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Current in the Grounded Conductor

Posted By Michael Johnston, Monday, May 01, 2006
Updated: Monday, February 11, 2013

Recently there have been several questions about the locations of grounding connections to the grounded conductor. Before looking at some of the requirements specific to grounding locations and the reasons behind those rules, let’s review some basic rules that apply to the grounded conductor. I guess the first thing that should be pointed out is that the grounded conductor is often a service or system neutral conductor. For the purposes of this article we’ll address this conductor as the grounded conductor. Remember, neutral conductors covered byNECrules are almost always grounded conductors, but grounded conductors of systems are not always neutral conductors. For example, for a multiphase system where one of the phase conductors is grounded and is not a neutral of the system, the grounded conductor is a phase conductor. This type of grounded system is 3-phase, delta-connected and is called a corner-grounded system, because one corner of the delta is grounded (figure 1).

Remember that all the rules in the NEC that apply to grounded conductors are applicable to a grounded phase conductor in this case as well. This includes the rules in 230.90(B) and 240.22 that normally do not permit an overcurrent device to be inserted in a grounded conductor, and also the identification requirements of 200.6. Note there are exceptions for overcurrent devices that open all conductors of the circuit including the grounded conductor, and conditions where the overcurrent device is used for overload protection for a motor. We’ll concentrate on the general requirements in this article.

Figure 1. This diagram of various grounded systems shows which conductor of each system is grounded to meet the requirements in 250.26. Note the typical corner-grounded, 3-phase, delta system showing one phase grounded.


Figure 2. Grounded conductors are required to be identified in accordance with Section 200.6.

Customarily, premises wiring systems are supplied by a system that has a conductor that is intentionally grounded as indicated in Section 200.3. Grounded conductors of electrical systems are required to be identified in a manner specified in 200.6. The grounded conductor of a premises wiring service or system is typically identified by the colors white or gray. Section 200.6(A) provides the identification alternatives for sizes 6 AWG and smaller, and 200.6(B) covers identification requirements for conductors larger than 6 AWG (figure 2).

Current in the Grounded Conductor

The Code addresses general sizing of the grounded (neutral) conductors in terms of ampacity in Section 220.61. Basically, this requirement indicates that the grounded conductor shall be sized to carry the maximum unbalanced load determined by Article 220. This applies to both feeder neutral conductors and service neutral conductors. When discussing the minimum requirements of the Code, the grounded (neutral) conductor must be no smaller than that required by 220.61, and in no case can it be smaller than the required grounding electrode conductor for service or system as indicated in 250.24(C). Section 215.2(A)(1) requires feeder grounded conductors to be at least the minimum size specified in 250.122. When discussing current in the grounded conductor that supplies a service, there are two types of currents to consider. In normal operating conditions, the grounded (neutral) conductor carries only the maximum unbalanced return current to the source windings. During ground-fault conditions, the grounded (neutral) conductor must carry the amount of short-circuit current that the source is capable of delivering for the duration of time it takes an overcurrent protective device to operate and clear the faulted condition. So clearly, we’ve established that the grounded conductor (typically a neutral conductor) serves two primary purposes.

Keeping Current on the Intended Path

As we’ve already established, the grounded (neutral) conductor can carry varying amounts of load current in normal operation and huge amounts during ground-fault conditions prior to overcurrent device operation. So if the grounded (neutral) conductor carries current in normal operation, then there should be efforts to maintain the insulation integrity of the grounded (neutral) conductor and to generally avoid multiple connections to grounded items. This helps to ensure that any current in the grounded conductor remains in that conductor and is insulated from other conductor paths. Ordinarily, grounded service conductors are required to be insulated but there are a few exceptions to this rule provided in theNEC.

What About Objectionable Current?

Article 250 includes a few key requirements that apply to grounded conductor connections that when properly applied can minimize the amount of objectionable current present in conductive paths, including the equipment grounding conductors, that are not usually intended for normal operating current. The Code addresses this objectionable current by requiring the grounding of systems, circuit conductors, surge arresters, surge protective devices, and non-current-carrying materials and equipment be installed and arranged in a manner that prevents objectionable current over the equipment grounding conductors or other grounding paths [see Section 250.6].

Section 250.6(B) provides a few alternative remedies for stopping objectionable current, but clearly requires the effective ground-fault current path required by 250.4(A)(5) and the ground-fault current path required by 250.4(B)(4) to remain intact and effective. It is important that the remedies or solutions attempted to reduce objectionable current meet the basic requirements of 250.6(B), which includes maintaining the equipment grounding conductor path. This ensures effective grounding of equipment and also ensures the integrity of the effective ground-fault current path to facilitate overcurrent device operation should a ground-fault condition develop.

