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.
Article 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.
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.
Correct 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).
Insulation 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:
- When grounding limits the voltage to less than 150 V
- 3-phase, 4-wire, wye connected system
- 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.
Identifying grounded conductors
For insulated grounded conductors of 6 AWG or less, the conductor must be manufactured to meet the identification requirements. Namely, it must have a continuous white or gray outer finish or three white stripes along the entire length of the conductor. There are other options for mineral-insulated (MI) cable, photovoltaic (PV) power systems, fixture wires, and aerial cable.
- MI due to nature of construction requires re-identification at terminals
- Single-conductor, sunlight-resistant outdoor cable for PV systems allows distinctive white markings at terminations (690.31)
- Fixture wire allows ridges for the grounded conductor among other color options detailed in 400.22
- Aerial cable may be identified by a ridge on cable.
Conductors larger than 6 AWG also require white or gray outer finish or three white stripes along the entire length of the conductor, but there is an option to re-identify the conductor by wrapping the cable with white or gray at termination points. Paint, tape, or shrink tube are good options. Just be sure the identification completely circles the conductor.
Where grounded conductors of two different systems are in the same raceway or enclosure, they must be identified differently from each other (200.6 D). Identification requirements are the same as explained above, plus there is an option to have a colored stripe other than green running along the insulation. Color code labels are required at junction boxes and at termination equipment. An example color code table:
Alternate uses for conductors
Conductors for systems greater than 50 V, identified by the manufacturer with white or gray, may be used for alternate purposes under the following conditions:
- Conductors within a cable assembly that are not switched. Supply to the switch is allowed but return from switch is not.
- Flexible cords as permitted in 400.22.
Where the grounded conductor is repurposed, it must be identified by encircling the conductor with a color other than white, gray, or green. Red tape is commonly used and paint is listed as an option; red shrink tube is a cleaner installation and it doesn’t fall off. The white conductor can only be used as the ”energized” leg to the switch. Troubleshooting the circuit and finding potential to ground on the white conductor clues the technician that this is a switch leg. Another advantage is that you end up with standard color coding at the device. This helps avoid confusion as to the purpose of the white conductor at the switch because it is energized under operating conditions. This practice is going to be less common because the 2011 NEC now requires that the grounded conductor also be carried to the switch and standard nonmetallic cable assemblies have only one neutral. 404.2 C is outside the scope of this article (see Figure 4).
This receptacle is switched and common nonmetallic cable was used for a switch leg. Red tape was used to re-identify the ungrounded conductor. At the switch, while the circuit is energized, voltage can be found on the white repurposed wire.
For devices 30 amp and less, terminals must be identified. Ungrounded terminals must be distinguishable from the grounded terminals. Receptacles, plugs, and connectors require white metal or a “W” or “white” label. If the terminal is not visible, as in the case of push-pin devices, the label is required. Identification of terminals for devices has been in practice for a long time. Even this old two-pin device has white terminals for the grounded connection.
Where leads are provided with a screw shell, they must have a white or gray colored jacket for the conductor attached to the screw shell. Appliance termination requirements have a separate point. Appliances with line-to-ground connections either hardwired or cord and cap require the grounded terminal to be identified.
Looking forward to the 2014 Edition of the NEC, one option to be voted on is whether gray stripes are an acceptable means of identification. For seven points where white stripes are allowed, gray stripes have been proposed as an option. Point 200.2 revisions have been proposed by a high-voltage task group looking to improve consistency when discussing 1000 V system divisions. Another proposal was to add a “fine print note” to 200.1 Scope with reference to 250.26 for when a grounded conductor is a neutral conductor. However, this proposal was rejected because, according to the code-making panel, “whether a grounded conductor is a neutral or not is not relevant to the requirements of Article 200.”
Identification of the grounded conductor is straightforward once you have taken the time to understand Article 200. Mistakes not only cause confusion during commissioning, but also can be deadly.
John Schuring is an electrical engineer at CH2M Hill with more than 20 years of engineering experience. He works primarily in industrial applications and start-up of facilities.
Several important definitions from NEC Article 100 are:
Grounded conductor. A system or circuit conductor that is intentionally grounded.
Grounding conductor, equipment (EGC). The conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.
Neutral conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Neutral point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system. Informational note: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential
National Electric Code Handbook, 2011
NFPA 70: Report on Comments-June 2013
NFPA 70: 2014 Report on Proposals
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