what is the proper way to ground multiple disconnects on a single service entrance

Learning Objectives

• Learn the proper electric grounding terminologies.
• Sympathise National Electrical Code grounding and bonding requirements for solidly grounded alternating electric current low-voltage systems (below i,000 volts).
• Prevent common grounding and bonding design and construction errors.

Electrical grounding and bonding is 1 of the many misunderstood topics of discussion in the design and construction manufacture. There are two main reasons for understanding grounding and applying the right design for grounding and bonding: safety and correct operation of sensitive electronic equipment.

NFPA 70: National Electrical Code Article 250 covers the minimum requirements for grounding and bonding and, although the NEC lists requirements to bide by, it should not be taken every bit a blueprint manual. Some terms and requirements discussed may be true for the European standards, however, the intent of this article is to clarify grounding and bonding design seen in the United States.

Grounding and bonding requirements

Article 250 is a circuitous portion of the NEC and covers many different types of systems: grounded systems (less than 50 volts, 50 to 1,000 volts and greater than 1,000 volts), ungrounded systems, systems greater than ane,000 volts, impedance grounded neutral systems, straight current systems, separately derived systems and grounding of instrument and meters/relays. The intent of this article is to hash out the requirements of solidly grounded, alternating current electrical systems less than 1,000 volts.

Effigy 1: The analogy of grounding systems shows connection from utility to load. Courtesy: CDM Smith

Grounding and bonding practices are important and required per NEC because when washed properly, it will protect personnel from electric shock hazards and ensure electric system operation. These practices perform the following functions:

• Keeps equipment enclosures and other normal metal parts stable and therefore, safe to touch.
• Limits unintended voltage on the electrical organisation imposed past lightning, line surges or unintentional contact with higher-voltage lines.
• Bonds electrical equipment together to establish a depression impedance path (effective ground-fault electric current path) from the error location back to supply source to facilitate the functioning of overcurrent devices.
• Establishes a stable voltage to basis during functioning, including curt circuits.
• Keeps electromagnetic interferences from causing misoperation.
• Prevents objectionable current.

The requirements for grounding and bonding begin at the service. The NEC requires the grounded conductor(s) to exist routed with the ungrounded conductors to the service entrance equipment and it shall connect to the grounded conductor(s) concluding or bus. The grounded service conductor is required to be continued to a grounding electrode conductor at each service. The chief bonding jumper shall connect the grounded conductor to equipment-grounding conductors and the service archway enclosure via the grounded conductor's terminal or bus.

The GEC shall be used to connect the EGCs, the service equipment enclosures and where the system is grounded, the grounded service conductor to the grounding electrodes. Figure 1 details the grounding organisation connections.

Effigy 2: Ground rod spacing is shown in this illustrations. Courtesy: CDM Smith

The minimum sizes of the grounded conductor, EGC and GEC are determined based on NEC Tabular array 250.102(C)(1), Table 250.122 and Table 250.66, respectively. The sizes for the main bonding jumpers, supply side bonding jumpers and system bonding jumpers can likewise be sized from Table 250.102(C)(i).

Although the grounded conductor is connected on the supply side, it shall not be connected to the EGCs or reconnected to footing on the load side of the service disconnection means except as otherwise permitted in the 2017 NEC Article 250.142(B).

Common errors

There are a few errors commonly seen in design or during construction due to a lack of understanding or misconception apropos grounding, bonding and the NEC Article 250. A few usually seen errors are:

Error 1: Using the wrong tables for EGC, grounded conductor or GEC.

The sizing methods detailed in the NEC are the minimum requirements and it may not be adequate for the telescopic and size of the project. Large available brusk-circuit currents may require larger usher sizes than the minimum NEC requirements.

The EGC should be sized per Table 250.122. A full-sized EGC is required to prevent overloading and possible burnout of the conductor if a ground fault occurs along ane of the parallel branches. The EGC is sized in accordance with Tabular array 250.122 based on the rating of the overcurrent protective device upstream that protects the conductors routed with the EGC.

However, the sizes for EGC in Table 250.122 does non account for voltage drop. Therefore, ungrounded conductors shall be sized while taking into account the voltage driblet and per 250.122(B), the EGC shall be increased in size proportionately to the upsized ungrounded conductors. For example, given a 480-volt branch feeder excursion billow rated 150 amperes, the EGC shall be sized vi AWG copper or 4 AWG aluminum for a voltage drop of at near 3%.

The grounded conductor at the service should be sized in accordance with Table 250.102(C)(1), based on the size of largest ungrounded conductor or equivalent area for parallel conductors. This table can also exist used to size the chief bonding jumper, system bonding jumper and supply-side bonding jumper for Air conditioning systems. As stated in the notes of Table 250.102(C)(1), for ungrounded conductors larger than 1,100 kcmil copper or 1,750 kcmil aluminum, the conductor shall take an area not less than 12.5% of the area of the largest ungrounded supply conductor or equivalent area for parallel supply conductors. If the ungrounded conductors are installed in parallel in two or more sets, the grounded usher shall too be installed in parallel.

