Code Quandaries

Apr 1, 2010 12:00 PM, By Mike Holt, NEC Consultant

Your most pressing National Electrical Code (NEC) questions answered. All questions and answers are based on the 2008 NEC

Q. What are the bonding requirements for service raceways and enclosures containing service conductors?

A. The requirements are contained in 250.92, and read as follows:

Fig. 1. Enclosures and raceways can be bonded to the service neutral conductor.

(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.

Author's comment: 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, as shown in Fig. 1.

Author's comment: 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 wrench tight.

Fig. 2. 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.

(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)].

Author's comment: 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, as shown in Fig. 2.

Q. When and where am I required to install plenum-rated cable?

A. 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. Refer to the following sections of the NEC, depending on which type of cable you're installing:

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)]

Q. How close to a bathtub/shower compartment can I locate a light switch?

A. Switches can be located next to, but not within, a bathtub, hydromassage bathtub, or shower space [404.4, 680.70, and 680.72].

Q. Can you please go over the rules for raceway bending when working with electrical metallic tubing (EMT)?

A. Raceway bends must not be made in any manner that would damage the raceway, or significantly change its internal diameter (i.e., 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. 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]. There is no maximum distance between pull boxes, because this is a design issue, not a safety issue.

Q. Please explain the grounding requirements for a service that consist of up to six switches or six circuit breakers mounted in a group of separate enclosures.

A. 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)].

Q. Can 14-2 NM cable be used for the travelers of a 3-way switch?

A. Yes, according to 404.2(A), wiring for 3-way and 4-way switching must be done so that only the ungrounded conductors are switched. In other words, the neutral conductor must not be switched. The white insulated conductor within a cable assembly can be used for single-pole, 3-way, or 4-way switch loops, if it's permanently re-identified to indicate its use as an ungrounded conductor at each location where the conductor is visible and accessible [200.7(C)(2)].

Where a metal raceway or metal-clad cable contains the ungrounded conductors for switches, the wiring must be arranged to avoid heating the surrounding metal by induction. This is accomplished by installing all circuit conductors in the same raceway in accordance with 300.3(B) and 300.20(A), or ensuring that they’re all within the same cable.

Exception: A neutral conductor isn't required in the same raceway or cable with travelers and switch leg (switch loops) conductors.

Q. We are being told that a 16 AWG conductor is too small to use for a control conductor. Is this true?

A. No. Conductors of sizes 18 AWG and 16 AWG installed in a raceway, enclosure, or listed cable are permitted if they don't supply a load that exceeds the ampacities given in 402.5. Conductors of 14 AWG and larger must not supply loads greater than the ampacities given in 310.15 [725.49(A)].

In addition, overcurrent protection for conductors 14 AWG and larger must be in accordance with the conductor ampacity, without applying the ampacity adjustment or correction factors of 310.15, and overcurrent protection must not exceed 7 amperes for 18 AWG conductors and 10 amperes for 16 AWG.

Q. Some of our transformers have factory-installed bonding straps from the X0 terminal to the transformer enclosure. Looking at them, they seem very small. How do we know that these straps are big enough?

A. Product evaluation for safety is typically performed by a testing laboratory, which publishes a list of equipment that meets a nationally recognized test standard. Products and materials listed, labeled, or identified by a testing laboratory are generally approved by the AHJ. According to 90.7, "It is the intent of this Code that factory-installed internal wiring or the construction of equipment need not be inspected at the time of installation of the equipment, except to detect alterations or damage, if the equipment has been listed by a qualified electrical testing laboratory that is recognized as having the facilities described in the preceding paragraph and that requires suitability for installation in accordance with this Code."