Maryland Master Electrician

Key Concepts

Ohm's Law is the foundation of every calculation question in this section: - V = I × R (Voltage = Current × Resistance) - I = V / R - R = V / I

Power Formulas (Watt's Law): - P = V × I - P = I² × R - P = V² / R

AC Circuit Theory: - Impedance (Z): Total opposition to AC current flow; combines resistance (R) and reactance (X). Z = √(R² + X²) - Inductive Reactance: X_L = 2πfL (f = frequency in Hz, L = inductance in Henrys) - Capacitive Reactance: X_C = 1 / (2πfC) - Power Factor (PF): PF = True Power (W) / Apparent Power (VA) = cos θ; ranges 0 to 1 - True Power (W) vs. Apparent Power (VA) vs. Reactive Power (VAR): Know the power triangle relationship.

NEC Articles 100, 110, 200–210, 215–225, 230, 240, 250–285 | 14 Questions (16%)

Key Concepts

Article 100 – Definitions Know these cold: Ampacity (max current a conductor can carry continuously without exceeding its temperature rating), Branch Circuit (conductors between the final OCPD and the outlet), AFCI (protects against arc-fault characteristics), Bonding vs. Grounding (bonding = electrical continuity; grounding = intentional connection to earth), Readily Accessible (no tools, ladders, or obstacles required), and AHJ (authority responsible for enforcing code).

Article 110 – Requirements for Electrical Installations - Working space in front of electrical equipment: 110.26 — minimum 3 ft deep, 30 in. wide (or equipment width, whichever is greater), 6½ ft high for equipment 600V or less. - Dedicated equipment space above panelboards extends 6 ft above the equipment or to the structural ceiling, whichever is lower. - All equipment must be listed (110.3) and installed per its listing and labeling instructions. - Conductor temperature ratings: most terminals rated 60°C or 75°C — use the lower of the terminal or conductor rating.

Key Concepts

Article 300 — General Requirements

• 300.3(A): Single conductors must be installed as part of a recognized wiring method. They cannot be used arbitrarily on their own.
• 300.3(B): All conductors of the same circuit — including grounded conductors, EGCs, and bonding conductors — must be contained within the same raceway, cable tray, trench, cable, or cord. This prevents inductive heating from canceling magnetic fields.
• 300.3(B)(1): Parallel conductor installations must keep all parallel conductors of the same phase/neutral within the same raceway (each raceway treated separately).
• 300.3(C)(1): AC and DC conductors rated 1000V or less may share the same raceway, but all conductors must be insulated to the highest voltage present.
• 300.2(A): Chapter 3 wiring methods apply to 1000V or less unless a section specifically permits higher voltage.
• 300.4(A): Cables and raceways through wood framing must be at least 32 mm (1¼ in.) from the nearest edge of a stud or joist. If that distance can't be maintained, a 1.6 mm (1/16 in.) steel plate is required.…

Key Concepts

Flexible Cords & Cables (Article 400) - Flexible cords must be suitable for conditions of use and location (400.3). Type and construction must match Table 400.4. - Ampacity tables: Use Table 400.5(A)(1) and (2) for up to 3 current-carrying conductors; apply derating from Table 400.5(A)(3) when more than 3 current-carrying conductors are present. - Neutral conductors carrying only unbalanced current do NOT count as current-carrying conductors. - On a 4-wire, 3-phase wye circuit where more than 50% of the load is nonlinear, the neutral IS a current-carrying conductor. - Temperature correction factors from Table 310.15(B)(1) apply when ambient exceeds 30°C (86°F). Cords rated 105°C use the 90°C column for correction. - Equipment grounding conductors are never counted as current-carrying conductors.

Key Concepts

Hazardous Location Classification (Articles 500–504)

Hazardous locations are classified by the type of hazard (Class), likelihood of hazard being present (Division or Zone), and specific material (Group).

• Class I — Flammable gases, vapors, or flammable liquid-produced vapors (e.g., gasoline, propane)
• Class II — Combustible dusts (e.g., grain, coal, metal dust)
• Class III — Ignitible fibers/flyings (e.g., sawdust, textile lint)

Divisions: - Division 1 — Hazardous material present under normal operating conditions - Division 2 — Hazardous material present only under abnormal conditions (failure, accident)

Documentation (500.4): All hazardous classified locations must be properly documented and that documentation must be available to anyone authorized to design, install, inspect, maintain, or operate equipment there.

Key Concepts

Article 600 – Electric Signs and Outline Lighting

• Listing required (600.3): All fixed, mobile, or portable electric signs, section signs, outline lighting, PV-powered signs, and retrofit kits must be listed and labeled regardless of voltage. Exceptions: field-installed skeleton tubing (not required to be listed) and outline lighting consisting of listed luminaires wired per Chapter 3.
• Markings (600.4): Signs must be marked with manufacturer's name/trademark, input voltage, and current rating. Signs with retrofitted illumination systems must be marked that the illumination system was replaced, including the kit provider and installer's identifier. Signs with incandescent lampholders must show maximum lamp wattage in letters at least 6 mm (¼ in.) high, visible during relamping.
• Disconnecting means (600.6): Each sign must have an externally operable disconnect within sight or capable of being locked in the open position. Location must allow service/maintenance personnel to safely work on the equipment.

Article 610 – Cranes and Hoists

Key Concepts

Article 700 – Emergency Systems

Emergency systems must restore power within 10 seconds of normal supply failure (700.12). This is the hardest deadline in the NEC — no exceptions without auxiliary power bridging the gap.

Sources of power (700.12): - Storage batteries must supply the load for a minimum of 1½ hours without voltage dropping below 87½% of normal. Automotive-type batteries are prohibited. - Generator sets (internal combustion): Must have an on-site fuel supply sufficient for not less than 2 hours of operation. Prime movers cannot rely solely on a public utility gas system or municipal water for cooling. - If a generator takes more than 10 seconds to come online, an auxiliary power supply must bridge the gap (700.12(D)(4)).

Wiring requirements (700.10): - Emergency circuit wiring must be kept entirely independent of other wiring. - In specific occupancies (assembly >1,000 persons; buildings >75 ft; educational >300 occupants), feeder-circuit wiring must meet one of: - Protected by approved automatic fire protection system - Listed electrical circuit protective system with **2-hour fire…

Key Concepts

Branch-Circuit & Feeder Loads (Article 220) - Dwelling unit general lighting load: 3 VA per sq ft (Table 220.12) - Small appliance circuits: 1,500 VA each, minimum two required (220.52(A)) - Laundry circuit: 1,500 VA (220.52(B)) - Demand factors for general lighting in dwellings (Table 220.42): First 3,000 VA at 100%; 3,001–120,000 VA at 35%; remainder at 25% - Optional method (220.82–220.85): Applies to single-family dwellings; uses 100% of first 10 kVA of total load, then 40% of remainder. Air conditioning vs. heating — use the larger load only. - Multifamily demand factors: Apply Table 220.84 based on number of units.

Motor Circuit Calculations (Article 430) - Single motor conductor sizing: 125% of FLA (430.22) - Multiple motors on one feeder: 125% of largest motor FLA + 100% of all others (430.24) - Overload protection (430.32): Standard motors — 115% of FLA for motors with SF ≥ 1.15 or motors marked with a temperature rise ≤ 40°C; all others 125% of FLA - If overload device is not adequate, increase to 140%…