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Originally published in NECDIGEST, Fall 2003
By John M. Caloggero
Grounding Requirements for 600 Volts Or Less
The NEC requirements pertaining to grounding have always been a mystifying segment of the Code. Why do we ground electrical systems and what function is it intended to achieve? What is the intended function of equipment grounding conductors? What systems are required by Code to be grounded? In the summer issue of necdigest we provided the historical background for grounding electrical systems. In this issue we will address grounding requirements for 600 volts or less and try to dispel some of the mystery with a technical rationale.
The first questions asked about grounding usually are:
What is the purpose of grounding an electrical system to mother earth or something that serves in place of earth, and how much current can soil (earth) conduct?
A very common response is that the purpose of connecting a current-carrying conductor to ground is so that the fuse will blow or the circuit breaker will trip when the hot leg faults to ground. Wrong! The proof is when you intentionally take an ungrounded conductor and stick it in the earth while connected to a 15-ampere circuit and nothing happens. The amount of current that flows is in the milliampere range, which is why the earth alone cannot serve as the ˝sole equipment grounding conductor or fault current pathţ. The earth is a very poor conductor.
One of the most important items in comprehending the requirements for grounding is to know the terminology and definitions provided in Article 100, Definitions. The terminology and definitions provided in Article 100 are specific to the Code and have very precise meanings. There is a big difference between ˝grounded conductor and grounding conductor.
In addition, the meaning of commonly used words and what they are meant to signify in the context of the requirement depends upon the rationale that generated the requirement. For example, the term ˝soleţ as used throughout Article 250 of the NEC, means singularly, by itself, alone. For example, the earth alone cannot serve as the ground-fault return path; there must be a conductor of some type that provides a low impedance ground-fault path from the fault to the power source.
Now, let═s discuss some of the most commonly used terms when explaining grounding.
Grounded: This is when something is connected to mother earth, or to some conducting body that serves in place of the earth, such as the steel frame of a high-rise building on a concrete footing, metal conduit, metal electrical enclosures, or equipment grounding conductors. It can be achieved intentionally or accidentally. For example, when an electrical system is ˝grounded,ţ a designated current-carrying conductor is intentionally connected to earth. An example of an accidental ground is when the conductor insulation is damaged and the metal comes in contact with the earth, or a conducting material that is in contact with the earth.
Grounded conductor: This is the system or circuit conductor that is intentionally connected to earth or something serving in place of the earth. Grounding of an electrical system is achieved by connecting a current-carrying conductor to a grounding electrode system.
Grounding conductor: This is the conductor used to connect equipment or the grounded circuit of a wiring system to the grounding electrode or multiple electrodes.
Grounding electrode conductor: This is the conductor used to connect the grounding electrode(s) to the equipment-grounding conductor, to the grounded conductor, or to both. This connection can be at the service, at each building or structure where supplied from a common service, or at the source of a separately derived system. In other words, this is the main conductor that ties the grounded conductor to earth.
Equipment grounding conductor: This is the conductor used to connect the non-current-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor or both. This connection can be made at the service equipment or at the source of a separately derived system.
Grounding electrode(s): These are the devices that serve to make physical contact between the grounding electrode conductor and the earth. Only specified items are allowed to serve as grounding electrodes, such as a metal underground water pipe, metal frame of a building or structure, concrete encased bars or rods, a bare copper conductor of a specific length and cross-sectional are, rods and pipes of a specific diameter and length, and steel or iron plates.
Grounding electrode systems: Where more than one of the above grounding electrodes exists on the premises, they must all be bonded together with an appropriate sized conductor to form the grounding electrode system.
Separately derived system: This is a premises wiring system whose power is derived from batteries, solar photovoltaic system, a generator, transformer, or converter windings, and that has no intentional, direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
Why Ground the System?
The following discussion pertains to service supplied systems, as opposed to separately derived systems. There are several reasons for connecting one of the current-carrying conductors of the electrical system to the earth or to some conductive element that is effectively connected to earth. See Section 250.4(A)(1).
- System grounding or earthing as it is called in other countries, stabilized the voltage relative to earth or to grounded objects. The voltage measured between an ungrounded conductor (the live wire) and earth or grounded objects will always be the same value.
- If the utility lines or service drop conductors receive a lightning strike, an effective ground on one of the current-carrying conductors will cause the voltage to be pulled down to earth potential (minimum voltage differential), thereby reducing the shock hazard to personnel and the fire hazard to buildings or structures.
- Since low voltage and high voltage conductors are installed on the same poles, damage to the supporting members might cause contact between high and low voltage conductors. Where transformers are installed, there exists the likelihood that there can be insulation failure between the primary and secondary windings. Having one of the systems grounded will pull the voltage down to the potential of the earth.
