Craftsman Electricity flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

Voltage equals current multiplied by resistance. In a resistive circuit, V = I × R. Rearranged, I = V/R and R = V/I. It is the foundation for all circuit analysis.

Three-phase power formula

For a balanced three-phase load, power in watts equals the square root of three times line-to-line voltage times line current times power factor: P = √3 × V × I × PF.

Series circuit resistance

Total resistance is the sum of individual resistances: R_total = R1 + R2 + R3. Current is identical through every element, and voltage divides in proportion to each resistance.

Parallel circuit current

Voltage is the same across each branch, and total current is the sum of branch currents. Branch current divides inversely with branch resistance, so the lowest resistance carries the most current.

Inductive reactance

Opposition to AC current from an inductor, calculated as X_L = 2πfL. It rises with frequency and inductance. It stores energy in a magnetic field and lags current behind voltage by 90 degrees.

Conductor resistance formula

Resistance equals resistivity times length divided by cross-sectional area: R = ρL/A. Doubling length doubles resistance; doubling area halves resistance. Temperature also raises resistance in metals.

NFPA 70E arc flash categories

PPE categories correspond to incident energy: Category 1 requires 4 cal/cm² arc-rated clothing, Category 2 requires 8 cal/cm², Category 3 requires 25 cal/cm², and Category 4 requires 40 cal/cm².

Electrically safe work condition

A state achieved by disconnecting all sources, verifying absence of voltage with a tested meter, applying lockout and tagout, and grounding for stored or induced energy before work begins.

GFCI protection requirements

NEC 210.8 requires ground-fault circuit interrupter protection for 125 V receptacles in bathrooms, garages, outdoors, kitchens, laundry areas, basements, and within six feet of sinks in dwellings.

NEC working space depth

Minimum clear depth in front of electrical equipment is 3 feet for 0-150 V to ground, 3.5 feet for 151-300 V, and 4 feet for 301-600 V, with 30-inch width and 6.5-foot height.

Limited approach boundary

The shock-protection boundary inside which only qualified persons may enter. At 50 to 300 V it is 3 feet 6 inches for fixed circuit parts and 10 feet for exposed movable conductors.

Box fill calculation

Per NEC 314.16, each 14 AWG conductor counts 2.00 cubic inches, 12 AWG 2.25, and 10 AWG 2.50. Each device yoke counts as two conductors and each cable clamp as one.

Conduit fill percentages

NEC Chapter 9 Table 1 limits raceway fill to 53 percent for one conductor, 31 percent for two conductors, and 40 percent for three or more conductors of the total interior cross-sectional area.

EMT support spacing

Electrical metallic tubing must be supported within 3 feet of each box or termination and at intervals not exceeding 10 feet along the run, per NEC 358.30.

NM-B cable restrictions

Nonmetallic sheathed cable is permitted in dry concealed locations in dwellings and many commercial buildings, but is prohibited where exposed to physical damage and in hazardous classified locations.

Standard overcurrent device ratings

NEC 240.6 lists standard ampere ratings including 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, and 400 amperes.

Equipment grounding conductor sizing

NEC Table 250.122 sizes the equipment grounding conductor by the overcurrent device rating: 15 A uses 14 AWG copper, 20 A uses 12 AWG, 60 A uses 10 AWG, and 200 A uses 6 AWG.

Service neutral bonding

NEC 250.24 requires the grounded service conductor to be bonded to the grounding electrode conductor at the service disconnect only. Downstream panels must use a separate ground bar.

Grounding electrode system

NEC 250.50 requires bonding together all available electrodes: metal water pipe with 10 feet in contact, ground rod, concrete-encased electrode, and ground ring, forming a single grounding electrode system.

Motor overload protection

NEC 430.32 permits overload devices sized at 125 percent of full-load current for motors with a service factor of 1.15 or greater, and 115 percent for motors with a lower service factor.

Motor short-circuit protection

NEC 430.52 allows an inverse-time breaker up to 250 percent of motor full-load current, a dual-element fuse up to 175 percent, and a non-time-delay fuse up to 300 percent.

HVAC nameplate MCA and MOCP

The minimum circuit ampacity sets conductor size, and the maximum overcurrent protective device sets the breaker or fuse size. Both values appear on the equipment nameplate and must not be exceeded.

Thermostat terminal designations

R is 24 V hot, C is common, W is heat, Y is cooling, and G is fan. Heat pumps add O or B for the reversing valve and W2 or E for auxiliary or emergency heat.

Transformer kVA formula

Single-phase kVA equals volts times amps divided by 1000. Three-phase kVA equals the square root of three times line-to-line volts times line amps divided by 1000.

Transformer turns ratio

Primary voltage divided by secondary voltage equals primary turns divided by secondary turns. Current ratio is the inverse, and power is conserved except for losses.

Transformer short-circuit current

Secondary short-circuit current equals full-load secondary amperes divided by the percent impedance expressed as a decimal. A 5 percent impedance transformer yields 20 times full-load current.

Transformer overcurrent protection

NEC 450.3 permits primary-only protection up to 125 percent of rated primary current for transformers over 1000 V, and up to 250 percent for transformers rated 600 V and below.

Dwelling general lighting load

NEC 220.12 assigns 3 volt-amperes per square foot for dwelling unit general lighting. Demand factors in Table 220.42 then reduce the load above 3000 volt-amperes.

Receptacle spacing rule

NEC 210.52 requires that no point along a dwelling wall be more than six feet from a receptacle, measured along the floor line, and kitchen countertop receptacles within 24 inches of any point.

Lockout tagout sequence

Per OSHA 1910.147: notify affected employees, shut down equipment, isolate all energy sources, apply locks and tags, release stored energy, and verify zero energy before work.

Motor insulation resistance test

A megohmmeter applies 500 or 1000 V DC to measure insulation resistance. A common minimum is one megohm per kilovolt of rated voltage plus one megohm, corrected to 40 °C.