NEC Articles 90 through 210
  National Electrical Code Questions and Answers

Article 90 - Introduction to the National Electrical Code


Q. List the types of occupancies and areas that must comply with the NEC.

Section 90-2(a). The answer to this question is pretty straightforward: The NEC generally applies to most, but not all, electrical installations. The NEC also covers temporary and permanent installations.

Q. List the types of occupancies and areas that are exempt from the NEC.

Section 90-2(b). Again this question is pretty simple: The NEC does not apply to utility wiring for power, telephone, and cable TV that are under exclusive control of the utility. Power and communication wiring not under the exclusive control of the utility must be installed according to the NEC. Q. Who enforces the Code and what are their responsibilities?

Section 90-4. The authority having jurisdiction, often a government official of a building department for a city, county, state or federal agency, is responsible for enforcing the Code. Other persons who may be responsible are insurance inspectors and fire marshals

The inspector's responsibilities are to enforce the requirements of the Code, determine product approval, give special permission where necessary, and permit alternate material and installation methods to insure a safe installation. In addition, the inspector must insure that electrical products are installed according to the manufacturer's instructions and without unauthorized modifications.

Q. What is the purpose of listing and labeling electrical equipment?

Section 90-7. Listing and labeling of electrical equipment provides the basis for equipment approval by the inspector. Listing and labeling of equipment significantly reduces the need for inspectors to check the internal wiring of such equipment as appliances, motors, devices, etc. Additional information about this subject can be found in Sections 110-2 and 110-3

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Article 110 - Requirements for Electrical Installations


Q. Explain the guidelines for approval of equipment.

Section 110-3(a) In judging equipment, considerations such as the following shall be evaluated:

  • (1) Suitability for installation and use in conformity with the provisions of this Code.
  • (FPN): Suitability of equipment use may be identified by a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application. Suitability of equipment may be evidenced by listing or labeling.
  • (2) Mechanical strength and durability, including, for parts designed to enclose and protect other equipment, the adequacy of the protection thus provided.
  • (3) Wire-bending and connection space.
  • (4) Electrical insulation.
  • (5) Heating effects under normal conditions of use and also under abnormal conditions likely to arise in service.
  • (6) Arcing effects.
  • (7) Classification by type, size, voltage, current capacity, and specific use.
  • (8) Other factors that contribute to the practical safeguarding of persons using or likely to come in contact with the equipment.

Q. Explain the reasons and dangers that equipment must have an interrupting rating sufficient for the available fault current.

Section 110-9. Short circuits and ground faults permit a tremendous amount of current to flow through conductors and equipment. If the current flow exceeds the rating of the equipment intended to open the circuit at fault levels, the equipment can explode. Equipment that interrupts the circuit under a fault condition must have an interrupting rating sufficient for available fault current on the line side of the equipment. See 240-60(c) and 240-83(c). Interrupting rating is the rating of equipment to open under a fault condition, such as a fuse, circuit breaker, switch, and so forth. Withstand rating [110-10] is the component's ability to withstand the current passing through the equipment. Example: A 1 horsepower controller for a motor is rated 5,000 amperes (testing laboratory information); if the let-through current exceeds 5,000 amperes the equipment can be destroyed. Note: If the fault current exceeds the interrupting or withstand rating of the equipment, electrical explosions caused by equipment failures can create temperatures greater than 1,000,000 degrees. Not only is there danger from explosions, but there is also danger from flash burns. (COOPER/Busman Fuse in St. Louis, Missouri, can provide additional information on the subject of interrupting and withstand ratings. Information is available in slides, workbooks, and videos of exploding equipment.)

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Q. Explain the rules that apply to conductor termination's.

Section 110-14(a). In general conductors must terminate in a proper manner. The terminals must be rated for the conductor material, copper and aluminum cannot be mixed, inhibitors and compounds must be identified for the conductors, and termination's must be torque according to the manufacturer's instruction [110-3(b)]. Naturally the termination's must be tight, and only one conductor is permitted under a terminal unless the terminal is identified otherwise.

Q. Explain the rules that apply to conductor splices.

Section 110-14(b). Conductors must be spliced in an approved manner [110-14(b)] with fitting identified for the purpose and the splice must be insulated (except for ground wires). It is not necessary to twist wires together before the use of a wirenut but the free ends of unused wires must be insulated. Wire connectors or other splicing means for direct earth burial must be listed for such use. Note: The conductor must be sized according to the lowest of the following: equipment rating, device rating, terminal temperature rating, or conductor rating. This means that if THHN (90 degree C) terminates on a 60 degree C terminal the conductor must be sized according to the 60 degree C rating, not the 90 degree C rating using Table 310-16. However, for ampacity derating we use the higher ampacity as listed in Table 310-16.

