NEC Questions and Answers based on 2008 NEC
January 2010 Part 1

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. When is plenum rated cable required?
A1. Control, signaling, and communications cables installed in spaces used for environmental air (such as the spaces above a suspended ceiling or below a raised floor) must be suitable for plenum use [300.22(C)]. This requirement doesn’t apply to habitable rooms or areas of buildings, the prime purpose of which isn’t air handling. See the following sections, depending on which type of cable is applicable.
–   CATV, 820.179(A)
–   Communications, 800.21
–   Control and Signaling, 725.154(A)
–   Fire Alarm, 760.7
–   Optical Fiber Cables and Raceways, 770.154(A)
–   Sound Systems, 640.9(C) and 725.154(A)

Q2. Can a receptacle be used as a splicing device? In other words, can we use all four screws (hot and neutral) on the receptacle?
A2. Yes, but continuity of the neutral conductor of a multiwire branch circuit must not be interrupted by the removal of a wiring device. In these applications the neutral conductors must be spliced together, and a “pigtail” must be provided for the wiring device. The opening of the ungrounded conductors, or the neutral conductor of a 2-wire circuit during the replacement of a device, doesn’t cause a safety hazard, so pigtailing of these conductors isn’t required [110.14(B)].

Caution: If the continuity of the neutral conductor of a multiwire circuit is interrupted (open), the resultant over- or undervoltage can cause a fire and/or destruction of electrical equipment.

Q3. How close to a bathtub/shower compartment can a light switch be located?
A3. Switches can be located next to, but not within, a bathtub, hydromassage bathtub, or shower space [404.4, 680.70, and 680.72].

Q4. What are the bonding requirements for service raceways and enclosures containing service conductors?
A4. The requirements are contained in 250.92 as follows:
(A) Bonding of Services. The metal parts of equipment indicated in (A)(1) and (A)(2) must be bonded together in accordance with 250.92(B).
(1)  Metal raceways containing service conductors.
(2)  Metal enclosures containing service conductors.

Note: Metal raceways or metal enclosures containing feeder and branch-circuit conductors are required to be connected to the circuit equipment grounding conductor in accordance with 250.86.

(B) Methods of Bonding. Metal raceways and metal enclosures containing service conductors must be bonded by one of the following methods:
(1) Neutral Conductor. By bonding the metal parts to the service neutral conductor.

Note:
•   A main bonding jumper is required to bond the service disconnect to the service neutral conductor [250.24(B) and 250.28].
•   At service equipment, the service neutral conductor provides the effective ground-fault current path to the power supply [250.24(C)]; therefore, an equipment grounding conductor isn’t required to be installed within PVC conduit containing service-entrance conductors [250.142(A)(1) and 352.60 Ex 2].

(2) Threaded Fittings or Entries. By using threaded couplings or threaded entries made up wrenchtight.
(3) Threadless Fittings. By using threadless raceway couplings and connectors made up tight.
(4) Bonding Fittings. When a metal service raceway terminates to an enclosure with a ringed knockout, a listed bonding wedge or bushing with a bonding jumper must be used to bond one end of the service raceway to the service neutral conductor. The bonding jumper used for this purpose must be sized in accordance with Table 250.66, based on the area of the service conductors within the raceway [250.92(B)(4) and 250.102(C)].

Note:
•   When a metal raceway containing service conductors terminates to an enclosure without a ringed knockout, a bonding-type locknut can be used.
•   A bonding locknut differs from a standard locknut in that it has a bonding screw with a sharp point that drives into the metal enclosure to ensure a solid connection.
•   Bonding one end of a service raceway to the service neutral provides the low-impedance fault current path to the source.

Q5. What are the bonding requirements for metal raceways?
A5. For raceways contain conductors less than 250V, other than service conductors, 250.96 applies: Metal parts intended to serve as equipment grounding conductors including raceways, cables, equipment, and enclosures must be bonded together to ensure they have the capacity to conduct safely any fault current likely to be imposed on them [110.10, 250.4(A)(5), and Note to Table 250.122].

Nonconductive coatings such as paint, lacquer, and enamel on equipment must be removed to ensure an effective ground-fault current path, or the termination fittings must be designed so as to make such removal unnecessary [250.12].

Note: The practice of driving a locknut tight with a screwdriver and pliers is considered sufficient in removing paint and other nonconductive finishes to ensure an effective ground-fault current path.

In addition, for raceways containing conductors operating at over 250V, we must comply with 250.97: Metal raceways or cables containing 277V and/or 480V feeder or branch circuits terminating at ringed knockouts must be bonded to the metal enclosure with a bonding jumper sized in accordance with 250.122, based on the rating of the circuit overcurrent device [250.102(D)].