For parallel sets, the equivalent size of the largest ungrounded supply conductor(s) shall be determined by the largest sum of the areas of the corresponding conductors of each set. For example, given that the electrical service is supplied past five sets of 500 kcmil copper conductors, the grounded conductor required in each set shall be 350 kcmil copper. The total equivalent surface area of the parallel supply conductors in each prepare is ii,500 kcmil (five times 500 kcmil given five parallel ungrounded conductors). Considering the equivalent expanse is higher up one,100 kcmil for copper, the grounded conductor(s) shall have an expanse non less than 12.v%. This is an area of roughly 312.5 kcmil, which according to Table 8 of Chapter nine in the 2017 NEC, is 350 kcmil copper.

Effigy 3: This compares a separately derived system (right) to a nonseparately derived organisation. Courtesy: CDM Smith

The GEC should be sized per Table 250.66. The notes at the bottom of Table 250.66 needs to exist considered if at that place are multiple service entrance conductors or no service archway conductors. Given the number of service entrance conductors, the size is determined either by the largest ungrounded service-entrance conductor or the equivalent area for parallel conductors. The size of the GEC is also dependent on the material of the usher and its connection to specified electrodes in Commodity 250.66(A) through (C). The allowed materials are copper, aluminum, copper-clad aluminum and items allowable in Article 250.68(C).

For case, given that the electrical service is supplied by ane set of 500 kcmil copper conductors, the GEC per Table 250.66 shall be 1/0 AWG copper. The location for GEC installation is at the service, at each building or construction where supplied by a feeder(s) or branch circuit(s) or at a separately derived system.

To reiterate, the GEC is the connexion of the arrangement grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode arrangement. This leads on to fault No. 2, errors in the grounding electrode system, which is normally seen in design and construction.

Error 2: Meeting only bare minimum NEC requirements for grounding electrode system that may non satisfy projection telescopic.

The grounding electrode system is made up of grounding electrodes that are present at each edifice or construction served that are bonded together. The items that qualify equally a grounding electrode are detailed in Article 250.52, which includes physical-encased electrode, basis ring encircling the building or structure, rod and pipe electrodes, plate electrodes and other listed electrodes. The NEC details the minimum requirements only non necessarily the pattern or construction requirement that allows for a functional system depending on the project telescopic.

These are the usually seen issues in grounding electrode arrangement that follows the NEC, but does not satisfy project scope:

• Not installing a third grounding electrode. The NEC requires a minimum of two grounding electrodes, unless 1 electrode has a resistance to earth less than 25 ohms. However, commonly in structure, the footing resistance is not measured again after a supplemental grounding electrode is installed. Therefore, the ground resistance of 25 ohms is non confirmed as having been met. Per the NEC, two electrodes would run into code, but this doesn't guarantee a low electrode-to-globe resistance. Including a grounding ring with multiple grounding electrodes is considered a best practice to ensure depression resistance. Too, specifications should too crave basis resistance measurements to be taken after grounding electrode system is installed to determine if additional electrodes are required.
• Assuasive 25 ohms ground resistance considering it is immune by code.
  • The NEC only requires 25 ohms footing resistance; nonetheless, the industry recognizes a lower resistance value may be more than desirable. International Electrical Testing Association ATS-201313 recommends five ohms or less for big industrial systems.
• Installing grounding electrodes (in particular, rods) half dozen feet autonomously because that is the minimum separation required past code.
  • Each footing rod has its own zone of influence as shown in Figure two. The optimal spacing between rods should be twice the length of the ground rod. When the zones overlap, the net resistance of each rod increase, thus making the ground organization less effective.

There are many considerations that need to be taken into account when designing and installing grounding electrode systems. These are:

• Size of service.
• Types of loads that will be connected.
• Soils: the resistivity is affected past table salt, moisture, temperature and depth.

While considering all of the to a higher place factors, some of the all-time practices seen in the manufacture are using footing rings around buildings, ground triangles at smaller services, exothermic welds for curtained or cached connections and ground rods and installing ground testing/inspection wells that allow easy access for ground resistance testing.

Figure 4: This is a service entrance primary breaker with a four-wire load. The line side is at the top with the white neutral conductors and the load side is at the bottom with greyness neutral conductors. Courtesy: CDM Smith

Mistake iii: Bonding grounded conductor (neutral) to ground bar at multiple locations.

Per Article 250.142, the neutral to basis connection is allowed on the supply side or within the enclosure of the AC service disconnecting ways. This connection is as well allowed at separately derived systems. If the grounded conductor is grounded again on the load side of the service, the connection between the grounded conductor and the EGC on the load side of the service places the EGC in a parallel excursion path with the grounded conductor.

