National Electrical Code (NFPA 70)
Comprehensive Study & Field Reference

• 1. Scope & Fundamentals
• 2. Definitions & Key Articles
• 3. Wiring Methods & Materials
• 4. Conduit, Raceway & Box Fill
• 5. Cable Types, Uses & Markings
• 6. Grounding & Bonding
• 7. Services, Feeders, Branch Circuits
• 8. Load Calculations
• 9. Low Voltage & Fire Alarm (Art. 725, 760, 770)
• 10. Boxes, Devices & Special Occupancies
• 11. Inspection, Testing & Safety
• 12. Examples, Field Scenarios & Quick Reference

1. Scope & Fundamentals

• NEC is the standard for safe installation of electrical wiring and equipment in the US.
• Covers 600V and below (most chapters); special articles for higher voltages, low voltage, and signaling.
• Not retroactive unless specifically stated; existing installations may be “grandfathered” if safe.
• AHJ (Authority Having Jurisdiction) can approve alternate methods if safety is assured.
• NEC is updated every three years; always check your local/state amendments!

2. Definitions & Key Articles

Term	Definition	Code Article
Branch Circuit	Conductors between final overcurrent device and outlets	Art. 100, 210
Feeder	Between service equipment and branch circuit panels	Art. 100, 215
Service	Entrance conductors and equipment to utility supply	Art. 100, 230
Grounded (Neutral) Conductor	System conductor intended to carry current under normal operation	Art. 100, 200
Bonding	Permanent joining of conductive parts to ensure continuity	Art. 100, 250
Overcurrent Device	Protects circuit by opening when current is excessive	Art. 100, 240
Raceway	Enclosed channel (conduit, tray, duct, etc.)	Art. 100, 300

• Key Articles:
  • 110 – Requirements for Electrical Installations
  • 210 – Branch Circuits
  • 250 – Grounding and Bonding
  • 300 – Wiring Methods
  • 725 – Class 1, 2, 3 Circuits
  • 760 – Fire Alarm Systems
  • 770 – Optical Fiber

3. Wiring Methods & Materials

• Art. 300 covers general requirements for wiring methods; specific types in later articles.
• Approved wiring methods:
  • Nonmetallic-sheathed cable (NM, Romex), conduit (EMT, IMC, RMC), flexible cable, MC cable, raceways, surface metal/plastic raceways.
• Minimum wire size for power/light: #14 AWG copper (#12 for 20A circuits, #10 for 30A).
• All conductors of the same circuit must be run together in the same raceway/cable (no separating neutrals/grounds).
• Insulation types: THHN, THWN, XHHW, UF, etc. (see Table 310.16 for ampacity & temperature ratings).

Cable/Conduit	Common Uses	Notes
NM (Romex)	Dry, residential only	Not in wet or exposed
MC	Commercial, exposed, some wet	Must be listed, armor is not ground
EMT	Conduit, indoor/commercial	Not for direct burial
PVC	Underground, wet areas	Must be sunlight-resistant, sch. 40/80

Installer Tip: Mark all wires at terminations. Never strip more than code allows; all unused box knockouts must be plugged.

4. Conduit, Raceway & Box Fill

• Conduit Types: EMT (thinwall), RMC (rigid), IMC, FMC (flex), LFMC (liquidtight flex), PVC, ENT (smurf tube).
• Fill:
  • Max 40% fill for one conductor, 31% for two, 53% for more (see Table 1, Ch. 9 and Annex C for trade sizes).
• Support spacing: EMT every 10 ft, within 3 ft of boxes; FMC/LFMC every 4.5 ft.
• Box Fill:
  • Box volume must equal or exceed required volume for wires/devices; each #14 wire = 2 cu in, #12 = 2.25 cu in, each device = 2x wire count.
  • Ground wire(s) count as one, no matter how many.
• Fittings must be listed for the raceway and installed per manufacturer instructions (e.g. locknuts, bushings).

