NEC Study Guide

Master the most-tested articles and formulas for your electrician exam

💡 Pro Tip: Bookmark this page and use it alongside your NEC code book. For open-book exams, knowing WHERE to look is just as important as what to look for.

Using This Guide

This study guide covers the most frequently tested NEC articles on electrician licensing exams. Each section includes key concepts, common exam questions, and where to find details in the code book.

Essential Formulas

Ohm's Law

V = I × R

I = V ÷ R

R = V ÷ I

V = Voltage (volts), I = Current (amperes), R = Resistance (ohms)

Power Formulas

P = V × I

P = I² × R

P = V² ÷ R

P = Power (watts), V = Voltage, I = Current, R = Resistance

Voltage Drop

VD = 2 × K × I × L ÷ CM

K = 12.9 (copper) or 21.2 (aluminum)
I = Current (amps), L = One-way length (feet)
CM = Circular mils

Box Fill (314.16)

#14 wire = 2.00 in³
#12 wire = 2.25 in³
#10 wire = 2.50 in³
#8 wire = 3.00 in³

Add 2 volumes for each device (yoke/strap); 1 volume for each: clamp (once per box), fixture stud/hickey, equipment ground (once per box)

Article 90: Introduction

Purpose, scope, and arrangement of the NEC

Foundation

Key Concepts

90.1 Purpose: Practical safeguarding of persons and property from hazards arising from the use of electricity
90.2 Scope: Covers installation of electrical conductors, equipment, and raceways in:
- Public and private premises (buildings, structures, yards)
- Industrial substations
- Installations of conductors and equipment connected to the utility supply
90.3 Code Arrangement: Chapters 1-4 apply generally; Chapters 5-7 supplement or modify general rules; Chapter 8 (communications) stands alone; Chapter 9 contains tables
90.5 Mandatory Rules vs. Permissive Rules:
- "Shall" = mandatory requirement
- "Shall be permitted" = permissive (allowed but not required)

Common Exam Question

"The NEC is not intended as a design specification or an instruction manual for untrained persons." (90.1(C))

Article 110: Requirements for Electrical Installations

General requirements for examination, installation, use, and equipment

High-Frequency

Key Concepts

110.3 Examination, Identification, Installation, and Use: Equipment must be listed and labeled, installed per instructions
110.12 Mechanical Execution of Work: Electrical equipment must be installed in a neat and workmanlike manner
110.14 Electrical Connections:
- Terminal connections must be made without damage to conductors
- Terminals for aluminum conductors must be identified for aluminum
- Splices and terminations must be done with listed devices
110.26 Spaces About Electrical Equipment (600V or less):
- Depth of working space: 3 feet minimum (Condition 1)
- Width of working space: 30 inches or width of equipment (whichever is greater)
- Height of working space: 6½ feet minimum
- Headroom: 6½ feet or height of equipment (whichever is greater)

Table 110.26(A)(1) - Working Space

Condition	0-150V	151-600V
1 (exposed live parts on one side)	3 ft	3 ft
2 (exposed on both sides)	3 ft	3½ ft
3 (exposed + grounded wall opposite)	3 ft	4 ft

Exam Tip

Working space questions are extremely common. Memorize the 3 feet minimum and the conditions in Table 110.26(A)(1).

Article 210: Branch Circuits

Requirements for branch circuits supplying lighting and appliance loads

High-Frequency

Key Concepts

210.8 GFCI Protection: Required in dwellings for receptacles in:
- Bathrooms (all receptacles)
- Garages and accessory buildings
- Outdoors (all receptacles)
- Crawl spaces and unfinished basements
- Kitchens (countertop receptacles)
- Sinks (within 6 feet)
- Laundry areas
210.11 Branch-Circuit Required:
- Small appliance circuits: minimum 2 circuits for kitchen/dining
- Laundry circuit: minimum 1 circuit (20A)
- Bathroom circuit: minimum 1 circuit (20A)
210.12 AFCI Protection: Required for all 120V, 15A & 20A branch circuits supplying outlets in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar rooms or areas
210.19 Conductors - Minimum Ampacity:
- Must have ampacity ≥ 125% of continuous load + 100% of non-continuous
- Voltage drop: 3% max on branch circuits (5% total including feeder)
210.52 Dwelling Unit Receptacle Outlets:
- No point along wall > 6 feet from receptacle
- Wall space ≥2 feet requires receptacle
- Kitchen countertop: receptacle every 4 feet
- Island countertop ≥24" × 12" requires receptacle

