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NEC Questions and Answers - Based on 2011 NEC
May 2013
By Mike Holt for EC&M Magazine
Here’s the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent, so you can see how you did.
Q1. What are the rules on conductor box fill and can you give me some examples?
A1. Boxes containing 6 AWG and smaller conductors must be sized to provide sufficient free space for all conductors, devices, and fittings. In no case can the volume of the box, as calculated in 314.16(A), be less than the volume requirement as calculated in 314.16(B).
Conduit bodies must be sized in accordance with 314.16(C).
Author’s Comment: The requirements for sizing boxes and conduit bodies containing conductors 4 AWG and larger are contained in 314.28. The requirements for sizing handhole enclosures are contained in 314.30(A).
The volume of a box includes the total volume of its assembled parts, including plaster rings, extension rings, and domed covers that are either marked with their volume in cubic inches (cu in.), or are made from boxes listed in Table 314.16(A).
The calculated conductor volume determined by (1) through (5) and Table 314.16(B) are added together to determine the total volume of the conductors, devices, and fittings. Raceway and cable fittings, including locknuts and bushings, aren’t counted for box fill calculations.
Each unbroken conductor that runs through a box, and each conductor that terminates in a box, is counted as a single conductor volume in accordance with Table 314.16(B) [314.16(B)(1)].
Each loop or coil of unbroken conductor having a length of at least twice the minimum length required for free conductors in 300.14 must be counted as two conductor volumes. Conductors that originate and terminate within the box, such as pigtails, aren’t counted at all.
Author’s Comment: According to 300.14, at least 6 in. of free conductor, measured from the point in the box where the conductors enter the enclosure, must be left at each outlet, junction, and switch point for splices or terminations of luminaires or devices.
Ex: Equipment grounding conductors, and up to four 16 AWG and smaller fixture wires, can be omitted from box fill calculations if they enter the box from a domed luminaire or similar canopy, such as a ceiling paddle fan canopy.
One or more internal cable clamps count as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box. Cable connectors that have their clamping mechanism outside of the box aren’t counted [314.16(B)(2)].
Each luminaire stud or luminaire hickey counts as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box [314.16(B)(3)].
Author’s Comment: Luminaire stems don’t need to be counted as a conductor volume.
Each single-gang device yoke (regardless of the ampere rating of the device) counts as two conductor volumes, based on the largest conductor that terminates on the device in accordance with Table 314.16(B) [314.16(B)(4)].
Each multigang-device yoke counts as two conductor volumes for each gang, based on the largest conductor that terminates on the device in accordance with Table 314.16(B).
Author’s Comment: A device that’s too wide for mounting in a single-gang box, as described in Table 314.16(A), is counted based on the number of gangs required for the device.
All equipment grounding conductors in a box count as a single conductor volume in accordance with Table 314.16(B), based on the largest equipment grounding conductor that enters the box. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)], count as a single conductor volume in accordance with Table 314.16(B) [314.16(B)(5)].
Author’s Comment: Conductor insulation isn’t a factor that’s considered when determining box volume calculations.
Question: How many 14 AWG conductors can be pulled through a 4 in. square x 2 1⁄2 in. deep box with a plaster ring with a marking of 3.60 cu in.? The box contains two receptacles, five 12 AWG conductors, and two 12 AWG equipment grounding conductors.
(a) 3 (b) 5 (c) 7 (d) 9
Answer: (b) 5
Step 1: Determine the volume of the box assembly [314.16(A)]:
Box 30.30 cu in. + 3.60 cu in. plaster ring = 33.90 cu in.
A 4 x 4 x 2 1⁄8 in. box will have a gross volume of 34 cu in., but the interior volume is 30.30 cu in., as listed in Table 314.16(A).
Step 2: Determine the volume of the devices and conductors in the box:
Two—receptacles 4—12 AWG
Five—12 AWG 5—12 AWG
Two—12 AWG Grounds 1—12 AWG
Total 10—12 AWG x 2.25 cu in. = 22.50 cu in.
Step 3: Determine the remaining volume permitted for the 14 AWG conductors:
33.90 cu in. - 22.50 cu in. = 11.40 cu in.
Step 4: Determine the number of 14 AWG conductors permitted in the remaining volume:
14 AWG = 2.00 cu in. each [Table 314.16(B)]
11.40 cu in./2.00 cu in. = 5 conductors
Q2. What wiring methods does the NEC permit in a Class I Division 1 location?
A2. Only the following wiring methods are permitted within a Class I, Division 1 location [501.10(A)]:
(a) Threaded rigid metal conduit or intermediate metal conduit with explosionproof fittings.
(b) MI cable terminated with fittings listed for the location.
(c) In industrial establishments with restricted public access where only qualified persons will service the installation, MC-HL cable listed for use in Class I, Zone 1, or Division 1 locations, with a gas/vaportight continuous corrugated metallic sheath, an overall jacket of suitable polymeric material, a separate equipment grounding conductor(s) in accordance with 250.122, and terminated with fittings listed for the application. Such cable must comply with Part II of Article 330.
Author’s Comment: Article 100 defines a “Qualified Person” as one who has skills and knowledge related to the construction and operation of the equipment and has received safety training on the hazards involved.
(d) In industrial establishments with restricted public access where only qualified persons will service the installation, ITC-HL cable terminated with fittings listed for the location.
Explosionproof flexible metal connections listed for the location are permitted in a Class I, Division 1 location when necessary for vibration, movement, or flexible cords for difficult bends in accordance with 501.140 if terminated with cord connectors listed for the location [501.10(A)(2)].
Boxes and fittings must be approved by the authority having jurisdiction for Class I, Division 1 locations [501.10(A)(3)].