Wire Gauge	Single Device Box (cu in)	Double Device Box (cu in)
#14	18	30
#12	20	32
#10	22	34

5. Cable Types, Uses & Markings

• NM (Romex): dry, protected; MC: commercial, flexible; AC: armored; UF: direct burial, wet.
• Article 310: Insulation types and temperature ratings. THHN/THWN = 90°C dry/75°C wet; XHHW/XHHW-2 = high temp, wet/dry.
• Cable ampacity derated for ambient temps >86°F, >3 conductors in raceway/cable (see Table 310.16, 310.15).
• Markings: All cables must show type, size, voltage, temp rating, and manufacturer.
• Voltage drop: Max 3% on branch, 5% total (branch + feeder).

Exam Note: All wire splices must be in accessible junction boxes – no buried splices allowed.

6. Grounding & Bonding

• Article 250 governs grounding and bonding requirements.
• Grounding Electrode: Ground rod, UFER (rebar in concrete), metal water pipe, or building steel.
• Grounding electrode conductor size based on largest service conductor (see Table 250.66).
• Bonding: Ensures electrical continuity for all metallic enclosures, raceways, and water/gas piping.
• Grounded (neutral) conductors must be isolated from equipment grounds after main service disconnect.
• Color Codes: Green/bare = ground, white/gray = neutral, other = hot (black, red, blue, etc.).
• All subpanels: separate neutral and ground bars; do NOT bond ground and neutral after main.

Safety: “Bootleg” grounds (neutral-to-ground jumpers in subpanels) are a major code violation and shock hazard!

7. Services, Feeders, Branch Circuits

• Service entrance conductors must be rated for the calculated load and protected by properly sized main OCPD (overcurrent protective device).
• Main disconnect required at service entrance (grouped if multiple); must be labeled and accessible.
• Feeder conductors: run from service to subpanels, sized per calculated load, derated for length, temperature, and number of conductors.
• Branch circuits: Supply outlets/devices; most residential 15A (#14) or 20A (#12), commercial often 20A or 30A.
• Dedicated Circuits: Certain loads (A/C, dryers, kitchen appliances) require dedicated circuits.
• GFCI: Required for bathrooms, kitchens, garages, outdoors, crawl spaces, unfinished basements, laundry.
• AFCI: Required for all 120V, 15A/20A outlets in habitable rooms (living, beds, family, dining, dens, etc.).
• Receptacle Spacing: Max 12 ft apart; wall >2 ft wide must have an outlet.

Room/Area	GFCI?	AFCI?
Bathroom	Yes	No
Kitchen	Yes (all counters)	Yes
Bedroom	No	Yes
Garage	Yes	No
Outdoor	Yes	No

Field Tip: All OCPDs must be marked for “series rating” if used for that purpose. Never install larger breaker “just to stop nuisance trips.”

8. Load Calculations

• NEC requires proper load calculations for all services/feeders/branch circuits (Article 220).
• Demand factors reduce total calculated load (see Table 220.42, 220.44, 220.56).
• Lighting load (general): 3 VA/ft² for dwelling; higher for commercial (see Table 220.12).
• Small appliance/countertop: Minimum 2 20A circuits for kitchens/dining/laundry.
• Motor circuits: Must include locked-rotor (starting) current and continuous load adjustment (125%).
• Voltage Drop: Not code, but recommended max 3% branch/feeder, 5% total.

Calculation	Formula/Notes
Service/feeder size (A)	Total VA ÷ voltage ÷ 0.8 (for continuous load)
Branch circuit min size	Load (A) × 1.25 for continuous, plus largest motor
VA to amps (single phase)	VA ÷ volts = A
VA to amps (three phase)	VA ÷ (volts × √3) = A

Exam Example: 1200 ft² house, 3 VA/ft² = 3600 VA, plus kitchen/laundry circuits, appliances, HVAC, water heater, etc.

9. Low Voltage & Fire Alarm (Art. 725, 760, 770)

• Article 725: Class 1, 2, 3 circuits (control, security, access, HVAC, etc.).
• Article 760: Fire Alarm
  • PLFA (Power-Limited Fire Alarm): Must use listed FPL/FPLR/FPLP cable
  • Conduit not always required unless subject to physical damage; follow local code
  • Red color not strictly required by code, but almost universally enforced
• Article 770: Optical Fiber
  • OFNR/OFNP: Nonconductive riser/plenum fiber cable
• Low voltage must not share conduit with power conductors except as allowed by listing
• Plenum-rated cable (CMP) required for return-air spaces; riser (CMR) for floor-to-floor unless in conduit

Fire Alarm Tip: Use EOL (end-of-line) resistors for zone supervision on Class B circuits. See your fire alarm plans and NFPA 72 for spacing and device requirements!