Common Calculation

Example: A 20A circuit supplies a continuous load of 12A and a non-continuous load of 5A. Minimum conductor ampacity = (12A × 1.25) + 5A = 20A. Use #12 AWG copper (20A rated).

Article 220: Branch-Circuit, Feeder, and Service Load Calculations

Methods for calculating loads on electrical systems

Critical

Key Concepts

220.12 Lighting Load:
- Dwelling units: 3 VA per sq ft
- Offices: 3.5 VA per sq ft (Table 220.12)
- Minimum branch-circuit load = VA ÷ Voltage
220.14 Other Loads - All Occupancies:
- Small appliance circuits: 1,500 VA each (minimum 2 required)
- Laundry circuit: 1,500 VA
- Fixed appliances: nameplate rating
220.42 General Lighting Demand Factors (Table 220.42):
- First 3,000 VA: 100%
- 3,001-120,000 VA: 35%
- Over 120,000 VA: 25%
220.52 Small-Appliance and Laundry Loads:
- Small appliance + laundry load: 1,500 VA × number of circuits
- Apply demand factor: First 3,000 VA @ 100%, remainder @ 35%
220.53 Appliance Load - Dwelling Unit (Table 220.53):
- 4 or more fastened-in-place appliances: Apply 75% demand factor
- Does not include electric ranges, dryers, HVAC, water heaters
220.54 Electric Cooking Equipment (Table 220.55):
- 1 range (≤12 kW): 8,000 VA demand
- Use Column C demand factors for multiple units
220.54 Electric Dryers: 5,000 VA or nameplate (whichever is larger) per unit; demand factors apply for multiple units

Dwelling Unit Load Calculation (Simplified)

1. Lighting: 3 VA/sq ft × floor area
2. Small appliance: 3,000 VA minimum (2 circuits × 1,500)
3. Laundry: 1,500 VA
4. Apply demand factor to #1-3 (Table 220.42)
5. Add ranges, dryers (with demand factors)
6. Add HVAC, water heater @ 100%
7. Add fastened-in-place appliances @ 75% (if 4+)
8. Total VA ÷ Voltage = Minimum service ampacity

Exam Alert

Load calculation problems appear on almost EVERY exam. Master the sequence and demand factors cold. Practice with sample calculations.

Article 250: Grounding and Bonding

Requirements for system and equipment grounding

Critical

Key Concepts

250.4 General Requirements:
- Grounded systems: Limit voltage from lightning, surges; stabilize voltage during normal operation
- Equipment grounding: Provide low-impedance path for fault current to facilitate overcurrent device operation
250.24 Grounding Service-Supplied Systems:
- Grounded conductor brought to service disconnect
- Main bonding jumper connects grounded conductor to equipment grounding conductor and service enclosure
250.52 Grounding Electrodes: Types include:
- Metal underground water pipe (≥10 ft in direct contact with earth)
- Metal frame of building or structure (if effectively grounded)
- Concrete-encased electrode (Ufer ground): #4 AWG copper, 20 ft minimum
- Ground rod(s): 8 feet minimum depth
250.53 Grounding Electrode System Installation:
- All available electrodes must be bonded together
- Supplemental electrode required if sole electrode is rod/pipe/plate
- Rod resistance > 25 ohms: add second rod, minimum 6 feet apart
250.66 Size of Grounding Electrode Conductor (GEC):
- Based on largest service-entrance conductor (Table 250.66)
- Example: 2/0 AWG service = #4 copper GEC
- GEC to concrete-encased electrode: #4 copper minimum
- GEC to ground rod: #6 copper maximum required
250.122 Size of Equipment Grounding Conductor (EGC):
- Based on rating of overcurrent device (Table 250.122)
- Example: 20A breaker = #12 copper EGC minimum
- If circuit conductors upsized, EGC must be proportionally increased

