Oklahoma Limited Electrical Contractor

Key Concepts

Scope note: The SOURCE material for this topic covers needs analysis and estimating (NEC Article 220 load calculations, plus load calculation for structural additions). It contains no permit, inspection, or structural-integrity provisions, so none are stated below.

Voltages and rounding - Nominal system voltages used for calculating branch-circuit and feeder loads: 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts — NEC 220.5(A). Note the correct designation is 480Y/277, not "277/480." - Calculations are permitted to be rounded to the nearest whole ampere, decimal fractions smaller than 0.5 dropped — NEC 220.5(B). - For dwelling units, the calculated floor area **shall not include open porches or unfinished areas not adaptable for future use as a…

Key Concepts

Series circuits (Ugly's, Series Circuits, Book Page 3) - Total current equals the current in any other part of the circuit: I_T = I₁ = I₂ = I₃. - Total voltage is the sum of the voltages across all parts: E_T = E₁ + E₂ + E₃. - Total resistance is the sum of all resistances: R_T = R₁ + R₂ + R₃.

Parallel circuits (Ugly's, Parallel Circuits, Book Page 5) - Total current is the sum of the branch currents: I_T = I₁ + I₂ + I₃. - Total voltage across any branch equals that of any other branch and the total: E_T = E₁ = E₂ = E₃. - Note: the total resistance is always…

Limited Electrical Contractor

Key Concepts

a. Approved Methods of Installation of Electrical Equipment (general, Article 300 / 110)

• Bored holes in wood members — The edge of the bored hole must be not less than 32 mm (1¼ in.) from the edges of the wood member. Where this distance cannot be maintained, protect with a steel plate, sleeve, or equivalent at least 1.6 mm (1/16 in.) thick. (NEC 300.4(A)(1))
• Under metal-corrugated sheet roof decking — Cable, raceway, or box must be installed and supported with not less than 38 mm (1-1/2 in.) measured from the lowest surface of the roof decking to the top of the cable, raceway, or box. (NEC 300.4(E))
• **Fittings for 4…

Key Concepts

Services (Art. 230) - A building or structure shall be supplied by only one service unless permitted in 230.2(A) through (D) (230.2). Additional services are permitted for special conditions (fire pumps, emergency and standby systems), special occupancies, capacity in excess of 2000 amperes at 1000 volts or less (230.2(C)), and different voltages, frequencies, or phases, or different uses such as different rate schedules (230.2(D)). - Minimum size of overhead service conductors: 8 AWG copper or 6 AWG aluminum/copper-clad aluminum (230.23(B)). - Vertical clearance above a roof surface: 2.6 m (8 ft 6 in.), maintained 900 mm (3 ft) in all directions from the roof edge (230.24(A)). - Point of attachment: never **less than 3.0 m (10 ft)…

Limited Electrical Contractor

Key Concepts

Small conductor protection (240.4(D)) — Unless permitted in 240.4(E) or (G), overcurrent protection shall not exceed the following, after correction factors for ambient temperature and number of conductors: - 18 AWG copper — 7 A, if continuous loads ≤ 5.6 A and protection is by branch-circuit-rated CBs/fuses listed and marked for 18 AWG copper, or Class CC, CF, J, or T fuses — 240.4(D)(1) - 16 AWG copper — 10 A, if continuous loads ≤ 8 A and protection is by branch-circuit-rated CBs/fuses listed and marked for 16 AWG copper, or Class CC, CF, J, or T fuses — 240.4(D)(2) - 14 AWG copper-clad aluminum — 10 A (continuous loads ≤ 8 A) —…

Key Concepts

General requirements (subtopic a)

• Permitted connection methods (250.8(A)): listed pressure connectors, terminal bars, pressure connectors listed as grounding and bonding equipment, exothermic welding, machine screw-type fasteners engaging not less than two threads or secured with a nut, thread-forming machine screws engaging not less than two threads in the enclosure, connections part of a listed assembly, other listed means. Connection devices or fittings that depend solely on solder shall not be used (250.8(B)).
• Clean surfaces (250.12): nonconductive coatings (paint, lacquer, enamel) shall be removed from threads and other contact surfaces to ensure electrical continuity, or fittings designed to make removal unnecessary shall be used.
• Effective ground-fault current path (250.4(A)(5)): "The earth shall not be considered…

Limited Electrical Contractor

Key Concepts

Bending radius (inner edge of the bend) - Type AC: not less than five times the diameter of the cable — 320.24 - Type MC, interlocked armor or corrugated sheath: seven times the external diameter of the metallic sheath — 330.24(B). Smooth sheath: 10× (≤ 3/4 in. dia.), 12× (> 3/4 in. to 1½ in.), 15× (> 1½ in.) — 330.24(A). Shielded conductors: 12× one individual conductor or 7× the multiconductor cable, whichever is greater — 330.24(C) - Types NM/NMC, SE/USE, UF: five times the diameter — 334.24, 338.24, 340.24 - Type MI: 5× sheath diameter (≤ 3/4 in.); 10× (> 3/4 in. to 1 in.) — 332.24

