The purpose statement of the NEC, section 90.1(A) states, “The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.”
In other words, it all comes back to protecting people and property from the inherent hazards of using electricity. This is directly evident when we take a look at the grounding and bonding requirements within the NEC. These are two key concepts used for protection. Think of grounding and bonding as the foundation of a safe electrical installation.
Section 250.4 lays out the performance requirements of grounding and bonding electrical systems. The rest of Article 250 covers how to achieve this desired level of protection. Similar to the purpose statement of the NEC, we are given guidance toward the intended results and then a set of rules to follow. Contractors can also consult Table 250.3 for a list of other articles that contain these requirements.
So, why do we ground systems and bond equipment together? Grounding is the act of connecting the electrical system or equipment to the earth or a conductive object that extends the connection to the earth. Bonding is connecting things together with a conductive path to establish electrical continuity. Both are foundational safety concepts in the NEC, and often talked about in the same breath, but they are distinctly different concepts.
Let’s tackle grounding first. As Section 250.4(A)(1) explains, electrical systems connect to ground to limit voltage imposed from lightning strikes, line surges, high-voltage crossovers and to stabilize voltage-to-ground under normal operation. Then equipment is connected to the system to limit voltage-to-ground at the equipment. This is done through a conductor that extends the connection from the equipment to the grounding electrode conductor back at either the service or the source of a separately derived system. This ensures control of the voltage.
However, we need to establish if the system needs a conductor connected to ground. This is the difference between a grounded system and an ungrounded system. Both are allowed by the NEC; however, there are specific times when one is required over the other. The rules for which systems must be grounded are relatively straight forward. The systems required to be grounded are:
- Any system that can be grounded so that the maximum voltage-to-ground doesn’t exceed 150V
- A 3-phase, 4-wire Wye system that uses the neutral as a circuit conductor, and
- A 3-phase, 4-wire Delta system that has a circuit conductor that connects to the midpoint of one of the phase windings
In other words, if it is likely that a system will be supplying 120V or line to neutral loads, it will need to be grounded unless specifically permitted or required to not be grounded by 250.21 or 250.22.
In order to establish this connection to ground, an installer must establish a network of conductive items. This is the grounding electrode system. There are some electrodes that are part of the building construction and others that must be installed. Either way, the NEC requires all electrodes present on the premises to be included in the system. The permitted electrodes can be found in section 250.52.
This system serves to establish that connection to ground stabilizes voltage. A common misconception is that electricity seeks the ground, but it is actually trying to return to its point of origin. For a grounded system, this might mean some current will take a certain path, but it cannot be relied on to take the place of an effective ground-fault current path. To establish the effective ground-fault current path, we need to turn to bonding.
If fault current can take unexpected paths, it will be difficult to apply the rules of physics to increase safety. As electricians, it is our job to harness the flow of electrons through a wiring system in order to perform work. It is no different in abnormal conditions. We know certain things happen with electricity because of science. The best way to protect from electrical hazards is to turn the power off, right? Code writers know this and have incorporated requirements aimed at the automatic de-energization of the circuit under abnormal or fault conditions
This is where the concept of the effective ground-fault current path becomes the star of the safety show. This is defined in the NEC as an intentionally constructed, low-impedance path designed to carry current underground-fault conditions from the fault to the source. It facilitates the operation of automatic overcurrent protection devices or ground detectors for ungrounded systems.
This means part of the system design provides a complete path from the furthest reaches of the premise wiring system back to the source of power. However, unlike the grounding electrode system, the effective ground-fault current path must have low enough impedance or resistance to facilitate the automatic operation of the overcurrent protective device (OCPD).
With a given applied voltage and reducing the opposition to current, more current will flow, which reduces it to a negligible value and the current goes up very quickly. OCPDs open faster with more current until the current exceeds the interrupting rating. This low opposition to current flow and direct path back to the source spikes the current high enough to be within the instantaneous trip range of the OCPD. When this low impedance is not achieved, such as through the earth, there is no guarantee that the OCPD will open when it should.
The main components of this effective ground-fault current path are made up of the equipment grounding conductors such as bonding jumpers (main, system, supply-side, equipment) and service or system grounded conductors. Equipment grounding conductors are the effective ground-fault current path at the feeder and branch circuit levels of the premise wiring system, and it must be sized in accordance with Table 250.122, which is based on OCPD size. Discontinuous portions of the equipment grounding conduction (EGC) are connected by equipment bonding jumpers that are sized from this same table. While the EGC system connects equipment to the earth to limit the voltage-to-ground at the equipment, it also serves a dual role and bonds noncurrent carrying metal parts of the system together to connect them to the effective ground-fault current path.
Once the EGC delivers fault current to the branch circuit or feeder distribution equipment it has served its function. From there, it needs to transfer to the conductor that will serve as the fault current path back to the source. This is done through the use of the main system and supply-side bonding jumpers. However, because these are bonding jumpers, they are connecting two components of the system together. They are connecting the EGC system to the grounded conductor. Because of this function, there is not an OCPD component to how we size these. Rather, these jumpers are sized based on how much current the system itself can supply through the ungrounded conductors. These conductors are installed as a fault current path and are integral to the EGC being able to perform its duties.
The last link in the chain gets back to the source, and the service level is often a transformer on a pole or located somewhere outside of the building. We make the connection from the EGC to the grounded conductor in the first disconnecting means on the premises and rely on the grounded conductor to deliver the fault current back to the source. All of these bonding jumpers and the grounded conductor are sized based on the potential fault current that can be supplied by the source. Table 250.102(C) sizes fault current paths based on what size ungrounded conductors are installed. Grounded conductors might also serve as neutral conductors, and this requires other considerations based on how much neutral current the conductor will carry. The two different roles of the neutral conductor must be compared and the larger of the two must be used.
In addition to serving as a fault current path, bonding is often used to keep an environment at the same potential. In certain environments, small changes in voltage can have dire consequences.
When grounding or bonding, or performing any electrical work governed by the NEC, remember the purpose of the code: to keep everyone safe. Understanding why you’re applying codes and standards, will help you determine how you perform the work.