ICC G21 Low-Voltage Electrician Exam (2017 NEC)

2017 NEC / Ugly's Electrical References

Key Concepts

Ohm's Law and power. Amperes = Watts ÷ Volts; Ohms = Volts ÷ Amperes; E = I × R; P = E × I; P = E² ÷ R; R = E² ÷ P (Ugly's Electrical References, Ohm's Law / Ohm's Law Power Formulas, Book Pages 1–2). A watt is the power used when 1 volt causes 1 ampere to flow in a circuit (Ugly's, To Find Watts).

Series circuits. Rule 1: total current equals the current in any other part of the circuit — IT = I1 = I2 = I3, etc. (Ugly's, Series Circuits).

Three-phase multiplier. Three-phase formulas use 1.73, the approximate square root of 3, which accounts for the 120-degree phase relationship between the three phases (Ugly's, Book Page 15; Book Page 19).

Apparent vs. true power. kVA is apparent power — "power before used, such as the rating of a transformer"; kilowatts is true power (Ugly's, To Find Kilovolt-Amperes, Page 23; Page 15 formula chart).

Units and prefixes. Frequency of an AC system is measured in hertz (Hz); standard U.S. power frequency is 60 Hz. Milli = 1/1,000 (350 mA = 0.35 A); kilo = 1,000 (1 kVA = 1,000 VA).

Distribution system identification. 120/240-volt, single-phase, three-wire: line one ungrounded black, line two ungrounded red; grounded neutral white or gray per NEC 200.6(A). 277/480-volt, three-phase, four-wire wye: C phase is yellow. 120/208-volt wye: 120 volts phase-to-neutral, 208 volts phase-to-phase (Ugly's, Common Electrical Distribution Systems). On a 4-wire delta system where the midpoint of one phase winding is grounded, the high leg (typically B phase) must be durably and permanently marked orange or by other effective means (NEC…

Articles 725 and 760 — Class 1, 2, and 3 circuits and fire alarm systems

Key Concepts

Circuit classifications (Article 725) - A Class 2 circuit is the portion of the wiring system between the load side of a Class 2 power source and the connected equipment; due to its power limitations it considers safety from a fire initiation standpoint and provides acceptable protection from electric shock. A Class 3 circuit also limits fire initiation, but because higher voltage and current are permitted, additional safeguards are specified for shock protection. [725.2, Definitions] - Article 725 covers security system circuits, access control circuits, sound circuits, nurse call circuits, intercom circuits, some computer network systems, some lighting dimmer control circuits, and some low-voltage control circuits from listed appliances or listed computer equipment. [common examples; the code text does not list them] - Class 1 circuits are classified as either Class 1 power-limited (per 725.41(A)) or Class 1 remote-control and signaling (per 725.41(B)). [725.41] - Class 1 power-limited: source rated not more than 30 volts and 1000 volt-amperes. [725.41(A)] Class 1 remote-control and signaling: shall not exceed 600 volts; source power output need not be limited. [725.41(B)] - Non-transformer Class 1 power-limited sources: overcurrent device rated not more than 167 percent of the VA rating divided by rated voltage; the device may be integral to the supply but shall not be interchangeable with devices of higher ratings. VA_max limited to 2500 volt-amperes; I_max × V_max not over 10,000 VA, with any OCPD bypassed. VA_max is measured after one minute; where a listed current-limiting impedance is used with a stored energy source (storage battery), I_max limits apply after 5 seconds. [725.41(A), (A)(2)] -…

Key Concepts

Article 810 — Antennas - Lead-in conductors attached to buildings must be installed so they cannot swing closer than 600 mm (2 ft) to conductors of circuits of 250 volts or less between conductors — 810.18(A). - Clearance between lead-in conductors and any conductor forming part of a lightning protection system: not less than 1.8 m (6 ft) — 810.18(A). - Indoor antennas and indoor lead-ins shall not be run nearer than 50 mm (2 in.) to conductors of other wiring systems, unless those conductors are in metal raceway or cable armor, or permanently separated by a continuous and firmly fixed nonconductor — 810.18(B). - Each lead-in conductor from an outdoor antenna requires a listed antenna discharge unit unless the antenna is in a zone of protection or otherwise protected from lightning exposure — 810.20(A).

