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NEC Questions and Answers - Based on the 2014 NEC®
November 2014
By Mike Holt for EC&M Magazine
Try your best to answer these questions without looking at the answers. You are welcome to use these questions as a quiz. Feel free to print and email to your peers.
Note: These questions are based on the 2014 NEC®. The underlined text indicates a change to the Code rule for the 2014 NEC. |
Q1. What is the mounting height requirement for circuit breakers and disconnects?
A1. Circuit breakers and fuses must be readily accessible according to 240.4(A), and they must be installed so the center of the grip of the operating handle of the fuse switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform, unless the installation is for:
(1) Busways, as provided in 368.17(C).
(2) Supplementary overcurrent devices aren’t required to be readily accessible [240.10].
(3) For overcurrent devices, as described in 225.40 and 230.92.
(4) Overcurrent devices located next to equipment can be mounted above 6 ft 7 in., if accessible by portable means [404.8(A) Ex 2].
(C) Not Exposed to Physical Damage. Overcurrent devices must not be exposed to physical damage.
Note: Electrical equipment must be suitable for the environment, and consideration must be given to the presence of corrosive gases, fumes, vapors, liquids, or chemicals that have a deteriorating effect on conductors or equipment [110.11].
(D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices must not be located near easily ignitible material, such as in clothes closets.
(E) Not in Bathrooms. Overcurrent devices aren’t permitted to be located in the bathrooms of dwelling units, dormitories, or guest rooms or guest suites of hotels or motels.
Author’s Comment: The service disconnecting means must not be located in a bathroom, even in commercial or industrial facilities [230.70(A)(2)].
(F) Over Steps. Overcurrent devices must not be located over the steps of a stairway.
Author’s Comment: Clearly, it’s difficult for electricians to safely work on electrical equipment that’s located on uneven surfaces such as over stairways.
Not all disconnects have fuses or a circuit breaker. Section 404.8(A) requires that switches and circuit breakers used as switches must be capable of being operated from a readily accessible location. They must also be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform [240.24(A)].
Author’s Comment: The disconnecting means for a mobile home must be installed so the bottom of the enclosure isn’t less than 2 ft above the finished grade or working platform [550.32(F)].
Q2. Can you explain the 10 foot feeder tap rule?
A2. Tap conductors are conductors, other than service conductors, that have overcurrent protection ahead of the point of supply that exceeds the value permitted for similar conductors.
Conductors can be tapped to a feeder as specified in 240.21(B)(1) through (B)(5). The “next size up protection rule” of 240.4(B) isn’t permitted for tap conductors [240.21(B)].
Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(1)]:
(1) The ampacity of the tap conductor must not be less than:
a. The calculated load in accordance with Article 220, and
b. The rating of the overcurrent device supplied by the tap conductors.
Ex: Listed equipment, such as a surge protection device, can have their conductors sized in accordance with the manufacturer’s instructions.
(2) The tap conductors must not extend beyond the equipment they supply.
(3) The tap conductors are installed in a raceway when they leave the enclosure.
(4) The tap conductors must have an ampacity not less than 10 percent of the rating of the overcurrent device that protects the feeder.
Note: See 408.36 for the overcurrent protection requirements for panelboards.
Ten-Foot Tap Rule
Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeder that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?
• 200A: 3/0 AWG is rated 200A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).
• 150A: 1/0 AWG is rated 150A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).
• 30A: 8 AWG rated 40A at 60°C. The tap conductors from the 400A feeder to the 30A overcurrent device can’t be less than 40A
(10 percent of the rating of the 400A feeder overcurrent device.
Q3. Can communication cables be secured to the suspended ceiling support wires?
A3. Equipment and communications cabling must be installed in a neat and workmanlike manner. Exposed communications cables must be supported by the structural components of the building so that the communications cable won’t be damaged by normal building use. Cables must be secured with straps, staples, cable ties, hangers, or similar fittings designed and installed so as not to damage the communications cable [800.24] .
Communications raceways and communications cable assemblies must be securely fastened in place and ceiling-support wires or the ceiling grid must not be used to support communications raceways or communications cables [300.11 and 800.24].
Author’s Comment: Raceways and cables can be supported by independent support wires attached to the suspended ceiling in accordance with 300.11(A).
When using cable ties to secure or support communications cables in a plenum space, they must be listed for use in a plenum.
Author's Comment: See 820.24 for the same requirements for coaxial cable support in a suspended ceiling.