- Grounding the system reduces the stress on electrical insulation. For example, in a grounded three-phase, four-wire, 480 wye connected system, the maximum voltage stress applied to the system is 277 volts relative to earth or a grounded object. However, if the system is ungrounded, the voltage could be 480 volts or greater under a ground-fault situation.
- An additional ground-fault return path is provided, although it is not the main path for operating the overcurrent device.
Systems To Be Solidly Grounded (less than 50 V)
The NEC, in Section 250.20 22 identifies alternating current electrical systems and circuits that are required to be grounded. This section is broken down into four areas, (A) AC circuits of less than 50 volts, (B) AC systems of 50 volts to 1000 volts, (C) AC systems of 1000 volts and greater, and (D) separately derived systems.
Alternating current circuits operating at less than 50 volts are required to have one of their current-carrying conductors grounded under the following conditions.
- Transformers supplied with at voltage in excess of 150 volts to ground and with a secondary voltage of less than 50 volts. The secondary must be grounded.
- Transformers supplied from an ungrounded electrical system with a secondary voltage of less than 50 volts, one of the conductors on the secondary must be grounded. An example of this is a 3-phase, delta supply of greater than 150 volts to ground; the voltage to ground for an ungrounded delta system is the line-to-line voltage.
- Circuits rated less than 50 volts that are run as overhead conductors, are required to have one of their circuit conductors grounded. This is to ensure that under accidental contact between high and low voltage conductors, as well as lightning strikes, the voltage will be pulled down to earth potential.
Systems To Be Solidly Grounded (50 to 1000 V)
The NEC in Section 150.20(B) identifies alternating-current systems that must be grounded where the voltage is 50 to 1000 volts as follows:
- Where the system can be grounded so that the maximum voltage does not exceed 150 volts when measured from an ungrounded conductor to ground. A step down transformer that reduces the voltage from 240/480/550/ to 120 volts, regardless of whether it is a 2-wire or 3-wire secondary, must be grounded.
- Any 3-phase, 4-wire, wye connected system in which the neutral is used as a current-carrying conductor must have the neutral grounded. Examples of some of the system voltages are 208Y/120, 480Y/277, and 600Y/347.
- Circuits in systems other than those specified in (1) and (2) are allowed to be grounded, but they must follow the requirements in Article 250.
- Anytime the premises wiring is supplied from a system that is intentionally grounded, the grounded conductor that is run to the premises wiring must be grounded again at the building or structure.
Where an installation is supplied from a 3-phase, 3-wire, wye ungrounded system, with a line-to-line voltage greater than 150 volts, it is not required to be grounded. However, it is good engineering design to ground the common point of the wye configuration at the transformer and route it to the premises wiring service equipment enclosure where it must be grounded again.
Which Conductor Do We Ground?
The conductor that must be grounded is based on the configuration of the supply system as follows:
- Any two-wire system originating from the secondary of a two-wire transformer in which the voltage does not exceed 150 volts, either one of the conductors must be grounded. [Section 250.26(1)]
- Any three-wire system originating from the secondary of a three-wire transformer in which the neutral is a current-carrying conductor common to the other two legs, the common or neutral conductor must be grounded. [Section 250.26(3)]
- Any three-phase, four-wire, wye connected system in which the neutral is used as a current-carrying conductor, the common or neutral conductor must be grounded. [Section 250.26(3)]
- Any three-phase, four-wire, delta connected system in which a common or neutral conductor is tapped from the center of one of the transformer windings that is connected between two phases, the common or neutral conductor must be grounded. [Section 250.26(5)]
Where Do We Make The Grounding Connection?
The installer or designer has the choice of where the grounding connection can be made, provided it is located within specific parameters. Remember one of the reasons for grounding that was mentioned earlier. Should an accidental contact between the high and low voltage system or a hit from a lightning strike occur, the ideal situation is that the voltage or lightning strike is directed to ground outside the building or structure. Therefore, the NEC allows a grounding connection to be made outside, on the load-side of the service drop near the weatherhead. Additionally, the connection can be made anywhere from the load-side of the service-point and the terminal or bus in at the service disconnecting means. The most common location for the grounding connection is in the main service disconnect enclosure [Section 250.24(A)(1)].
It is important to remember, do not make a grounding connection on the load-side of the service disconnect, except under special conditions that will be discussed in a future column.
In the next edition, we will continue to follow the trail through the grounding labyrinth and explain how to size the grounding conductor, the grounding electrode conductor, and making taps to the grounding electrode conductor.