Article 800 — Communications circuits - Primary protector required on each aerial/underground circuit within the block containing the building served that is exposed to power conductors operating at over 300 volts to ground — 800.90(A). Located as close as practicable to the point of entrance; mobile home service equipment or disconnecting means within 9.0 m (30 ft) of the exterior wall satisfies this — 800.90(B). - Fused-type protector = arrester between each line conductor and ground, a fuse in series with each line conductor, and a mounting; terminals marked line, instrument, and ground — 800.90(A)(2). The general listed primary protector is an arrester between each line conductor and ground, terminals marked line and ground — 800.170(A). - Fuseless protectors permitted only under 800.90(A)(1)(a)–(e), e.g., grounded metallic sheath cable whose conductors safely fuse above the capacity of the protector and its bonding/grounding electrode…

Key Concepts

• Class 1 circuit definition: the portion of the wiring system between the load side of the overcurrent device or power-limited supply and the connected equipment (725.2). A Class 2 circuit is defined from the load side of a Class 2 power source (725.2).
• Circuit requirements by class: Class 1 circuits comply with Parts I and II of Article 725; Class 2 and Class 3 circuits comply with Parts I and III (725.35).
• Safety-control equipment: remote-control circuits for safety-control equipment must be classified as Class 1 if failure of the equipment to operate introduces a direct fire or life hazard. Room thermostats, water temperature regulating devices, and similar controls for household heating/air conditioning are not safety-control equipment (725.31(A)).
• Physical protection: where damage to such remote-control circuits would introduce a hazard, all conductors must be installed in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, Type MI cable, or Type MC cable, or be otherwise suitably protected from physical damage (725.31(B)).
• Class 1 conductor sizes: 16 AWG and 18 AWG are permitted (725.49(A); commentary to 725.1).
• Class 1 power source limits: Class 1 power-limited circuits are supplied from a source rated not more than 30 volts and 1000 volt-amperes (725.41(A)); transformers supplying them comply with the applicable sections of Parts I and II of Article 450 (725.41(A)(1)). Class 1 remote-control and signaling circuits shall not exceed 600 volts, and source power output need not be limited (725.41(B)).
• Motor control circuits referred out of Article 725: control circuits tapped from the load side of the motor branch-circuit protective device comply with Article 430, Part VI, as specified in 430.72(A)…

Key Concepts

Cable listing and marking (770.179, Table 770.179) - Optical fiber cables installed in buildings shall be listed and identified per 770.179(A) through (F) and marked per Table 770.179 (770.113(A), 770.179). - All optical fiber cables shall have a temperature rating of not less than 60°C (140°F); a rating exceeding 60°C must be marked on the cable jacket (770.179). - Type suitability: OFNP/OFCP — plenum, ducts, and other spaces used for environmental air, fire-resistant and low-smoke [770.179(A)]; OFNR/OFCR — vertical run in a shaft or floor to floor [770.179(B)]; OFNG/OFCG — general purpose except risers and plenums [770.179(C)]; OFN/OFC — general purpose except risers, plenums, and other spaces used for environmental air [770.179(D)]. - Circuit integrity cables carry the suffix "CI" [770.179(E)(1)].

Installation locations within buildings (770.113, Table 770.154(a)) - Ducts specifically fabricated for environmental air [770.113(B)]: up to 1.22 m (4 ft) of OFNP and OFCP if directly associated with the air distribution system; all types permitted in raceways installed per 300.22(B). - Other spaces used for environmental air (plenums) [770.113(C)]: only OFNP and OFCP may be installed directly, in plenum communications raceways, in plenum cable routing assemblies, or supported by open metallic cable trays [770.113(C)(1)–(4)]. All types are permitted in metal raceway complying with 300.22(C) or in solid bottom metal cable trays with solid metal covers [770.113(C)(5)–(7)]. - Table 770.154(a) shows "N" for OFNR/OFCR and OFNG/OFCG/OFN/OFC in other spaces used for environmental air, plenum raceways, plenum cable routing assemblies, and open metal cable trays; risers in vertical runs are "N" for OFNG/OFCG/OFN/OFC. - Optical fiber cables shall not be strapped, taped, or attached to the exterior of any conduit or raceway as a means of support…