Editor's note: In this month's article, Mike Holt discusses the 2011 NE Code changes that affect grounding products, the necessary protection for all cable and raceway installations, and direct-burial applications.
250.53(A) Rod, Pipe, and Plate Electrodes
The 25-ohm rule has been relocated and greatly clarified. Editorial changes to this section have also been made, and the Informational Note has been revised.
250.53 Grounding Electrode Installation Requirements.
(A) Rod, Pipe, or Plate Electrodes.
Below Permanent Moisture Level. If practicable, rod, pipe, and plate electrodes must be embedded below the permanent moisture level and be free from nonconductive coatings such as paint or enamel.
Supplemental Electrode. A single rod, pipe, or plate electrode must be supplemented by an additional electrode that's bonded to one of the following:
The single rod, pipe, or plate electrode
The grounding electrode conductor of the single electrode
The neutral service-entrance conductor
The nonflexible grounded service raceway
The service enclosure
Ex.: If a single rod, pipe, or plate grounding electrode has an earth contact resistance of 25 ohms or less, the supplemental electrode isn't required.
Spacing. The supplemental electrode for a single rod, pipe, or plate electrode must be installed not less than 6 ft from the single electrode. (Fig. 8)
Note: The efficiency of paralleling electrodes is improved by spacing them at least twice the length of the longest rod.
Analysis: The long-standing rule that a ground rod as well as a pipe or plate electrode must have a resistance to earth of 25 ohms or less or be supplemented by an additional electrode was well understood until recent revisions to the NEC created confusion. These revisions left the Code user trying to figure out if a concrete-encased electrode required a supplement and when a ground rod is actually required. Revisions to this section now match the standard industry practice of (when required) driving two ground rods instead of testing the resistance of a single driven rod. The 25-ohm language is now written as an exception, recognizing this practice. Code users will notice that 250.56 has been deleted as a result of this change, but the technical provisions contained therein haven't disappeared.
The Informational Note explaining the logic of spacing ground rods more than 6 ft apart, while technically true, didn't provide any guidance as to what the spacing should be. This change clarifies that the driven rods should be at least twice the length of the longer of the two rods. For example, two rods 8 ft in length should be driven at least 16 ft apart. It's worth remembering that this is an Informational Note, not a Code requirement [90.5(C)].
250.121 Use of Equipment Grounding Conductors
A new section was added to prohibit the use of the equipment grounding conductor as a grounding electrode conductor.
250.121 Use of Equipment Grounding Conductors. An equipment grounding conductor isn't permitted to be used as a grounding electrode conductor.
Analysis: The grounding electrode conductor (GEC) is intended to help direct lightning-induced energy to the earth, while an equipment grounding conductor (EGC) is intended to provide a low-impedance ground-fault current path to the source to operate overcurrent devices in the event of a ground fault. The requirements for sizing are also different. An EGC is sized in accordance with 250.122, while a GEC is sized using 250.66. Because these conductors have different rules, different sizing requirements, and different installation requirements, this section was added to clarify that one conductor can't fill the roles of both an EGC and a GEC.
300.4 Protection Against Physical Damage
This rule has been revised to be more technically accurate.
300.4 Protection Against Physical Damage. Conductors, raceways, and cables must be protected against physical damage [110.27(B)].
Analysis: Previous editions of the NEC have required protection of conductors where subject to physical damage. While most Code users understand this rule is intended to apply to all conductors in all wiring methods, it didn't clearly state that. This revision makes it clear that all conductors in all wiring methods must be protected from physical damage.
The rule on protecting raceways under metal-corrugated sheet roof decking has been expanded.
300.4(E) Wiring Under Roof Decking. Cables, raceways, and enclosures under metal-corrugated sheet roof decking must not be located within 1½ in. of the roof decking, measured from the lowest surface of the roof decking to the top of the cable, raceway, or box. In addition, cables, raceways, and enclosures aren't permitted in concealed locations of metal-corrugated sheet decking type roofing.
Ex: Spacing from roof decking doesn't apply to rigid metal conduit and intermediate metal conduit.
Analysis: New to the 2008 NEC was a requirement for the protection of most raceways when installed within 1½ in. of the roof deck. Although this 2008 rule change went a long way toward protecting wiring systems from damaging roofing screws that can penetrate the raceways, it left out one critical part of the installation — boxes. With this change, it's clear that the Code is concerned not only with protecting the raceways, but also the boxes.