Quick Reference: Common EGC Sizes (Table 250.122)

15A: #14 Cu

20A: #12 Cu

30A: #10 Cu

40-60A: #10 Cu

70-100A: #8 Cu

110-200A: #6 Cu

225-300A: #4 Cu

Grounding vs. Bonding

Grounding: Connection to earth (GEC to electrode)
Bonding: Connection between conductive parts to maintain common potential (EGC, bonding jumpers)

Article 310: Conductors for General Wiring

Conductor ampacity, insulation, and installation requirements

High-Frequency

Key Concepts

310.12 Conductor Identification:
- Grounded (neutral): white or gray (or white stripe on other than green)
- Equipment grounding: green, green with yellow stripe, or bare
- Ungrounded (hot): any color except white, gray, or green
310.15 Ampacities: Based on Table 310.16 (formerly Table 310.15(B)(16))
- Max OCPD per 240.4(D): #14 Cu = 15A, #12 Cu = 20A, #10 Cu = 30A. Ampacity at 75°C: #8 Cu = 50A
- Ambient temperature correction factors apply (> 86°F or > 30°C)
- Adjustment factors for > 3 current-carrying conductors in raceway/cable
310.15(B)(3)(a) Adjustment Factors: More than 3 current-carrying conductors:
- 4-6 conductors: 80% of ampacity
- 7-9 conductors: 70%
- 10-20 conductors: 50%
- Neutrals carrying only unbalanced current don't count
310.10(H) Temperature Limitations:
- Conductor ampacity based on lowest temperature rating in circuit
- 60°C terminations common on 15A-100A breakers
- 75°C terminations on equipment rated > 100A
- Use 75°C column for most calculations (unless otherwise specified)

Quick Reference: Copper Conductor Ampacity (75°C)

#14: 20A*

#12: 25A*

#10: 35A

#8: 50A

#6: 65A

#4: 85A

#3: 100A

#2: 115A

#1: 130A

#1/0: 150A

#2/0: 175A

#3/0: 200A

*Limited by 240.4(D): #14 = 15A max, #12 = 20A max for branch circuits

Exam Tip

Memorize the ampacity table for common sizes (#14 through #3/0). Many exam questions test this directly or require it for load calculations.

Article 430: Motors, Motor Circuits, and Controllers

Requirements for motor installations and protection

Critical

Key Concepts

430.6 Ampacity/Motor Rating Determination:
- Use Table 430.247-250 for full-load current (FLA)
- Do NOT use nameplate rating for conductor/switch sizing
- Nameplate only used for overload protection sizing
430.22 Conductor Size - Single Motor:
- Conductors must be ≥125% of motor FLA (from tables)
- Example: 10 HP, 230V, 3-phase motor = 28A FLA → 28A × 1.25 = 35A → #8 Cu (50A)
430.24 Conductor Size - Multiple Motors:
- Conductors = (largest motor FLA × 125%) + sum of all other motor FLAs
430.32 Overload Protection:
- Based on nameplate current rating
- Service factor ≥1.15: 125% of nameplate
- Service factor < 1.15: 115% of nameplate
430.52 Branch-Circuit Short-Circuit and Ground-Fault Protection:
- Inverse time breaker: maximum 250% of motor FLA
- Dual-element fuse: maximum 175% of motor FLA
- Instantaneous trip breaker: maximum 800-1300% (Table 430.52)
- If standard size doesn't allow motor to start, next size up permitted
430.110 Disconnecting Means:
- Must disconnect motor and controller from all ungrounded conductors
- Rated ≥115% of motor FLA
- Must be in sight of controller (or lockable)

