NEC Questions and Answers - Based on 2011 NEC®
December 2012

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 is the NEC rule regarding labeling of electrical service equipment with the maximum available fault current?

A1. Service equipment in other than dwelling units must be legibly field-marked with the maximum available fault current, including the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved [110.24(A)].

When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current [110.24(B)].

Ex: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

Q2. What is the Code rule for overcurrent protection of feeder conductors?

A2. Feeder overcurrent devices must have a rating of not less than 125 percent of the continuous loads, plus 100 percent of the noncontinuous loads [215.3].

Author’s Comment: See 215.2(A)(1) for feeder conductor sizing requirements.

Ex: If the assembly and the overcurrent device are both listed for operation at 100 percent of its rating, the overcurrent device can be sized at 100 percent of the continuous load.

Author’s Comment: Equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.

Question: What size feeder conductors are required for a 200A continuous load if the terminals are rated 75ºC?
(a)
2/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(b)
3/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(c)
4/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(d)
250 kcmil ungrounded conductors and a 3/0 AWG neutral conductor

Answer: (d) 250 kcmil AWG ungrounded conductors and a 3/0 AWG neutral conductor
Since the load is 200A continuous, the feeder conductors must have an ampacity of not less than 250A (200A x 1.25). The neutral conductor is sized to the 200A continuous load according to the 75ºC column of Table 310.15(B)(16). According to the 75ºC column of Table 310.15(B)(16), 250 kcmil has an ampacity of 255A, and 3/0 has an ampacity of 200A.

Q3. What is the Code rule regarding installing feeder and branch circuit conductors in the same raceway as service conductors?

A3. Service conductors must not be installed in the same raceway or cable with feeder or branch-circuit conductors [230.7].

Warning: Overcurrent protection for the feeder or branch-circuit conductors can be bypassed if service conductors are mixed with feeder or branch-circuit conductors in the same raceway and a fault occurs between the service and feeder or branch-circuit conductors.

Author’s Comments:
•  This rule doesn’t prohibit the mixing of service, feeder, and branch-circuit conductors in the same service equipment enclosure.
•  This requirement may be the root of the misconception that “line” and “load” conductors must not be installed in the same raceway. It’s true that service conductors must not be installed in the same raceway with feeder or branch-circuit conductors, but line and load conductors for feeders and branch circuits can be in the same raceway or enclosure.

Q4. When does the Code allow a building to be served by more than one service?

A4. A building/structure can only be served by one service drop or service lateral, except as permitted by (A) through (D) [230.2].
 (A) Special Conditions. Additional services are permitted for the following:
(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)].

(B) Special Occupancies. By special permission, additional services are permitted for:
(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.

(C) Capacity Requirements. Additional services are permitted:
(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.”

(D) Different Characteristics. Additional services are permitted for different voltages, frequencies, or phases, or for different uses, such as for different electricity rate schedules.

(E) Identification of Multiple Services. 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.

Q5. What does the Code require in regard to mechanical strength and minimum size of overhead conductors?

A5. Conductors 10 AWG and larger are permitted for overhead spans up to 50 ft long. For spans over 50 ft in length, the minimum size conductor is 8 AWG, unless supported by a messenger wire [225.6(A)(1)].

Q6. What is the Code rule regarding disconnects for ungrounded conductors of multi-wire branch circuits?

A6. Each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)].

Note: Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.15(B)(1)].

Caution: This rule is intended to prevent people from working on energized circuits they thought were disconnected.

Q7. What is the Code rule regarding the limitation of locations allowed for the installation of overcurrent devices?

A7. Overcurrent devices must not be exposed to physical damage [240.24(C)].

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].

Overcurrent devices must not be located near easily ignitible material, such as in clothes closets [240.24(D)].

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 [240.24(E)].

Author’s Comment: The service disconnecting means must not be located in a bathroom, even in commercial or industrial facilities [230.70(A)(2)].

Overcurrent devices must not be located over the steps of a stairway [240.24(F)].

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.

Q8. What kind of marking does the Code require to identify the high-leg of a three-phase, 4-wire delta connected system?

A8. On a 4-wire, delta-connected, three-phase system, where the midpoint of one phase winding of the secondary is grounded (a high-leg system), the conductor with 208V to ground must be durably and permanently marked by an outer finish orange in color, or other effective means [110.15]. Such identification must be placed at each point on the system where a connection is made if the neutral conductor is present [230.56].

Author’s Comments:
•  The high-leg conductor is also called the “wild leg,” “stinger leg,” or “bastard leg.”