Q. Explain the rules that apply to working space for electrical equipment.

Section 110-16. Working space includes the necessary width and depth for examination, adjustment, servicing, or maintenance of equipment "while energized" [110-16]. The minimum working space is 30 inches wide and 36 inches deep, but no less than necessary for swinging doors to open 90 degrees. Equipment can be flush to one side of a wall, because the 30-inch dimension is not measured from the center of the equipment. There shall be no storage in the working space and there must be an accessible entrance to this space. There also must be illumination and sufficient headroom. The minimum headroom of working space about service equipment, switchboards, panelboards, or motor control centers shall be 6 1/2 feet. Where the electrical equipment exceeds 6 1/2 feet in height, the minimum headroom shall not be less than the height of the equipment.

Q. Explain the rules that apply to working space entrances.

Section 110-16(c). The general rule is that one entrance of "sufficient area" is required. Check with the inspector for what is considered "sufficient area." Two entrances not less than 24 inches wide and 6 1/2 feet high are required for equipment rated 1,200 amperes or more and over 6 feet wide. The entrances must be at both ends of the equipment. Exception: Only one entrance is required where the location permits a continuous and unobstructed way of exit travel or where the work space required by Section 110-16(a) is doubled. When there is only one entrance as permitted by this exception, the equipment must be located so that the edge of the entrance is no closer to the equipment than the distance given in Table 110-16(a).

Q. Explain the rules that apply to identification of disconnects and circuits.

Section 110-22. All disconnects, branch circuits, feeders, and services must be legibly marked to indicate its purpose. See Section 384-13. Where it is totally obvious what the disconnect is for, identification is not required. Examples would be a service main to a single family dwelling, a single disconnect next to a single motor, a single disconnect at an air-conditioning unit, and other very obvious locations.

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Article 200 - Use and Identification of Grounded Conductors


Q. Explain the difference between a grounded and a neutral conductor.

Generally the "neutral" is the center point of a Wye three-phase transformer. Most other grounded conductors are not "neutrals." There are very few rules in the NEC that apply to the neutral conductor, but many rules that apply to the grounded conductor. It is important that the student understand that when the term "neutral" is used in the field, the conductor is probably a grounded conductor and not a "neutral." In reality, this is not a big deal, but the student should know the difference. The student needs to learn to call the "white" wire the grounded conductor, not the neutral. The reason this conductor is called the grounded conductor will be explained in the Article 250 Questions

Q. Explain the rules associated with identification of the neutral (grounded) conductor.

Section 200-6. The grounded conductor No. 6 AWG, or smaller must be continuously identified with white or gray insulation. Grounded conductors No. 4 and larger, can be any color (except green) if reidentified with a white finish or white tape (not gray). The white conductor in a cable, such as NM or AC (BX), can be used WITHOUT reidentification for the feed to 1-pole, 3-way, and 4-way switches. In addition, reidentification of the white conductor is not required when used for travelers between 3-way and 4-way switches. If the white or gray conductor is used for the switch leg, the white or gray conductor must be reidentified according to Sections 200-7 and 310-12(c).

Q. Explain the use of the white wire for the hot conductor.

Section 200-7. The general rule is that the colors white or gray can be used only for the grounded conductor. Section 200-7 Exception 1 permits a conductor with white or gray insulation to be used as the ungrounded (hot) conductor if reidentified with a color other than white, gray, or green. Exception 2 permits the white or gray conductor to be used for feeds to switches and travelers without reidentification

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Article 210 - Branch Circuits


Q. Explain the advantages of multiwire branch circuits.

Some of the advantages of multiwire branch circuits are: one less conductor; reduced voltage drop; and sometimes a smaller conduit size.

Q. Explain the disadvantages and dangers of multiwire branch circuits.

Some of the disadvantages of multiwire circuits are an intentional or accidental open grounded (neutral) conductor which could cause extreme variance in circuit voltage causing possible serious damage to equipment, and a shock hazard would exist if the grounded conductor is open and one side of a multiwire circuit is turned off (because the circuit is now a series circuit). For these reasons, Section 300-13(b) requires the grounded (neutral) conductor to be pigtailed.

  • Note. Three-phase, 4-wire power systems used to supply nonlinear loads cause a distortion of the phase and neutral currents, producing unwanted and potentially hazardous harmonic currents on the neutral. Nonlinear loads cause the neutral to carry as much as twice the phase current. The NEC contains no rules on sizing of branch circuit neutral conductors for nonlinear loads. Harmonic currents are present only in three-phase 4-wire Wye systems such as 208Y/120 and 480Y/277 volt systems, not 120/240 volt delta systems. For branch circuits you can reduce the heat on the neutral by installing separate grounded conductors for each phase or doubling the neutral conductor size.

Q. Explain the rules that apply to the replacement of nongrounding type receptacles.

Section 210-7. Grounding type receptacles must be used to replace nongrounding type receptacles [210-7(d)]. If no grounding means exist in the receptacle box, then nongrounding receptacles can used or a grounding type receptacle protected by a GFCI protective device (receptacle or breaker) can be used. When a grounding type receptacle is installed and the contacts or the receptacle are not grounded, the receptacle must be marked "GFCI protected and No Equipment Ground." Note. When replacing nongrounding type receptacles in areas that require GFCI protection [210-8], the replacement receptacles must be GFCI protected.