Motor Circuit Calculation Example

10 HP, 230V, 3-phase motor:
1. FLA from Table 430.250: 28A
2. Conductor ampacity: 28A × 1.25 = 35A → #8 Cu (50A)
3. Overload (SF 1.15): 28A × 1.25 = 35A
4. Branch circuit protection (inverse time breaker): 28A × 2.5 = 70A max

Exam Alert

Motor problems are complex but predictable. Always use Table values (not nameplate) for conductors/switches. Master the 125% multiplier and Table 430.52 percentages.

Article 240: Overcurrent Protection

Protecting conductors and equipment from excessive current

Critical

Key Concepts

240.4 Protection of Conductors:
- Conductors must be protected per their ampacity (Table 310.16)
- Overcurrent device (breaker/fuse) must not exceed conductor ampacity
- 240.4(D) Small Conductors: #14 Cu max 15A, #12 Cu max 20A, #10 Cu max 30A
- Next size up rule: If conductor ampacity doesn't match standard OCPD size AND loads are continuous, you may use next size up (max 800A)
240.6 Standard Ampere Ratings:
- 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110...
- Memorize common sizes: 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 175, 200
240.21 Location in Circuit: Overcurrent protection must be at point where conductor receives supply
- Tap rules allow limited distances without protection at tap point
- 10-foot tap rule: 10 feet max, ampacity ≥ combined loads, terminates in single breaker/set of fuses
- 25-foot tap rule: 25 feet max, ampacity ≥ 1/3 of feeder OCPD rating
240.24 Location of Overcurrent Devices:
- Must be readily accessible (not requiring ladder or removal of obstacles)
- Not in bathrooms (dwelling units)
- Not above stairs unless headroom ≥ 6½ feet
- Not near easily ignitable material

Standard OCPD Sizes (240.6)

15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 400A...

Exam Tip

Know the small conductor limits cold: #14=15A, #12=20A, #10=30A. These appear on almost every exam.

Article 300: General Requirements for Wiring Methods and Materials

Installation requirements applicable to all wiring methods

High-Frequency

Key Concepts

300.3 Conductors:
- All conductors of same circuit must be in same raceway/cable
- Prevents inductive heating from unbalanced magnetic fields
- Grounded, ungrounded, and EGC must run together
300.4 Protection Against Physical Damage:
- Cables through wood framing: 1¼" from face of stud OR steel plate protection
- Cables through metal framing: bushings/grommets required
- Cables parallel to framing: minimum 1¼" from edge OR protected
300.5 Underground Installations:
- Minimum burial depths vary by wiring method (Table 300.5): Direct burial cable = 24"; PVC conduit = 18"
- 120V GFCI-protected 20A circuits: 12" minimum burial depth
- Rigid metal conduit = 6" minimum (all locations)
- Emerging from grade: protect to 8 feet above grade
300.14 Length of Free Conductors:
- Minimum 6 inches of free conductor at outlet/junction boxes
- Must extend at least 3 inches outside box opening
300.22 Wiring in Ducts, Plenums, and Air-Handling Spaces:
- Limited wiring methods allowed in air-handling spaces
- Wiring above suspended ceilings used for environmental air: Must use plenum-rated cable

Common Mistake

Students often confuse burial depths. Memorize the table! Residential driveway ≠ street. Metal conduit gets shallow burial everywhere.

