = 8 AWG | 200A = 6 AWG
TRAP: EGC table (250.122) uses OCPD size. GEC table (250.66) uses conductor size. Don't confuse them.

Hard limits for small copper conductors:
14 AWG = 15A max OCPD | 12 AWG = 20A max | 10 AWG = 30A max
TRAP: A 20A breaker on 14 AWG wire is ALWAYS a violation, regardless of calculated load.

10-foot tap: length <= 10 ft, ampacity >= calculated load, terminates in single OCPD, enclosed in raceway.
25-foot tap: length <= 25 ft, ampacity >= 1/3 of feeder OCPD, terminates in single OCPD, protected from damage.
25-ft tap minimum ampacity = feeder OCPD rating / 3 TRAP: The 1/3 rule applies to 25-ft taps only. 10-ft taps must carry the full calculated load.

The most-referenced table in the NEC. Common values (75C column, copper):
14 AWG = 20A | 12 AWG = 25A | 10 AWG = 35A | 8 AWG = 50A | 6 AWG = 65A
4 AWG = 85A | 3 AWG = 100A | 2 AWG = 115A | 1/0 = 150A | 2/0 = 175A
TRAP: These are 75C values. Terminal ratings (240.4(D)) still cap 14 AWG at 15A and 12 AWG at 20A.

More than 3 current-carrying conductors in a raceway requires ampacity adjustment:
4-6 conductors = 80% | 7-9 = 70% | 10-20 = 50% | 21-30 = 45%
Adjusted ampacity = Table 310.16 value x derating factor TRAP: Neutrals carrying only unbalanced current are NOT counted. Equipment grounds are NOT counted.

Motor conductor: 125% of motor FLC (from Table 430.248/250, NOT nameplate). Motor OCPD: use Table 430.52 percentages (dual-element fuse: 175% of FLC; inverse-time breaker: 250% of FLC).
Motor conductor = FLC x 1.25 | Motor breaker = FLC x 2.50 (round UP to standard size) TRAP: ALWAYS use table FLC values, NEVER nameplate amps, for motor circuit calculations.