DM Electrical Exam cheat sheet

Ohm's law: E = I × R; power P = E × I = I²R = E²/R (DC resistive circuits).

Series circuit: R_total = R1 + R2 + …; current is identical through every element; voltage divides in proportion to resistance.

Parallel circuit: 1/R_total = 1/R1 + 1/R2 + …; voltage is identical across branches; branch currents sum to total current.

AC impedance: Z = √(R² + X²); inductive reactance X_L = 2πfL (lags 90°), capacitive reactance X_C = 1/(2πfC) (leads 90°).

Three-phase power: P = √3 × E_line × I_line × PF for a balanced load; single-phase P = E × I × PF.

Transformer voltage ratio equals turns ratio: E_primary/E_secondary = N_primary/N_secondary; power in ≈ power out (minus losses).

Conductor resistance rises with length and temperature and falls with increased circular-mil area: R = K × L / CM.

Effective (RMS) value of a sine wave = 0.707 × peak; average value = 0.637 × peak.

NEC is organized into Chapters 1–9; Chapter 1–4 rules apply generally, Chapters 5–7 cover special occupancies, equipment, and conditions, Chapter 9 holds tables.

Article 100 definitions govern throughout: a branch circuit is the circuit conductors between the final overcurrent device and the outlets.

Continuous loads are calculated at 125% of their rated current for branch-circuit and feeder sizing (NEC 210.19, 215.2).

Standard overcurrent device sizes per NEC 240.6 include 15, 20, 30, 40, 50, 60, 70, 100, 125, 150, 200 A.

Working clearance at equipment is governed by NEC 110.26 Table 110.26(A)(1): 3 ft for 0–150 V to ground, increasing with voltage and condition.

GFCI protection is required for personnel in bathrooms, garages, outdoors, kitchens, and similar wet or damp locations per NEC 210.8.

Conductor ampacity tables are NEC 310.16; ampacities are adjusted for ambient temperature and number of current-carrying conductors (310.15).

The NEC is not a design manual or installation specification; it is the minimum standard for safe installation, and local amendments may be stricter.

Current, not voltage, causes physiological damage; roughly 1 mA is perceptible, 10–20 mA causes muscle lock, and 100+ mA can be fatal.

Body resistance drops sharply when skin is wet, so shock severity rises with moisture, contact area, and duration.

Arc flash incident energy depends on available fault current, clearing time, and working distance; boundaries are defined in NFPA 70E.

Approach boundaries: limited, restricted, and arc flash boundary; energized work requires an energized electrical work permit unless justified as infeasible de-energized.

Lockout/tagout requires a verified zero-energy state, a lock per worker, and a test with a meter rated for the system voltage (CAT III/IV).

PPE categories are selected from an arc-rated clothing system matched to the calculated incident energy, not to voltage alone.

Class C fires involve energized electrical equipment; a conductive extinguishing agent must not be used until power is removed.

Grounding and bonding limit touch potential; equipment grounding conductors carry fault current back to the source to trip the overcurrent device.

NM-B cable is permitted in dry, protected locations; it is prohibited in wet locations, in masonry cavities, and where subject to physical damage.

EMT requires support within 3 ft of a box and at intervals not exceeding 10 ft; rigid metal conduit intervals are 10 ft with 3-ft box support.

Box fill is calculated per NEC 314.16 by counting conductors, devices, and grounds as volume allowances against the box's cubic-inch rating.

Conduit fill per Chapter 9 Table 1: 1 conductor 53%, 2 conductors 31%, over 2 conductors 40% of interior cross-sectional area.

Bend radius and conduit body sizing are governed by NEC 344/358 and Chapter 9 Table 2; total bends between pull points are limited to 360°.

Wire ampacity depends on insulation temperature rating (60/75/90 °C) and termination rating; the lower of conductor and terminal ratings governs.

Cable and raceway entries into enclosures require listed connectors; unused openings must be closed to maintain the enclosure rating.

Expansion, seismic, and corrosion considerations dictate fitting selection; dissimilar metals require compatible connectors to avoid galvanic corrosion.

Motor branch-circuit short-circuit protection is sized per NEC 430.52 (e.g., inverse-time breaker typically up to 250% of FLC), separate from overload protection.

Motor overload protection protects the motor from sustained overcurrent; the running overload device is sized to the motor's service factor and temperature rise.

Contactors switch load current; starters add overload relays; a disconnect must be within sight of the motor controller per NEC 430.102.

A transformer's primary protection is sized per NEC 450.3 based on transformer rating and primary/secondary voltage; secondary protection may be required.

Switch and receptacle ratings must not be less than the circuit's ampacity; a 15 A receptacle may be used on a 20 A circuit only where permitted by 210.21(B).

Control circuits are classified by voltage and source; a Class 2 circuit is power-limited and must be separated from power conductors per Article 725.

Device terminal markings (screw color, back-wire clamps) indicate conductor material and torque requirements; terminations must be torqued to the manufacturer's specification.

GFCI and AFCI devices protect against different hazards: GFCI against ground-fault shock, AFCI against arc-fault ignition.

Load calculation per NEC Article 220 uses general lighting VA per square foot, demand factors, and appliance/receptacle additions.

Standard dwelling lighting load is 3 VA/ft²; small-appliance and laundry branch circuits are 1,500 VA each, subject to demand factors.

Service and feeder sizing uses the calculated load divided by the supply voltage; account for the largest motor at 125% per NEC 430.24.

Voltage drop approximation: VD = (2 × K × I × L) / CM for single-phase, and √3 factor for three-phase; 3% branch and 5% total are common targets.

Panel schedule planning includes spare capacity, breaker space for future loads, and balancing phase loads to limit neutral current.

Estimating requires takeoff of conduit, conductors, boxes, devices, and labor units; labor units are typically expressed per device or per 100 ft.

Coordination studies set breaker trip settings so downstream devices clear faults before upstream devices, minimizing outage scope.

Short-circuit and available fault current studies verify equipment interrupting ratings (AIC) exceed the calculated available fault current.

A meter's CAT rating must meet or exceed the installation's overvoltage category; CAT III/IV for service and distribution equipment.

Continuity and resistance tests are performed with power off; voltage tests are performed with power on and the meter on the correct function.

Insulation resistance testing (megohmmeter) applies a DC test voltage and reads resistance in megohms; readings trend downward indicate degradation.

A clamp-on ammeter measures current without breaking the circuit; it must be clamped around a single conductor to read that conductor's current.

Ground fault location uses a ground-fault clamp or divided-circuit testing; a neutral-to-ground bond downstream of the service causes nuisance tripping.

Motor troubleshooting sequence: verify voltage at the disconnect, check overloads, measure winding resistance, then test insulation to ground.

Thermographic inspection identifies loose connections and overloaded conductors by temperature rise under load.

Preventive maintenance includes torque checks, cleaning, insulation testing, and exercising breakers and switches on a defined schedule.