Article 334: Nonmetallic-Sheathed Cable (Romex)

Type NM, NMC, and NMS cable installation

High-Frequency

Key Concepts

334.10 Uses Permitted:
- One- and two-family dwellings
- Multifamily dwellings (Type III, IV, V construction)
- Concealed or exposed work (in dry locations for Type NM)
- Type NMC: Permitted in damp/corrosive locations
334.12 Uses Not Permitted:
- Commercial garages (hazardous locations)
- Theaters and assembly occupancies (except dwelling portions)
- Motion picture studios
- Embedded in concrete, masonry, or aggregate
- Type NM in damp/wet locations
334.15 Exposed Work:
- Must follow building surface closely
- Protected from physical damage where subject to it
- Run through bored holes in joists, studs (1¼" from edge rule applies)
334.23 In Accessible Attics:
- Run across top of joists: Guard strips if within 7 feet of floor
- Run through joists/rafters: No protection needed
334.30 Securing and Supporting:
- Secured every 4½ feet
- Within 12 inches of every box, cabinet, fitting (except fixture whips)
- Two-wire cable with non-metallic sheath: Staples must not damage cable

Exam Tip

Romex questions focus on where it CAN'T be used. Know the prohibited locations and the 4½-foot/12-inch securing rules.

Article 680: Swimming Pools, Fountains, and Similar Installations

Special requirements for pool and spa installations

Special

Key Concepts

680.6 GFCI Protection: All electrical equipment associated with pool must be GFCI protected
680.7 Grounding and Bonding:
- Equipotential bonding required around pool
- Bond all metal parts within 5 feet of pool: Ladder, handrails, lights, pumps, metal surfaces
- Bonding conductor: #8 copper (solid, insulated, covered, or bare)
- Bonding grid: Encircles pool, 18-24" from inside wall, 4-6" below surface
680.22 Lighting Around Pools:
- Overhead lighting > 12 feet above max water level: No special rules
- 5-12 feet horizontally: Must be GFCI protected, installed ≥5 feet above max water level, and rigidly attached
- < 5 feet horizontally: Must be suitable for wet location + GFCI
680.26 Equipotential Bonding (Permanently Installed Pools):
- Metal parts within 5 feet of pool
- Metal fittings, ladder, slide, handrails
- Pool water (via stainless steel fitting)
- Does NOT require connection to equipment grounding conductor
680.42 Outdoor Spa/Hot Tub Receptacles:
- At least one 125V receptacle, 15-20A, GFCI protected, 6-10 feet from spa
- All receptacles within 10 feet must be GFCI protected

Safety Critical

Pool bonding saves lives. Every metal part within 5 feet must be bonded together. This is separate from grounding — it's about equalizing potential.

Article 690: Solar Photovoltaic (PV) Systems

Requirements for solar electric installations

Special

Key Concepts

690.4 General Requirements:
- PV systems must comply with general NEC + Article 690
- Maximum system voltage: Calculated per 690.7
690.7 Maximum Voltage:
- Based on open-circuit voltage (Voc) × temperature correction factor
- Crystalline silicon: Voc × 1.25 (cold weather increases voltage)
- For 600V systems: Max Voc after correction cannot exceed 600V
690.8 Circuit Sizing and Current:
- PV source circuit current = Module Isc × 125%
- PV output circuit current = Inverter continuous output × 125%
- Always multiply by 125% for continuous operation
690.12 Rapid Shutdown:
- PV systems on buildings must have rapid shutdown capability
- Controlled conductors must be limited to 80V within 30 seconds
- Controlled conductors > 1 foot from array: Limited to 80V within 30 seconds of shutdown
690.31 Wiring Methods:
- PV source/output circuits: Must use Chapter 3 wiring methods
- Exposed single conductors: Only in conduit or cable assemblies
- Common: PV wire, USE-2, THWN-2 in conduit
690.53 Direct Current (DC) Combining:
- Fuses or circuit breakers for each PV source circuit
- DC-rated overcurrent devices required

Growing Field

Solar installations are booming. Know the 125% multiplier for PV calculations and rapid shutdown requirements (new safety mandate).

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Study Tips

✓ Use this guide alongside your NEC code book for open-book exam practice
✓ Bookmark frequently used tables (110.26, 220.42, 250.66, 250.122, 310.16, 430.52)
✓ Practice finding articles quickly — speed matters on timed exams
✓ Work through calculation problems by hand, then verify with code
✓ Review Test Day Success Guide before your exam