Q3. What wiring methods does the NEC permit in a Class I Division 2 location?
A3. Only the following wiring methods are permitted within a Class I, Division 2 location [501.10(B)(1)]:
(1) Wiring methods permitted in Class I, Division 1 locations [501.10(A)].
(2) Enclosed gasketed busways and enclosed gasketed wireways.
(3) PLTC and Type PLTC-ER cable terminated with listed fittings in accordance with Article 725.
(4) ITC and ITC-ER cable terminated with listed fittings as permitted in 727.4.
(5) Types MC, MV, or TC cables including installation in cable tray systems and terminated with listed fittings.
(6) Where metallic conduit doesn’t provide sufficient corrosion resistance, reinforced thermosetting resin conduit and Schedule 80 PVC conduit can be used, but only in industrial establishments where maintenance and supervision ensure that only qualified persons service the installation.
If flexibility is necessary, the following wiring methods are permitted [501.10(B)(2)]:
(1) Listed flexible metal fittings.
(2) Flexible metal conduit with listed fittings.
(3) Liquidtight flexible metal conduit with listed fittings.
(4) Liquidtight flexible nonmetallic conduit with listed fittings.
(5) Flexible cords listed for extra-hard usage, containing an equipment grounding conductor, and terminated with listed fittings.
Author’s Comments:
• See the definition of “Grounding Conductor, Equipment” in Article 100.
• If flexible cords are used, they must comply with 501.140.
Informational Note: See 501.30(B) for equipment grounding requirements where flexible conduit is used.
According to 501.10(B)(4), general-purpose enclosures and fittings are permitted in Class I Division 2 locations unless the enclosure contains make-and-break contacts for meters, instruments, and relays [501.105(B)(1)], switches, circuit breakers, or motor controllers [501.115(B)(1)], signaling, alarm, remote-control, and communications systems [501.150(B)].
Author’s Comment: See the definition of “Controller” in Article 100.
Q4. What are the requirements for the grounding electrode system when a lightning protection system is installed on a building/structure?
A4. If a lightning protection system is installed on a building/structure, it must be bonded to the building/structure grounding electrode system [250.106].
Author’s Comment: The grounding electrode for a lightning protection system must not be used as the required grounding electrode system for the buildings or structures [250.60].
Informational Note 1: See NFPA 780—Standard for the Installation of Lightning Protection Systems, which contains detailed information on grounding, bonding, and side-flash distance from lightning protection systems.
Informational Note 2: To minimize the likelihood of arcing between metal parts because of induced voltage, metal raceways, enclosures, and other metal parts of electrical equipment may require bonding or spacing from the lightning protection conductors in accordance with NFPA 780—Standard for the Installation of Lightning Protection Systems.
Q5. What are the rules that permit more than one service to a building or structure?
A5. A building/structure can only be served by one service drop or service lateral, except as permitted by (A) through (D) [230.2].
Service laterals 1/0 AWG and larger run to the same location and connected together at their supply end, but not connected together at their load end, are considered to be a single service.
Additional services are permitted for the following special conditions [230.2(A)]:
(1) Fire pumps
(2) Emergency systems
(3) Legally required standby systems
Author’s Comment: A separate service for emergency and legally required systems is permitted only when approved by the authority having jurisdiction [700.12(D) and 701.11(D)].
(4) Optional standby power
(5) Parallel power production systems
(6) Systems designed for connection to multiple sources of supply for the purpose of enhanced reliability.
Author’s Comment: To minimize the possibility of accidental interruption, the disconnecting means for the fire pump, emergency system, or standby power system must be located remotely away from the normal power disconnect [230.72(B)].
By special permission, additional services are permitted for the following special occupancies [230.2(B)]:
(1) Multiple-occupancy buildings where there’s no available space for supply equipment accessible to all occupants, or
(2) A building or other structure so large that two or more supplies are necessary.
Additional services are permitted for the following special capacity requirements [230.2(C)]:
(1) If the capacity requirements exceed 2,000A, or
(2) If the load requirements of a single-phase installation exceed the utility’s capacity, or
(3) By special permission.
Author’s Comment: Special permission is defined in Article 100 as “the written consent of the authority having jurisdiction.”
Additional services are permitted for different voltages, frequencies, or phases, or for different uses, such as for different electricity rate schedules [230.2(D)].
If a building/structure is supplied by more than one service, or a combination of feeders and services, a permanent plaque or directory must be installed at each service and feeder disconnect location to denote all other services and feeders supplying that building/structure, and the area served by each [230.2(E)].
Q6. How do I size an optional standby generator when a manual or automatic transfer switch is installed?
A6. The calculated load on the standby source must be in accordance with Article 220 or by a method approved by the authority having jurisdiction [702.4(B)].
When using manual transfer equipment, the optional standby power source must have adequate capacity for all equipment intended to operate at one time as determined by the user [702.4(B)(1)].
Author’s Comment: When a manual transfer switch is used, the user of the optional standby system selects the loads to be connected to the system, which determines the system kVA/kW rating.
When using automatic transfer equipment, the optional standby power source must have adequate capacity to supply the full load transferred [702.4(B)(2)].
Author’s Comment: For a new installation, the load is determined by Article 220 or an alternate method approved by the AHJ. For existing facilities, the maximum demand data for one year or the average power demand of a 15-minute period over a minimum of 30 days can be used to size the power source [220.87].
A transfer switch is required for all fixed or portable optional power systems [702.5].
Ex: Temporary connection of a portable generator without transfer equipment is permitted where written safety procedures are in place and conditions of maintenance and supervision ensure that only qualified persons will service the installation, and where the normal supply is physically isolated by a lockable disconnecting means or by the disconnection of the normal supply conductors.
For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.
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