NEC Questions and Answers Based on the 2005 NEC I

By Mike Holt, for EC&M Magazine

Q1 What is the maximum number of 15 or 20A, 125V receptacle outlets permitted on a 20A, 120V general-purpose branch circuit in a commercial occupancy?

A1 For commercial occupancies, the NEC requires each receptacle outlet to be calculated at 180 VA [220.14(I)]. Therefore, the maximum number on a 20A circuit would be 13.

Circuit VA = Volts x Amperes
Circuit VA = 120V x 20A
Circuit VA = 2,400 VA
Number of Receptacles = 2,400 VA/180 VA
Number of Receptacles = 13

Note: According to the NEC Handbook, published by the NFPA, general-purpose receptacles aren't considered a continuous load.

Q2 What is the maximum number of 15 or 20A, 125V receptacle and lighting outlets permitted on a 15A, 120V general-purpose branch circuit in a dwelling unit?

A2 The NEC doesn't limit the number of receptacle and lighting outlets on a general-purpose branch circuit in a dwelling unit. See the NFPA's NEC Handbook for more information.

Although there's no limit on the number of lighting and/or receptacle outlets on dwelling general-purpose branch circuits, the NEC does require a minimum number of circuits to be installed for general-purpose receptacles and lighting outlets [210.11(A)]. In addition, the receptacle and lighting loads must be evenly distributed among the required circuits [210.11(B)].

Caution: Not likely, but there might be a local electrical requirement that limits the number of receptacles and lighting outlets on a general-purpose branch circuit.

Q3 What outlets in a dwelling unit are required to be AFCI protected?

A3 All 15 or 20A, 120V branch circuits that supply outlets in dwelling unit bedrooms, including those of mobile and manufactured homes [550.25] must be protected by a listed AFCI device [210.12(B)]. According to Article 100, an outlet is a point in the wiring system where electric current is taken to supply a load. This would include receptacle outlets, lighting outlets, as well as outlets for paddle fans and smoke detectors.

Note: The 120V circuit limitation means that AFCI protection isn't required for equipment rated 230V, such as a baseboard heater or room air-conditioner equipment.

Q4 What are the requirements for installing 15 or 20A receptacles in wet locations?

A4 All 15 and 20A receptacles in a wet location must be within an enclosure that is weatherproof when an attachment plug is inserted [406.8(B)].

According to Article 100, a wet location would be an area subject to saturation with water, and unprotected locations exposed to weather.

Q5 What are the Code limitations on using service entrance cable as a feeder or branch circuit?

A5 According to 338.10, all circuit conductors must be insulated, including the grounded neutral conductor. However, a bare conductor can be used for the grounded neutral conductor when the cable is used as a feeder to a remote building in accordance with 250.32.

Service-entrance cable used for interior branch circuits or feeders must be installed in accordance with Parts I and II of Article 334, excluding 334.80. Basically this means that Type SE cable must be installed as if it were nonmetallic-sheath cable, with the exclusion on conductor sizing as it relates to 60°C terminals.

Caution: Underground service-entrance cable (Type USE) cannot be used for interior wiring because it doesn't have flame-retardant insulation.

Q6 Is metal siding and metal framing members required to be grounded?

A6 The NEC does not specifically require metal siding or metal framing members to be bonded (not grounded) to an effective ground-fault current path [250.4(A)(4)]. However, exposed structural metal that forms a metal building frame must be bonded to one of the following (effective ground-fault path):

• Service equipment enclosure
• Grounded neutral service conductor
• Grounding electrode conductor where sized in accordance with Table 250.66
• One of the electrodes of the grounding electrode system

The bonding jumper for structural metal is sized to the conductors that supply the building or structure, in accordance with Table 250.66. In addition, the bonding jumper must be:

• Copper where within 18 in. of earth [250.64(A)].
• Securely fastened and not exposed to physical damage [250.64(B)].
• Installed without a splice or joint, unless spliced by compression connectors or by the exothermic welding process [250.64(C)].

Q7 What are the installation requirements for dwelling unit lighting outlets and switches?

A7 According to 210.70, at least one wall switch-controlled lighting outlet must be installed in every habitable room and bathroom of a dwelling unit. In other than kitchens and bathrooms, a receptacle controlled by a wall switch can be used instead of a lighting outlet. Lighting outlets can be controlled by occupancy sensors equipped with a manual override that permits the sensor to function as a wall switch.

The Code contains the location requirement for the wall switched-controlled lighting outlet, but it doesn't specify the location for the switch. Naturally, you wouldn't want to install a switch behind a door or other inconvenient location, but the NEC doesn't require you to relocate the switch to suit the swing of the door. When in doubt as to the best location to place a light switch, consult the job plans or ask the customer. If you're the boss and you don't know, check with the authority having jurisdiction.

In addition, not less than one wall switch-controlled lighting outlet must be installed in hallways, stairways, attached garages, and detached garages with electric power. And at least one wall switch-controlled lighting outlet must provide illumination on the exterior side of outdoor entrances or exits with grade level access.

Q8 Is a metallic tray required to be grounded?

A8 Metallic cable trays must be effectively bonded to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them [[250.96(A)]. This is accomplished by bonding the metal parts to an equipment grounding (bonding) conductor that is sized to the circuit protection device in accordance with 250.122 [392.7(A)]

Note: Metallic cable trays can serve as equipment grounding (bonding) conductors where continuous maintenance and supervision ensure that qualified persons service the installed cable tray system, and the [392.3(C) and 392.7(B)]:

• Cable tray sections and fittings are identified for grounding (bonding) purposes.
• Cable tray sections, fittings, and connected raceways are bonded to each other in accordance with 250.96 using bolted mechanical connectors or bonding jumpers sized in accordance with 250.102.

Q9 Does the NEC require the metal covers of handholes to be grounded? Could I use a ground rod for this purpose?

A9 Metal covers and other exposed conductive surfaces of handholes must be effectively bonded to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them [250.96(A)]. This is accomplished by bonding the metal parts to an equipment grounding (bonding) conductor that is sized to the circuit protection device in accordance with 250.122.

Caution: A ground rod does not provide the effective ground-fault path necessary to clear a ground fault. This is because the resistance of the earth is so high, very little current returns to the electrical supply source via the earth. New Graphic

Danger: The failure to bond metal handhole as well as manhole covers in accordance with 250.4(A)(3) and 250.96(A), has resulted in many unnecessary deaths.

Q10 We have installed access and security control equipment in a factory. The power source is in a breaker panel and the employees have learned how to disable the system by turning the breaker off and waiting for the battery to discharge. Does the code permit the use of a padlock on the panel door to restrict access?

A10 The NEC does not prohibit the locking of panel doors or the placing of a padlock on a circuit breaker to restrict access.

23. NEC Exam Practice Questions Textbook
 Learning to use the NEC is like learning to play the game of chess. To play the game well, you’ll need to study the rules, understand the subtle and complicated strategies, and then practice, practice, practice. This book contains 12 exams in code order, 12 exams in random order, and 1 final exam. Over 2,000 typical NEC Code practice questions from Articles 90 through 830. Product Code: 05PQ ISBN: 1-932685-28-6 Available March, 2005