Breakdown of Article 250 in the National Electrical Code®.
Questions about grounding and bonding? You’re not alone.
Article 250 in NFPA 70®: National Electrical Code is the most asked-about article in the NEC. Grounding is the safety belt of the electrical system; when something goes wrong, the equipment-grounding conductor is needed to operate the overcurrent protective device. Bonding ensures we are at the same potential as the earth, to mitigate any touch or step potentials that may exist. A recent nec connect Breakdown Session featuring NFPA Senior Electrical Specialist Gil Moniz explored changes in the 2014 NEC and answered Code users’ questions. This recap expands on the technical changes to Article 250 and includes the Q&A that followed the live session.
Article 250 covers general grounding and bonding requirements for all electrical installations not specifically mentioned in other articles such as Article 680: Swimming Pools
NEC Article 250 covers general requirements for grounding and bonding of electrical installations. For example, which systems are permitted to be grounded and ungrounded, which conductors are to be grounded, the location of grounding connections, the types and sizes of equipment grounding and bonding conductors, and methods of grounding and bonding. “The main reason for grounding and bonding is safety,” says Senior Electrical Specialist Gil Moniz, “Proper grounding and bonding is important because it protects the user of electricity from potentially fatal electrical shocks and injuries — and ensures equipment is protected in the event of a fault.”
Changes in Article 250 keep pace with advances in technology and address
new methods, new products, and recent developments in electrical safety.
The NEC is a living, breathing document. Changes to Article 250 respond to numerous forces within the field. Code-Making Panel 5, the Technical Committee responsible for Article 250, made between 50 and 60 changes in the 2014 NEC, including revising the phrases “less than 1000 volts” and “1000 volts and over” to “up to 1000 V” and “over 1000 V” to eliminate inconsistent use within the Article and with other product standards and international codes. Another terminology revision made throughout Article 250 was the addition of “switchgear” to the phrase “switchboard and panelboard.” This correlates with action taken
by CMP 9 to place a revised definition of what used to be “Metal-Enclosed Power Switchgear” in Article 100.
Marking of ungrounded systems 50V to 1000V is covered in 250.21.
Revised 250.21(C) now correlates with Article 408.3(F)(2) for consistent marking requirements for ungrounded systems to be legibly marked. This change incorporates the marking requirements from Article 408.3(F)(2) which require switchboard, switchgear, or panelboard containing an ungrounded AC electrical system to be legibly and permanently field marked as “Caution Ungrounded System Operating — _____Volts Between Conductors.”
Other changes to Article 250 continue to emphasize the differences between grounding and bonding.
As part of an ongoing process, changes to emphasize the distinction between grounding and bonding have been made over a number of Code cycles and continue in the 2014 NEC. Terminology has changed for greater clarity. Sometimes equipment grounding and bonding is the same, but sometimes it’s not. “When we’re talking about bonding, we’re connecting,” says Gil Moniz, “and when we’re talking about grounding we are providing a fault path for grounding.”
A new Table 250.102(C)(1) deals with the sizing of system bonding jumpers.
Previous to the 2014 NEC, multiple sections such as for main bonding jumpers, system bonding jumpers, supply side bonding jumpers, and grounded conductors referred to Table 250.66 for sizing the conductor or jumper. Now, Code users can access Table 250.102(C)(1) for sizing a main bonding jumper. Corresponding changes to related sections simplify the sizing requirements for fault-carrying conductors that are not sized using Table 250.122.
Another key change in 250.64 deals with grounding electrode conductor installation — securing and protection against physical damage.
Electrical inspectors and contractors will relate to this scenario: When connecting one grounding electrode to another grounding electrode, does the bonding jumper have to meet the depth requirements of 300.5? “This may not be a perfect situation if you’re running down the exterior wall of a building, down into the ground to meet the depth requirements, and then back up to connect to the grounding electrode,” says Gil Moniz. “That would actually impede the effectiveness of the grounding electrode conductor.” In the 2014 NEC, 250.64(B) is revised with the addition of a new sentence clarifying that the grounding electrode conductors and grounding electrode bonding jumpers are not required to comply with 300.5.