Q4. What are the rules in providing overcurrent protection for panelboards?
A4. Each panelboard must be provided with overcurrent protection located within, or at any point on the supply side of, the panelboard. The overcurrent device must have a rating not greater than that of the panelboard, and it can be located within or on the supply side of the panelboard [408.36].
Ex 1: Individual overcurrent protection isn’t required for panelboards used as service equipment where the service disconnecting means consists of up to six circuit breakers mounted in a single enclosure in accordance with 230.71.
When a panelboard is supplied from a transformer, as permitted in 240.21(C), the overcurrent protection for the panelboard must be on the secondary side of the transformer. The required overcurrent protection can be in a separate enclosure ahead of the panelboard, or it can be in the panelboard [408.36(B)].
Plug-in circuit breakers that are back-fed from field-installed conductors must be secured in place by an additional fastener that requires other than a pull to release the breaker from the panelboard [408.37(D)].
Author’s Comment: The purpose of the breaker fastener is to prevent the circuit breaker from being accidentally removed from the panelboard while energized, thereby exposing someone to dangerous voltage.
CAUTION: Circuit breakers marked “Line” and “Load” must be installed in accordance with listing or labeling instructions [110.3(B)]; therefore, these types of devices must not be back-fed.
Q5. Can cable wiring methods such as Type MC Cable be installed inside a raceway?
A5. MC Cable is allowed to be installed in any raceway as listed in 330.10(A)(7):
(A) General Uses.
(1) In branch circuits, feeders, and services
(2) In power, lighting, control, and signal circuits
(3) Indoors or outdoors
(4) Exposed or concealed
(5) Directly buried (if identified for the purpose)
(6) In a cable tray
(7) In a raceway
(8) As aerial cable on a messenger
(9) In hazardous locations as permitted in 501.10(B), 502.10(B), and 503.10
(10) Embedded in plaster or brick
(11) In wet locations, where a corrosion-resistant jacket is provided over the metal sheath and any of the following are met
a. The metallic covering is impervious to moisture.
b. A jacket is provided under the metal covering that’s moisture resistant.
c. The insulated conductors under the metallic covering are listed for use in wet locations.
(12) If single-conductor cables are used, all circuit conductors must be grouped together to minimize induced voltage on the sheath [300.3(B)].
(B) Specific Uses.
(1) Cable Tray. Type MC cable installed in a cable tray must comply with 392.10, 392.12, 392.18, 392.20, 392.22, 392.30, 392.46, 392.56, 392.60(C), and 392.80
(2) Direct Buried. Direct-buried cables must be protected in accordance with 300.5.
(3) Installed as Service-Entrance Cable. Type MC cable is permitted for service entrances, when installed in accordance with 230.43.
(4) Installed Outside of Buildings. Type MC cable installed outside of buildings must comply with 225.10, 396.10, and 396.12.
Note: The “Uses Permitted” isn’t an all-inclusive list, which indicates that other suitable uses are permitted if approved by the authority having jurisdiction.
Raceways can be used to provide protection from physical damage for other types of cables. Examples such as mast and downpipe sleeves for service and feeder cables. Raceway sleeves are used to protect branch circuits where subject to physical damage [300.15(C)]. Where NM Cable is run in unfinished basements and crawl spaces, it can be installed in a raceway for protection, providing that a suitable insulating bushing or adapter is provided where the cable enters the raceway [334.15(C].
Q6. How do we size the grounding electrode conductor (GEC) for a service having parallel conductors?
A6. Except as permitted in 250.66(A) through (C), the grounding electrode conductor (GEC) must be sized in accordance with Table 250.66.
The portion of the GEC connected to a ground rod is a minimum of 6 AWG, regardless of if the service conductors are paralleled or not [250.166(A)]. The same is true for a concrete-encased electrode [250.166(B) but the minimum size GEC is 4 AWG, and 2 AWG is the minimum size permitted for a ground ring [250.166(C)].
If a GEC is required to something other than what is required in 250.166(A) through (C), such as the metal frame of a building or an underground water pipe, Table 250.66 applies. The size of the GEC is based on either the actual size of the largest ungrounded conductor, or the equivalent area of ungrounded parallel conductors. For example, if a service is fed by three parallel sets of 600 kcmil conductors, the equivalent ungrounded service conductor is 600 kcmil x 3 parallel sets = 1,800 kcmil equivalent. Table 250.166 would require a 3/0 AWG GEC.
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