Note:
•   Bonding jumpers for raceways and cables containing 277V or 480V circuits are required at ringed knockout terminations to ensure the ground-fault current path has the capacity to safely conduct the maximum ground-fault current likely to be imposed [110.10, 250.4(A)(5), and 250.96(A)].
•   Ringed knockouts aren’t listed to withstand the heat generated by a 277V ground fault, which generates five times as much heat as a 120V ground fault.

Exception: A bonding jumper isn’t required where ringed knockouts aren’t encountered, knockouts are totally punched out, or where the box is listed to provide a reliable bonding connection.

Q6. Can you please go over the rules for raceway bending requirements for electrical metallic tubing (EMT)?
A6. Raceway bends must not be made in any manner that would damage the raceway, or significantly change its internal diameter (no kinks) [358.24]. The radius of the curve of the inner edge of any field bend must not be less than shown in Chapter 9, Table 2 for one-shot and full shoe benders.

Note: This typically isn’t a problem, because most benders are made to comply with this table.

To reduce the stress and friction on conductor insulation, the maximum number of bends (including offsets) between pull points can’t exceed 360° [358.26].

Note: There is no maximum distance between pull boxes because this is a design issue, not a safety issue.

Q7. Please explain the grounding requirements for a service that consists of six switches in six separate, but grouped.
A7. A grounding electrode conductor is permitted from each service disconnecting means sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor for each service disconnecting means. Or, a single grounding electrode conductor is permitted from a common location, sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor at the location where the connection is made [250.64(D)].

Q8. Can you please go over the rules for installing grounding electrode conductors?
A8. Most of the requirements for the grounding electrode conductor (GEC) are found in 250.64 as follows:
(A) Aluminum Conductors. Aluminum grounding electrode conductors must not be within 18 in. of earth.
(B) Conductor Protection. Where run exposed, ground­ing electrode conductors must be protected where subject to physical damage, and grounding electrode conductors 6 AWG copper and larger can be run exposed along the surface of the building if securely fastened and not subject to physical damage.
Grounding electrode conductors sized 8 AWG must be installed in rigid metal conduit, intermediate metal conduit, PVC conduit, or electrical metallic tubing.

Note: A ferrous metal raceway containing a grounding electrode conductor must be made electrically continuous by bonding each end of that type of raceway to the grounding electrode conductor [250.64(E)]. So it’s best to use PVC conduit.

(C) Continuous Run. The grounding electrode conductor is not permitted to be spliced except as permitted in (1) or (2):
(1)  Irreversible compression-type connectors listed for grounding, or by exothermic welding.
(2)  Sections of busbars connected together.
(D) Grounding Electrode Conductor for Multiple Service Disconnects.
(2) Grounding Electrode Conductor from Each Disconnect. A grounding electrode conductor is permitted from each service disconnecting means sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor for each service disconnecting means.
(3) One Grounding Electrode Conductor. A single grounding electrode conductor is permitted from a common location, sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor at the location where the connection is made.
(E) Ferrous Metal Enclosures Containing Grounding Electrode Conductors. To prevent inductive choking of grounding electrode conductors; ferrous raceways and enclosures containing grounding electrode conductors must have each end of the raceway or enclosure bonded to the grounding electrode conductor in accordance with 250.92(B).

Note: Nonferrous metal raceways, such as aluminum rigid metal conduit, enclosing the grounding electrode conductor aren’t required to meet the “bonding each end of the raceway to the grounding electrode conductor” provisions of this section.

Caution: The effectiveness of a grounding electrode is significantly reduced if a ferrous metal raceway containing a grounding electrode conductor isn’t bonded to the ferrous metal raceway at both ends. This is because a single conductor carrying high-frequency induced lightning current in a ferrous raceway causes the raceway to act as an inductor, which severely limits (chokes) the current flow through the grounding electrode conductor. ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) states: “An inductive choke can reduce the current flow by 97 percent.”

Note: To save a lot of time and effort, run the grounding electrode conductor exposed if it’s not subject to physical damage [250.64(B)], or enclose it in PVC conduit suitable for the application [352.10(F)].

(F) Termination to Grounding Electrode.
(1) Single Grounding Electrode Conductor. A single grounding electrode conductor is permitted to terminate to any grounding electrode of the grounding electrode system.
(2) Multiple Grounding Electrode Conductors. When multiple grounding electrode conductors are installed [250.64(D)(2)], each grounding electrode conductor is permitted to terminate to any grounding electrode of the grounding electrode system.
(3) Termination to Busbar. A grounding electrode conductor and grounding electrode bonding jumpers are permitted to terminate to a busbar sized not less than ¼ in. × 2 in. that is securely fastened at an accessible location. The terminations to the busbar must be made by a listed connector or by exothermic welding.