This rule was also changed to prohibit wiring methods from being installed in concealed locations above the roof decking. In some instances, installers place raceways above the roof deck prior to the insulation being installed, which results in the same potential for damage from roofing screws.
The requirement for protection of conductors 4 AWG and larger has been changed to add clarity.
300.4(G) Insulating Fittings. If raceways contain insulated circuit conductors 4 AWG and larger that enter an enclosure, the conductors must be protected from abrasion during and after installation by a fitting identified to provide a smooth, rounded insulating surface, such as an insulating bushing. (Fig. 9)
Ex.: Insulating bushings aren't required if a raceway terminates in a threaded raceway entry that provides a smooth, rounded, or flared surface for the conductors. An example would be a meter hub fitting or a Meyer's hub-type fitting.
Analysis: The term “substantial fitting” has been replaced with the term “identified” so that inspectors won't have to interpret the Code unnecessarily. The term “identified” is clearly defined in Art. 100, is used throughout the NEC, and takes interpretation out of the requirement. Although fittings that are designed to provide this protection are typically used to achieve compliance with this requirement, it could have been argued that fittings designed for another application could satisfy this rule. This change removes that argument by making the NEC a more prescriptive document.
A new protection requirement for structural (expansion) joints was added.
300.4(H) Structural Joints. A listed expansion/deflection fitting or other approved means must be used where a raceway crosses a structural joint intended for expansion, contraction or deflection.
Analysis: In larger commercial/industrial buildings, it isn't uncommon to see an expansion joint inside of the building. When these are encountered, the Code has never offered any guidance to the installer as it pertains to wiring methods. With this change, it becomes clear that a fitting or other approved means must be used to allow for expansion or deflection of the wiring method.
300.5 Underground Installations
Type MI and Type MC Cables are now allowed to be installed under buildings without a raceway.
300.5(C) Cables Under Buildings. Cables installed under a building must be installed in a raceway that extends past the outside walls of the building.
Ex. 2: Type MC Cable listed for direct burial is permitted under a building without installation in a raceway [330.10(A)(5)]. (Fig. 10)
Analysis: Although certain types of MC cable are listed for direct burial and concrete encasement, this rule has prohibited them from being installed underneath buildings. This change now allows cables to be installed under the floor slab of a building, which has been accepted by many inspectors for some time. Interestingly, other cables that are listed for this application, such as UF cable, aren't recognized by this change.
This change clarifies the use of single conductor cables installed in parallel.
300.5(I) Conductors Grouped Together. All conductors of the same circuit, including the equipment grounding conductor, must be inside the same raceway or in close proximity to each other. See 300.3(B).
Ex. 1: Conductors can be installed in parallel in raceways, multiconductor cables, or direct-buried single-conductor cables. Each raceway or multiconductor cable must contain all conductors of the same circuit, including the equipment grounding conductor. Each direct-buried single-conductor cable must be located in close proximity in the trench to the other single-conductor cables in the same parallel set of conductors, including equipment grounding conductors.
Ex. 2: Parallel circuit conductors installed in accordance with 310.10(H) of the same phase or neutral can be installed in underground PVC conduits, if inductive heating at raceway terminations is reduced by the use of aluminum locknuts and cutting a slot between the individual holes through which the conductors pass as required by 300.20(B).
Analysis: Conductors of the same circuit are required to be grouped in the same raceway or cable to help reduce the inductive reactance of the conductors. A very literal reading of previous Code editions didn't address the use of single-conductor cables, such as many USE cables, in parallel installations. The NEC now recognizes this practice while giving guidance on how to install these cables. The issues of inductive reactance are addressed by requiring these single-conductor cables to be installed within close proximity of each other.
300.11(A)(2) Nonfire-Rated Ceiling Assemblies
The rule requiring identification of electrical ceiling support wires has been expanded.
300.11(A)(2) Nonfire-Rated Ceiling Assembly. Wiring in a nonfire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceiling assembly. The independent support wires must be distinguishable from the suspended-ceiling support wires by color, tagging, or other effective means.
Analysis: Identification of ceiling-support wires used for electrical equipment was previously limited to those wires that are in fire-resistance-rated ceiling assemblies. In order to distinguish which wires are installed by the electrician and which are installed by the ceiling contractor, this change requires identification of the independent support wires for all ceiling systems, whether the ceiling assembly is fire-resistance rated or not.