Another issue that can arise out of multiple bonding locations is the gamble the grounded conductor existence asunder on the line side of the service. This could cause the EGC and all conductive parts connected to it to go energized considering the conductive path back to the source that would ordinarily let the overcurrent device to trip is not connected. In this case, the potential to basis of any exposed metal parts can be raised to line voltage, which can result in arcing and astringent stupor risk.

Error iv: Grounding and bonding blueprint for separately derived systems.

One common error in grounding and bonding design is the grounding of generators and whether a 3- or four-pole automatic transfer switch is used with a four-wire power system. Grounding a separately derived system is detailed in Article 250.thirty. The error in grounding and bonding design for separately derived systems stems from agreement the definition of a separately derived system. As shown in Effigy 3, a organisation is considered separately derived when the organization does non have a directly electric connectedness to the other supply system grounded conductor (neutral), other than through the bonding and equipment grounding conductor.

The generator too requires to exist straight connected to footing when it is considered a separately derived organization as shown beneath. If a four-pole ATS is used and the neutral is switched, the generator or secondary backup source becomes a separately derived system. Information technology should be noted that a three-pole ATS can exist used with a four-wire generator and also be considered a separately derived system if the electrical distribution system is a three-wire organisation. In this situation, the generator neutral would exist connected to footing, just a grounded (neutral) conductor would not be brought to the ATS.

Effigy 5: This is a delta-wye transformer with the high side coming in from the bottom and the secondary coming out from the top. Equally shown, the grounded conductor (neutral) is grounded at the transformer. Courtesy: CDM Smith

Grounding and bonding definitions

There are many requirements in NFPA lxx: National Electrical Code Article 250. A common reason for defoliation mainly stems from non understanding the proper definitions. Therefore, the outset footstep to understanding Commodity 250 is understanding the terminology within the NEC. Beneath are some terms taken from the 2017 edition of NEC Commodity 100 and clarifications for mentioned terms.

Bonded (bonding): Continued to plant electrical continuity and conductivity. Bonding is non to exist confused with grounding. Two pieces of equipment bonded together does not necessarily mean both pieces of equipment are grounded. Nonetheless, it assures that the metallic parts of the bonded equipment can form an electrically conductive path for electrical continuity.

Bonding jumper, supply side: A conductor installed on the supply side of a service or inside a service equipment enclosure(s) or for a separately derived system that ensures the required electric conductivity between metallic parts required to be electrically connected.

Bonding jumper, system: The connectedness between the grounded circuit usher and the supply-side bonding jumper or the equipment grounding conductor or both, at a separately derived organization.

Bonding conductor or jumper: A reliable conductor to ensure the required electrical electrical conductivity between metal parts required to be electrically connected.

Bonding jumper, main: The connectedness between the grounded circuit conductor and the equipment grounding conductor at the service.

Effective ground-fault current path: An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground mistake on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-error detectors. The globe is not considered equally an effective ground-error current path.

Equipment grounding usher: The conductive path(s) that provides a ground-fault current path and connects unremarkably noncurrent-conveying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor or both.

Ground: The globe.

Grounded usher: A arrangement or excursion conductor that is intentionally grounded (I.eastward., neutral conductor).

Grounding electrode: A conducting object through which a direct connection to world is established. Common grounding electrodes include rods, plates, pipes, footing rings, metal in-basis support structures and concrete-encased electrodes. All grounding electrodes at each edifice or structure shall be bonded together to form the grounding electrode system.

Grounding electrode conductor: A conductor used to connect the system grounded usher or the equipment to a grounding electrode or to a point on the grounding electrode system.

Footing-fault electric current path: An electrically conductive path from the indicate of a ground error on a wiring system through normally noncurrent-conveying conductors, equipment or the world to the electrical supply source. Examples of ground-mistake current paths are any combination of equipment grounding conductors, metallic raceways and electric equipment.

Grounded (grounding): Connected (connecting) to footing or to a conductive body that extends the footing connection. Grounding is non to exist dislocated with bonding. Equipment may be bonded together, but information technology is not considered grounded unless information technology is connected back to the basis.

Grounded, solidly: Connected to ground without inserting any resistor or impedance device.

Neutral usher: The conductor continued to the neutral indicate of a system that is intended to carry current under normal conditions.

Neutral point: The common indicate on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire arrangement or midpoint of a single-phase portion of a three-stage delta system or a midpoint of a three-wire, direct-current system.

Service: The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the bounds served.

Service equipment: The necessary equipment, unremarkably consisting of a circuit billow or switch and fuses and their accessories, located well-nigh the point of entrance of supply conductors to a building or other structure or an otherwise defined area and intended to constitute the primary command and means of cutoff of the supply.

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