•  Other important NEC rules relating to the high leg are as follows:
–  Panelboards. Since 1975, panelboards supplied by a 4-wire, delta-connected, three-phase system must have the high-leg conductor terminate to the “B” phase of a panelboard [408.3(E)]. Section 408.3(F)(1) requires panelboards to be field-marked with “Caution Phase B Has 208V to Ground.”
–  Disconnects. The NEC doesn’t specify the termination location for the high-leg conductor in switch equipment (Switches—Article 404), but the generally accepted practice is to terminate this conductor to the “B” phase.
–  Utility Equipment. The ANSI standard for meter equipment requires the high-leg conductor (208V to neutral) to terminate on the “C” (right) phase of the meter socket enclosure. This is because the demand meter needs 120V, and it obtains that voltage from the “B” phase.

•  Also hope the utility lineman isn’t color blind and doesn’t inadvertently cross the “orange” high-leg conductor (208V) with the red (120V) service conductor at the weatherhead. It’s happened before…

Warning: When replacing equipment in existing facilities that contain a high-leg conductor, care must be taken to ensure the high-leg conductor is replaced in its original location. Prior to 1975, the high-leg conductor was required to terminate on the “C” phase of panelboards and switchboards. Failure to re-terminate the high leg in accordance with the existing installation can result in 120V circuits being inadvertently connected to the 208V high leg, with disastrous results.

Q9. When a receptacle is installed to serve a specific appliance, what is the Code requirement regarding the location of the receptacle outlet?

A9. Receptacle outlets installed in a dwelling unit for a specific appliance, such as a clothes washer, dryer, range, garage door opener or refrigerator, must be within 6 ft of the intended location of the appliance [210.50(C)].

Q10. What is the Code rule requiring the installation of a receptacle outlet for a peninsula or island countertop space?

A10. At least one receptacle outlet must be installed at each island countertop space with a long dimension of 2 ft or more, and a short dimension of 1 ft or more [210.52(C)(2)].

At least one receptacle outlet must be installed at each peninsular countertop with a long dimension of 2 ft or more, and a short dimension of 1 ft or more, measured from the connecting edge [210.52(C)(3)].

Author’s Comment: The Code doesn’t require more than one receptacle outlet in an island or peninsular countertop space, regardless of the length of the countertop, unless the countertop is broken as described in 210.52(C)(4).

When breaks occur in countertop spaces for rangetops, refrigerators, or sinks, each countertop space is considered as a separate countertop for determining receptacle placement [210.54(C)(4)].

If a range, counter-mounted cooking unit, or sink is installed in an island or peninsular countertop, and the depth of the counter behind the range, counter-mounted cooking unit, or sink is less than 12 in., the countertop space is considered to be two separate countertop spaces.

Author’s Comment: GFCI protection is required for all 15A and 20A, 125V receptacles that supply kitchen countertop surfaces [210.8(A)(6)]. 

Q11. What is the Code rule for sizing overcurrent protection for branch circuits [210.20]?

A11. Branch-circuit overcurrent devices must have a rating of not less than 125 percent of the continuous loads, plus 100 percent of the noncontinuous loads [210.20(A)].

Branch-circuit conductors must be protected against overcurrent in accordance with 240.4 [220.20(B)].

Q12. What is the Code rule regarding supporting of festoon lighting?

A12. Overhead conductors for festoon lighting must not be smaller than 12 AWG, unless messenger wires support the conductors. The overhead conductors must be supported by messenger wire, with strain insulators, whenever the spans exceed 40 ft in length [225.6(B)].

Author’s Comment: Festoon lighting is a string of outdoor lights suspended between two points [Article 100]. It’s commonly used at carnivals, circuses, fairs, and Christmas tree lots [525.20(C)].

Q13. What does the Code require for minimum clearances of overhead feeder conductors?

A13. Overhead conductor spans must maintain vertical clearances as follows [225.18]:
(1) 10 ft above finished grade, sidewalks, platforms, or projections from which they might be accessible to pedestrians for 120V, 120/208V, 120/240V, or 240V circuits.

(2) 12 ft above residential property and driveways, and those commercial areas not subject to truck traffic for 120V, 120/208V, 120/240V, 240V, 277V, 277/480V, or 480V circuits.

(4) 18 ft over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles (such as those used for cultivation, grazing, forestry, and orchards).

(5) 24½ ft over track rails of railroads.

Author’s Comment: Overhead conductors located above pools, outdoor spas, outdoor hot tubs, diving structures, observation stands, towers, or platforms must be installed in accordance with the clearance requirements in 680.8.