Back to the Basics

It is important to carefully consider the various remedies applied to electrical systems in the interest of improving power quality. Often when it has been determined that there is excessive "noise” in the electrical grounding paths, facilities use the services of organizations in the business of handling power quality issues. One of the most important steps in resolving power quality issues starts with a complete analysis of the grounding and bonding system within the facility. This analysis should be conducted before any filtering equipment is applied in the system. Usually this analysis reveals inappropriate grounding connections made to the grounded conductor on the load side of the service disconnecting means. This contributes to the amount of objectionable current over conductive grounding paths, including equipment grounding conductor paths. Restoring the grounding and bonding connections within a facility to those required and allowed by the minimum requirements of the NEC goes a long way in handling problems associated with what the industry refers to as electrical noise or objectionable current in the grounding circuit. It should be noted that improper electrical grounding and bonding connections within a facility are not the only source of objectionable current or electrical noise in the system, but it’s a good place to start a diagnosis of these types of problems.

Grounding Connection Locations


Figure 3. A grounding connection made outside at the serving utility company transformer and also at the service disconnect

The first rule regarding locations of system grounding connections shows up in 250.24(A) for services. Section 250.24(A)(2) indicates that for a grounded system where the transformer is outside the building, at least one additional grounding connection from the grounded conductor is required. This is usually taken care of at the serving utility transformer location. The next grounding electrode conductor connection for the service is permitted to be made at any accessible location from the load end of the service drop or lateral to and including the terminal or bus to which the grounded service conductor is connected in the service disconnecting means. For a grounded system, this connection is typically made within the service equipment enclosure using a main bonding jumper (MBJ) [figure 3]. See the definition of bonding jumper, main in Article 100 (figure 4).


Figure 4. An example of main bonding jumper configurations in equipment that is suitable for use as service equipment

Service Equipment

Service equipment that is listed and identified as "suitable for use as service equipment” in accordance with the rules in 230.70(C) is equipped with grounding and bonding capabilities for accomplishing the requirements of this section. Section 250.24(A) requires that a grounding electrode conductor be connected to the grounded conductor of an ac service. Section 250.24(C) requires the service grounded conductor (usually a neutral) to be brought to and bonded to each service disconnecting means.


Figure 5. Legend for keeping the grounding (neutral) conductor insulated from grounding connections

Now that the grounded service conductor is terminated in the service equipment, connected to a grounding electrode, and bonded to the service disconnect enclosure, no additional grounding connections are permitted as a general rule [see Section 250.24(A)(5)]. From this point downstream toward the load at the feeder level and the branch-circuit level, the goal should be to keep the grounded (neutral) conductor insulated from grounding connections to meet the requirements in 110.7, 250.6(A), 250.24(A)(5), and 250.142(B) [figure 5].

Special Allowances

Be sure to read the FPN following 250.24(A)(5). Some specific conditions are provided where grounding connections to grounded conductors are permitted, but they are restrictive. The fine print note references 250.30(A), 250.32, and 250.142 for those specific conditions where grounding connections to the grounded conductor are permitted. It should also be mentioned that the frames of existing dryers and ranges are permitted to be grounded using the grounded conductor, but only when meeting the specific conditions of 250.140, Exception. Let’s concentrate on the general requirements of the Code for the purpose of this article, but remember that those allowances are there for future reference. Section 250.142(B), Exceptions 1 through 4, also provide restrictive conditions for using the grounded (neutral) conductor for grounding on the load side of the service disconnect. Be sure to become familiar with all of the conditions in these exceptions when applying them in the field.

Grounded Conductor As a Grounding and Bonding Means


Figure 6. The grounded conductor is permitted for grounding on the line side of the service disconnect in accordance with 250.142(A)(1) and also for bonding at the service equipment in accordance with 250.92(B)(1).

TheNECclearly permits the grounded conductor to be used for grounding on the line side of the service disconnect as indicated in 250.24(A) and 250.142(A). The grounded conductor is also permitted as a bonding means on the supply side of the service disconnecting means as provided in 250.92(B)(1) [figure 6].

Service Disconnect Location

As a review, the service disconnect location is required to be either outside the building or structure or inside nearest the point of entrance of the service conductors. TheCoderecognizes the grounded conductor for use in grounding and bonding in these situations because that is where the main bonding jumper is located. The grounded conductor and the service raceways and enclosures basically become one (electrically) at that point by virtue of the main bonding jumper connection. For this reason, it is recognized that any current returning to the source will be present in all conductive paths, but the amount returning over the high impedance paths is generally limited.

Current Paths


Figure 7. Current will divide over as many paths as are available to it to while returning to its source.

To keep with our basics, we understand that current will always try to return to its source. Current needs a complete circuit to be present in any conductive path. Current will divide over as many paths as are available to try to return to the source. The amount of current in any particular path (if there is more than one) is related to the amount of impedance in that particular path (figure 7).

Summary

For grounded systems, there must be a conductor that is grounded. This conductor is often a neutral of a system, but grounded conductors are not always neutrals of the system. The Code includes specific rules that indicate where the grounded conductor connection to ground is required to be located. Grounding connections on the load side of these identified locations are not permitted, other than by exceptions under restrictive conditions. Keeping normal current that is present in the grounded (neutral) conductor on the path it was intended is important from an overall safety concern as well as an important consideration when dealing with electrical current and noise over the grounding paths, including the equipment grounding conductors of the system.


Read more by Michael Johnston

Tags:  Featured  May-June 2006 

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