National Electrical Code 2005: Article 250 – Grounding and Bonding
By Chris Tapas, P.E., President, Tapas Engineering Services PC, Chicago -- Consulting-Specifying Engineer, 3/3/2005 12:00:00 AM MT
Editor’s Note: This is the third in a regular monthly column that covers significant new issues raised by the 2005 Edition of the National Electrical Code.
Over the past two code cycles, Article 250 of the 2005 NEC was extensively revised to make it better organized and easier to implement. Article 250 further clarifies the requirements for an electrical distribution system grounding system, including: requirements for providing paths to divert high voltage to the earth; requirements for the low-impedance fault current path to facilitate the operation of overcurrent protection devices; and how to remove dangerous voltage potentials between conductive parts of building components and electrical systems. An effectively grounded electrical distribution system is a start to providing a safe and hazard free environment to the personnel occupying and working in a facility.
The 2005 NEC has several changes to Article 250 and Article 100 – Definitions. The changes to the definitions are made to further refine the intent of the NEC's Article 250. Changes to Article 100 include a definition for grounding electrodes, system bonding jumper and further explanation on the definition of grounding electrode conductors. The changes to definitions in Article 250.2 are the expansion of the effective ground-fault current path definition to include “facilitation of the operation of an overcurrent protection device or ground fault detectors.”
The change to Article 250.8 further limits the methods of grounding or bonding equipment by adding the term “or connection devices” to last sentence, making it all encompassing.
Also, there is an editorial change for the title of Article 250.28 to “Main Bonding Jumper and System Bonding Jumper” from “Main Bonding Jumper.” The term system-bonding jumper was added to Article 100 – Definitions and discussed in an earlier column in this series. This is to avoid confusion when dealing with bonding jumpers and clarifies the method of sizing equipment-bonding jumpers when the separately derived system conductors are run in parallel.
A substantial addition to Article 250.118(5)(D) from the 2002 edition pertaining to flexible metal conduit is also included. The code requirement for using flexible metal conduit as a grounding conductor was enhanced to include the following:
a. The conduit is terminated in fittings listed for grounding.
b. The circuit conductors contained in the conduit are protected by over-current devices rated at 20 amperes or less.
c. The combined length of flexible mental conduit and flexible metallic tubing and liquid-tight path does not exceed 6 feet (1.8m)."
d. Where used to connect equipment where flexibility is necessary, an equipment-grounding conductor shall be installed.
These enhancements increase the specificity of the code requirement. These changes and several of the other changes in Article 250 not covered in this month's column remove and/or modify some of the language from the 2002 Edition of the NEC that could have been perceived as vague and open to wide interpretation.