Q14. Does the NEC allow the use of trees to support overhead feeder or branch circuit conductors?

A14. Trees or other vegetation must not be used for the support of overhead conductor spands. This applies to both feeders and branch circuits [225.26]. Section 410.36(G) does allow trees to be used for the support of luminaires.

Author’s Comment: Service-drop conductors installed by the electric utility must comply with the National Electrical Safety Code (NESC), not the National Electrical Code [90.2(B)(5)]. Overhead service conductors that aren’t under the exclusive control of the electric utility must be installed in accordance with the NEC.

Q15. What are the standard ampere ratings for overcurrent protection devices?

A15. Section 240.6(A) of the Code lists standard ratings as:
The standard ratings in amperes for fuses and inverse time breakers are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1,000, 1,200, 1,600, 2,000, 2,500, 3,000, 4,000, 5,000 and 6,000.

Additional standard ampere ratings for fuses include 1, 3, 6, 10, and 601.

Author’s Comment: Fuses rated less than 15A are sometimes required for the protection of fractional horsepower motor circuits [430.52], motor control circuits [430.72], small transformers [450.3(B)], and remote-control circuit conductors [725.43].

Q16. What does the Code specify regarding the maximum height of a switches and circuit breakers used as switches? 

A16.  Section 404.8(A) states 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)].

Ex 1: On busways, fusible switches and circuit breakers can be located at the same level as the busway where suitable means is provided to operate the handle of the device from the floor.

Ex 2: Switches and circuit breakers used as switches can be mounted above 6 ft 7 in. if they’re next to the equipment they supply, and are accessible by portable means [240.24(A)(4)].

Q17.  What type of circuit breaker does the Code allow to be used for switching duty of lighting circuits?

A17. Circuit breakers used to switch 120V or 277V fluorescent lighting circuits must be listed and marked SWD or HID. Circuit breakers used to switch high-intensity discharge lighting circuits must be listed and marked HID [240.83(D)].

Author’s Comments:
•  This rule applies only when the circuit breaker is used as the switch. If a general-use snap switch or contactor is used to control the lighting, this rule doesn’t apply.
•  UL 489, Standard for Molded Case Circuit Breakers, permits “HID” breakers to be rated up to 50A, whereas an “SWD” breaker can only be rated up to 20A. The tests for “HID” breakers include an endurance test at 75 percent power factor, whereas “SWD” breakers are endurance-tested at 100 percent power factor. The contacts and the spring of an “HID” breaker are of a heavier-duty material to dissipate the increased heat caused by the increased current flow in the circuit, because the “HID” luminaire takes a minute or two to ignite the lamp.

Q18.  What type of wiring methods does the Code allow?

A18. Only wiring methods recognized as suitable are included in the NEC, and they must be installed in accordance with the Code [110.8].

Author’s Comment: See Chapter 3 for power and lighting wiring methods, Chapter 7 for signaling, remote-control, and power-limited circuits, and Chapter 8 for communications circuits.

Q19. What does the NEC require for coloring of device terminals to which grounded or equipment grounding conductors are to be attached?

A19. Receptacles must have the terminal intended for connection to the grounded conductor identified by [200.10(B)]:

• A metal or metal coating that’s substantially white in color or marked by the word white or the letter W.
• If the terminal isn’t visible, the conductor entrance hole must be marked with the word white or the letter W.

Grounding-type receptacles, adapters, cord connections, and attachment plugs must have a means for connection of an equipment grounding conductor to the grounding pole. A terminal for connection to the grounding pole must be designated by one of the following [406.10(B)]:

• A green-colored hexagonal-headed or -shaped terminal screw or nut, not readily removable.
• A green-colored pressure wire connector body (a wire barrel).
• A similar green-colored connection device, in the case of adapters. The grounding terminal of a grounding adapter must be a green-colored rigid ear, lug, or similar device. The equipment grounding connection must be so designed that it cannot make contact with current-carrying parts of the receptacle, adapter, or attachment plug. The adapter must be polarized.
• If the terminal for the equipment grounding conductor is not visible, the conductor entrance hole must be marked with the word green or ground, the letters G or GR, a grounding symbol, or otherwise identified by a distinctive green color. If the terminal for the equipment grounding conductor is readily removable, the area adjacent to the terminal must be similarly marked.

Q20. What is the Code rule regarding the location of switches for stairway lighting?