10. Boxes, Devices & Special Occupancies

• All splices, taps, and devices must be in listed, accessible boxes (no hidden/covered splices allowed).
• Outlet boxes: Must be listed and sized for fill (see Ch. 9, Table 314.16(A)).
• Device mounting: Faceplates must cover box openings, be flush or extend to finished surface.
• Wet/damp locations: Use weatherproof boxes and covers; listed GFCI for protection.
• Hazardous locations (Art. 500+): Use explosion-proof boxes, conduit, and seals as required by class/division/group.
• Patient care (Art. 517): Hospital wiring, redundant ground, isolated circuits in critical areas.

Inspection: “No ground wire in box” = automatic fail. Hospital, commercial kitchens, and hazardous locations require special hardware and inspection.

11. Inspection, Testing & Safety

• All installations must be inspected and approved by AHJ before cover-up.
• Tests:
  • Continuity (all conductors, grounds, and neutrals)
  • Insulation resistance (megger for >1MΩ preferred for critical/industrial circuits)
  • Voltage, polarity, and GFCI/AFCI function (use outlet tester and breaker trip test)
• Lockout/Tagout: Required for all work on energized systems (OSHA 1910, NEC 110.25).
• PPE: Arc-rated clothing, gloves, and shields for working live above 50V.
• Label all panels/circuits. Directory required in all panelboards (NEC 110.22).

Field Safety: “Test before you touch.” Never trust a label or someone else’s word—verify de-energized with a meter.

12. Examples, Field Scenarios & Quick Reference

Example 1: Branch Circuit Design (Residential)

• Kitchen: Two 20A circuits for countertop/outlets, GFCI protected, AFCI at panel
• Lighting: 15A, #14, spaced per NEC, switches at all entries
• Refrigerator: Dedicated 15A or 20A circuit

Example 2: Fire Alarm NAC in Conduit

• Run 16AWG FPLP cable in 3/4" EMT with power conductors (separated by barrier if needed)
• Calculate voltage drop for 300 ft run, 1.2A: If voltage at last device >16V, pass; if not, upsize wire or split circuit

Example 3: Grounding/Bonding in Subpanel

• Ground and neutral separate after main service
• Install green bonding screw or strap for equipment ground only
• All panelboards labeled with panel and circuit directory

Example 4: MC Cable in Commercial Ceiling

• Supports every 6 ft, bends no tighter than 7x cable diameter
• Armor not considered equipment ground unless marked; green ground required
• Junction boxes accessible above ceiling tile; do not cover or bury

Example 5: Voltage Drop Calculation (Lighting Circuit)

• Run: 100 ft, #12 AWG, load: 12A, voltage: 120V
• Voltage drop: VD = 2 × K × I × D / CM
  (K=12.9 for copper, I=12A, D=100ft, CM=6530 for #12 AWG)
• Result: ~2.4V drop, within 3% allowable (3.6V for 120V)

Quick Reference

Requirement	NEC Value	Field Note
Service Ground Rod	8 ft min, ≤25Ω or supplement	Always use two rods unless you test!
Panel Working Space	36" deep × 30" wide × 78" high	No storage in working space
Outlet Box Fill	2.0 cu in (#14), 2.25 cu in (#12)	Each device = 2x wire fill
Conductor Ampacity	See Table 310.16	Adjust for ambient, # of wires
Minimum burial depth	18" for direct burial, 24" for PVC	Check local amendments!

• Voltage Drop Formula:
  VD = 2 × K × I × D / CM
  K=12.9 (copper), I=current (A), D=distance (ft), CM=circular mils (from NEC Table 8)
• Box Fill:
  • Each #14 wire: 2 cu in, #12: 2.25 cu in
  • Each device: 2x wire count
• All terminations: Torque to manufacturer specs; do not exceed.
• Test, tag, and document all GFCI/AFCI devices at install and during annual checks.