NEC Chapter 2: Wiring and protection

NFPA 70: National Electrical Code Chapter 2 (Article 200) covers the use and identification of grounded conductors, providing requirements for identification of terminals, wiring systems, and grounded conductors. This is a quick overview of the code.

08/14/2013


Figure 1: Two examples where a neutral point is not available in a grounded system are shown. All graphics courtesy: CH2M HillArticle 200 is a fundamental prerequisite to understanding Chapter 2 of NFPA 70: National Electrical Code. Article 200 covers the “Use and Identification of Grounded Conductors.” Notice that “neutral” is not part of the title and is not included in the scope language. We often refer to the grounded conductor as the neutral, but this is not always correct. 

Two examples of systems that are grounded where a neutral point is not available are shown in Figure 1. Alternatively, per 250.26, the neutral of alternating-current systems is grounded. Other sources, including IEEE142-Green Book and IEEE141-Red Book, refer to the grounded conductor as a grounded neutral. This article of the NEC avoids the term “neutral” and refers to the grounded neutral conductor as a grounded conductor. Article 200 covers conductor identification for grounded conductors, terminal identification, and grounded conductors in premise wiring. Please note that “green” grounding conductors are covered in Article 250.119. If you are looking for a historical perspective, read “The historical development of neutral grounding practices” by Edward Owen. 

We intentionally ground a source to provide a reference point for protection devices. Accurately identifying grounded conductors and terminals makes it possible to connect circuits with correct polarity and eliminate unintentional connections to the grounding system. This is critical because our grounding conductor path may include raceways, metal boxes, and other pathways. We don’t want to intentionally apply current to these items that are exposed to personnel. We also don’t want to create alternate paths back to the source.

Grounded conductors 

A grounded conductor is connected to the earth to create a voltage reference that is close to zero. Only the voltage drop due to the impedance of the circuit is measured. This reference is critical for operation of protective devices. Even though the voltage is close to zero, this conductor is carrying current and is dangerous when energized. The next time someone tries to beguile you into an argument about the difference between the green wire and the white wire (connected at the source), just ask him if he would like to test that theory by holding the white wire in one hand and the green in the other when current is flowing. We must bear in mind that the grounded conductor intentionally carries current during normal operation. 

Figure 2: This is an example of a component wired with reversed polarity. Courtesy: CH2M HillCorrect polarity is critical because a manufacturer’s equipment safety protection design may be defeated when connected improperly. For example, internal circuit protection can be eliminated when circuit polarity is not maintained. A fuse connected on the return side of the circuit is bypassed during a fault condition (see Figure 2). 

Lamp screw shells are specifically required to be connected to the grounded connector in point 200.10 C. The shell of a lamp socket is relatively exposed to human hands and should not provide a path to ground. Correct polarity for grounded conductors is required by the last point 200.11 

200.2 general defines insulation and continuity requirements for a grounded conductor. The practice of using a metallic structure to connect grounded conductors cannot be used. Improper connections create a dangerous potential voltage difference between grounded conductor and ground. Stray currents create a voltage on metal that is exposed to personnel. Grounded conductors must be connected to terminals specifically intended for grounded connections. In a panelboard downstream from the service, this is a separate busbar that is insulated from the metal enclosure. Remember: the enclosure is connected to the grounding system and must be kept separate from the grounded system except at the service (see 250.184 B.7). 

Figure 3: Grounded conductor insulation requirements for impedance grounded system are shown. Courtesy: CH2M HillInsulation rating for grounded conductors on 1000 V or less systems need to be rated the same as the phase conductors. This applies to solidly grounded and impedance grounded systems. For solidly grounded systems greater than 1000 V, the grounded conductor insulation rating is 600 V minimum. For impedance grounded systems greater than 1000 V, the grounded conductor insulation must be rated the same as the phase conductors (see Figure 3). 

The first sentence in 200.3 (Connection to Grounded System) is difficult to understand. It states: “Premise wiring shall not be electrically connected to a supply system unless the latter contains, for any ungrounded conductor of the interior system, a corresponding conductor that is grounded.” This article from the code comes from an older vernacular; it read almost the same back in 1938.

This section requires that the internal building wiring match the service. We are not allowed to connect or ground a system where the supply is not grounded. Article 250.20.B describes grounding requirements more clearly. Three conditions require grounding:

  1. When grounding limits the voltage to less than 150 V
  2. 3-phase, 4-wire, wye connected system
  3. 3-phase, 4-wire, delta connected system with a grounded midpoint connection. 

NEC handbook Exhibit 250.4 shows a grounded conductor at the source. Generally, the local utility provides a grounded service, but you should not assume that this is the case. I found an exception on a recent project in the Alaskan outback. Pole-top transformers are provided with a midpoint tap, but it is no longer connected to ground. With the original high-leg delta wiring unchanged, a moving crane provides a path back to ground with visible arcing.


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Pete , United States, 08/21/13 12:12 PM:

Great Article, I've tried to explain same, hundreds of times, this distills it very well.
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