Key Concepts

General raceway requirements - The total degrees of bends in a run shall not exceed 360 degrees between pull points — RMC 344.24(B), IMC 342.24(B), FMC 348.24(B), LFMC 350.24(B), PVC 352.24(B), EMT 358.24(B). A conduit body is a pull point, so each section between pull points may have up to 360° (NEC 358.26). - Indoor wet locations where walls are frequently washed: exposed wiring systems require at least a 1/4 in. (6 mm) airspace between the raceway and the supporting surface — 300.6(D). (Nonmetallic raceways on concrete/masonry are excepted.) - Bored holes in wood members: where the hole edge is less than 1-1/4 in. from the edge of the member, protect with a **steel plate at least…

Limited Electrical Contractor

Key Concepts

Location classification (Article 500)

• Class I — locations where flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are or may be present in air in quantities sufficient to produce explosive or ignitible mixtures — 500.5(B).
• Class I, Division 1 — ignitible concentrations can exist under normal operating conditions — 500.5(B)(1).
• Class II — hazardous because of the presence of combustible dust — 500.5(C).
• Class III, Division 1 — locations where ignitible fibers/flyings are handled, manufactured, or used; informational note cites cotton and other textile mills — 500.5(D)(1)(b).
• Ammonia refrigerant machinery rooms with adequate mechanical ventilation operating continuously or initiated by a detection system at a concentration **not…

Limited Electrical Contractor Exam

Key Concepts

Class 1 circuits (Art. 724) - Class 1 circuits shall be supplied from a source with a rated output of not more than 30 volts and 1000 volt-amperes — 724.40. - Overcurrent protection shall not exceed 7 amperes for 18 AWG and 10 amperes for 16 AWG; conductors 14 AWG and larger are protected per conductor ampacity — 724.43.

Class 2 and Class 3 circuits (Art. 725) - Class 2/Class 3 cables and conductors shall not be placed in any cable, cable tray, compartment, enclosure, raceway, or similar fitting with electric light, power, Class 1, or NPLFA conductors unless permitted by 725.136(B) through (I) — e.g., separation by a barrier — 725.136(A), (B).…

Key Concepts

Fixture installation (410, Part II) - Luminaires must be constructed, installed, or equipped with shades or guards so combustible material is not subjected to temperatures over 90°C (194°F) — 410.11 (also 410.97 for lampholders, 410.115(A) and 410.120 for recessed). - Clothes closets: minimum clearance from the nearest point of clothes closet storage space is 300 mm (12 in.) for surface-mounted incandescent or LED luminaires with a completely enclosed light source installed on the wall above the door or on the ceiling — 410.16(D). Storage space is bounded to 1.8 m (6 ft) or the highest hanging rod, 600 mm (24 in.) horizontally from side/back walls, and 300 mm (12 in.) or shelf width to the ceiling —…

Key Concepts

First aid and medical services (§1926.50) - The employer must ensure availability of medical personnel for advice and consultation (§1926.50(a)) and make provisions for prompt medical attention prior to commencement of the project (§1926.50(b)). - Where no infirmary, clinic, hospital, or physician is reasonably accessible in terms of time and distance, a person with a valid first-aid certificate from the U.S. Bureau of Mines, the American Red Cross, or equivalent training verifiable by documentary evidence must be available at the worksite (§1926.50(c)). - First aid kit contents go in a weatherproof container with individual sealed packages for each item type, and must be checked **before being sent out on each job and at least weekly on each…

Key Concepts

Which current to use. Conductor ampacity, switch ratings, and branch-circuit short-circuit/ground-fault (SC/GF) device ratings come from Table 430.247–Table 430.250, not the nameplate [430.6(A)(1)]. Nameplate current is used for separate motor overload protection [430.6(A)(2)]. For an inverse time breaker on a single motor, use the table FLC, not nameplate amps [430.52(B)].

Single motor conductors. Continuous duty: not less than 125% of motor FLC [430.22]. Special cases: - Multispeed, line side of controller: highest FLC on the nameplate; controller-to-motor: 125% of the winding(s) energized [430.22(B)] - Wye-start, delta-run: line side 125%; controller-to-motor 72% (58% × 1.25) [430.22(C)] - Part-winding: controller-to-motor 62.5% (50% × 1.25) [430.22(D)] - DC motor from rectified single-phase supply: 190% half-wave, 150% full-wave; rectifier input 125%…