A20. If the stairway between floor levels has six risers or more, a wall switch must be located at each floor level and at each landing level that includes an entryway to control the illumination for the stairway [210.70(A)(2)(c)].

Ex: Lighting outlets can be switched by a remote, central, or automatic control device.

Q21. What is the Code requirement for illumination of attics and under-floor spaces?

A21. At least one lighting outlet that contains a switch or is controlled by a wall switch must be installed in attics, underfloor spaces, utility rooms, and basements used for storage or containing equipment that requires servicing. The switch must be located at the usual point of entry to these spaces, and the lighting outlet must be located at or near the equipment that requires servicing [210.70(A)(3)].

Q22. What types of equipment grounding conductors does the NEC allow?  

A22. An equipment grounding conductor can be any one or a combination of the following [250.118]:

Note: The equipment grounding conductor is intended to serve as the effective ground-fault current path. See 250.2.

Author’s Comment: The effective ground-fault path is an intentionally constructed low-impedance conductive path designed to carry fault current from the point of a ground fault on a wiring system to the electrical supply source. Its purpose is to quickly remove dangerous voltage from a ground fault by opening the circuit overcurrent device [250.2].

(1) A bare or insulated copper or aluminum conductor sized in accordance with 250.122.

Author’s Comment: Examples include PVC conduit, Type NM cable, and Type MC cable with an equipment grounding conductor of the wire type.

(2) Rigid metal conduit (RMC).

(3) Intermediate metal conduit (IMC).

(4) Electrical metallic tubing (EMT).

(5) Listed flexible metal conduit (FMC) where:
a. The raceway terminates in listed fittings.
b. The circuit conductors are protected by an overcurrent device rated 20A or less.
c. The combined length of the flexible conduit in the same ground-fault current path doesn’t exceed 6 ft.
d. If flexibility is required to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor of the wire type must be installed with the circuit conductors in accordance with 250.102(E), and it must be sized in accordance with 250.122, based on the rating of the circuit overcurrent device.

(6) Listed liquidtight flexible metal conduit (LFMC) where:
a.  The raceway terminates in listed fittings.
b.  For 3⁄8 in. through ½ in., the circuit conductors are protected by an overcurrent device rated 20A or less.
c.  For ¾ in. through 1¼ in., the circuit conductors are protected by an overcurrent device rated 60A or less.
d. The combined length of the flexible conduit in the same ground-fault current path doesn’t exceed 6 ft.
e. If flexibility is required to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor of the wire type must be installed with the circuit conductors in accordance with 250.102(E), and it must be sized in accordance with 250.122, based on the rating of the circuit overcurrent device.

(8) The sheath of Type AC cable containing an aluminum bonding strip.

Author’s Comments:
•  The internal aluminum bonding strip isn’t an equipment grounding conductor, but it allows the interlocked armor to serve as an equipment grounding conductor because it reduces the impedance of the armored spirals to ensure that a ground fault will be cleared. It’s the aluminum bonding strip in combination with the cable armor that creates the circuit equipment grounding conductor. Once the bonding strip exits the cable, it can be cut off because it no longer serves any purpose.
•  The effective ground-fault current path must be maintained by the use of fittings specifically listed for Type AC cable [320.40]. See 300.12, 300.15, and 320.100.

(9) The copper sheath of Type MI cable.

(10) Type MC cable that provides an effective ground-fault current path in accordance with one or more of the following:
(a) It contains an insulated or uninsulated equipment grounding conductor in compliance with 250.118(1).
(b) The combined metallic sheath and uninsulated equipment grounding/bonding conductor of interlocked metal tape-type MC cable that’s listed and identified as an equipment grounding conductor.

Author’s Comment: Once the bare aluminum grounding/bonding conductor exits the cable, it can be cut off because it no longer serves any purpose. The effective ground-fault current path must be maintained by the use of fittings specifically listed for Type MCAP® cable [330.40]. See 300.12, 300.15, and 330.100.

(c) The metallic sheath or the combined metallic sheath and equipment grounding conductors of the smooth or corrugated tube-type MC cable that’s listed and identified as an equipment grounding conductor.

(11) Metallic cable trays where continuous maintenance and supervision ensure only qualified persons will service the cable tray, with cable tray and fittings identified for grounding and the cable tray, fittings [392.10], and raceways are bonded using bolted mechanical connectors or bonding jumpers sized and installed in accordance with 250.102 [392.60].

(13) Listed electrically continuous metal raceways, such as metal wireways [Article 376] or strut-type channel raceways [384.60].

(14) Surface metal raceways listed for grounding [Article 386].

For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.