Q. Explain the GFCI protection rules that apply to commercial occupancies.

Section 210-8(b)(1). GFCI protection is required for all 15 and 20 ampere, 125 volt, single-phase receptacles installed in bathrooms of nondwelling occupancies [210-8(b)]. There is no Code rule that requires receptacles to be installed in these locations, but if they are installed, they must be GFCI protected. Section 210-8(b)(2), GFCI protection is required for all 15 and 20 ampere, 125 volt single-phase receptacles installed on roofs for equipment, see 210-63.

  • Note. There are other locations in the NEC that require GFCI protection for receptacles, they include Sections 305-6, 422-8, 511-10, 517-19, 550-8, 551-71, and 551-41.

Q. Explain the rules that apply to circuits that have a single receptacle.

Single receptacle requirements: Only grounding type receptacles can be used for 15 and 20 ampere circuits and the ampacity of the receptacle shall not be less than the ampere rating of the circuit protection device and the load [210-7(a), 210-19(a), and 210-21(b)]. Single receptacles to dedicated branch circuits in areas that require GFCI protection are not required to be GFCI protected [210-8 Exceptions]. A duplex receptacle is not a single receptacle, but is considered a multioutlet receptacle; see FPN in Article 100 definition of receptacle.

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Q. Explain the rules that apply to circuits that have multi-receptacles.

Multioutlet receptacle circuits contain two or more receptacles and have the following requirements:

  • In general, multioutlet circuits are only permitted to be rated 15, 20, 30, 40, or 50 amperes [210-3].
  • Multiwire branch circuits in a dwelling unit terminating on a duplex (multioutlet) receptacle yoke, require a two-pole circuit breaker, this does not apply to nondwelling unit occupancies [210-4].
  • Only grounding type receptacles can be used for 15 and 20 ampere circuits [210-7] and the ampacity of the receptacle shall be according to Table 210-24.
  • The conductor ampacity for multioutlet receptacle circuits shall not be less than the branch circuit overcurrent protection device [210-19(a)]; 15 and 20 ampere multioutlet branch circuits permit receptacles and lights on the same circuit [210-23(a)], except for temporary power [305-4(d)].
  • Portable equipment, cord-and-plug connected utilization equipment shall not exceed 80 percent of the branch circuit rating; fixed equipment, or equipment fastened in place, shall not exceed 50 percent of the branch circuit rating when connected to circuits that supply lighting or portable equipment loads [210-23], and 20 ampere multioutlet receptacles are not permitted on 15 ampere branch circuits [Table 210-24].

Q. Explain the rules associated with the kitchen counter top for dwelling units.

Section 210-52(c). Kitchen countertops in dwelling units require two or more small appliance circuits [220-4(b)]. The countertops must have 15 or 20 ampere receptacles for each countertop space 12 inches or wider, located so that no point along the wall space more than 2 feet from a receptacle. Island and peninsular countertops require a receptacle for each 4 feet of countertop located above or within 12 inches below the countertop. Receptacles must be located so that no point along the center line is more than 24 inches from a receptacle. Each countertop surface is considered a separate counter. Receptacles shall not be placed face up. Inaccessible receptacles, and receptacles for appliances fastened in place or in dedicated spaces do not count. Receptacles that supply counter top surface appliances require GFCI protection.

Q. Explain the rules that apply to receptacles outside dwelling units.

Section 210-52(e). One-family dwellings require two 15 or 20 ampere, 125 volt, GFCI protected receptacles at grade level, one in front and one in the rear of the dwelling unit. Two-family dwellings require two 15 or 20 ampere, 125 volt, GFCI protected receptacles at grade level for each dwelling unit, one in the front and one in rear of the dwelling unit. Multifamily dwelling units do not require any receptacles outside, but if receptacles are installed, those accessible at grade level must be GFCI protected [210-8(a)].

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Q. Explain the rules that apply to heating, air-conditioning, and refrigeration (HACR) equipment, as applied to lighting and receptacles for commercial occupancies.

Sections 210-63 and 210-70. Commercial heating, air-conditioning, and refrigeration equipment require a 15 or 20 ampere GFCI receptacle within 25 feet of this equipment. The receptacle must be in an accessible location and cannot be connected to the load side of the equipment disconnect. A wall-switch-controlled lighting outlet is required at or near equipment in attics and underfloor spaces.

Q. Explain the rules that apply to dwelling unit lighting and switching.

Section 210-70. One wall-switch-controlled lighting outlet or receptacle must be installed in every habitable room. Lighting outlets (not receptacles) must be switched for bathrooms, kitchens, hallways, and stairways. A switched lighting outlet is required for attached garages, detached garages with electric power, and outdoor entrances or exits. Areas used for storage, or containing equipment requiring servicing, must have a lighting outlet located near the equipment, but the switch for these outlets is not required to be wall switched. When the difference between floor levels is six steps or more, 3-way and 4-way wall switches are required to control the lighting outlet for the interior stairways.

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