Article 110 in the National Electrical Code (NEC) covers general requirements for the examination and approval, installation and use, and access to spaces about electrical conductors and equipment; enclosures intended for personnel entry; and tunnel installations (110.1). While the scope in 110.1 mentions a number of subjects covered in Article 110, it only mentions a small percentage of the many topics actually in the article. Some other subjects covered include voltages; conductor sizes; interrupting rating; selective coordination; mechanical execution of work; electrical connections, which includes temperature limitations; arc-flash-hazard warning; markings, which includes the new section for field-applied hazard markings; various requirements pertaining to disconnecting means; and enclosure types.
Like many NEC articles, Article 110 contains installation requirements for 600 volts (V), nominal, or less as well as more than 600V, nominal. Part II, sections 110.26 through 110.28, contains requirements for 600V, nominal, or less. Part III, sections 110.30 through 110.40, contains requirements for more than 600V, nominal.
In the 2014 edition of the NEC, the voltage threshold was changed from 600V to 1,000V in several hundred places throughout the Code. Although it changed in many articles, the voltage threshold of 600V did not change to 1,000V in Article 110. Part I (sections 110.1 through 110.25) contains general requirements that are not associated with specific voltage levels.
Wiring integrity is covered in 110.7. “Integrity” means the state of being complete, whole or undiminished. (Reliability is a good synonym for integrity.) Once the wiring installation is finished, it shall be free from short circuits, ground faults or any connections to ground other than as required or permitted elsewhere in the NEC. The next section in Article 110 pertains to wiring methods. In accordance with 110.8, only wiring methods recognized as suitable are included in the Code. The recognized methods of wiring shall be permitted to be installed in any type of building or occupancy, except as otherwise provided in the Code.
The next section in Article 110 covers interrupting rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the line terminals of the equipment (110.9). The second paragraph states that equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted. Article 100 defines interrupting rating as the highest current at rated voltage that a device is identified to interrupt under standard test conditions. Overcurrent devices, such as fuses and circuit breakers, have interrupting ratings. In accordance with 240.83(C), every circuit breaker having an interrupting rating other than 5,000 amperes (A) shall have its interrupting rating shown on the circuit breaker (see Figure 1). Sometimes the interrupting rating is shown as AIC rating and kAIC rating.
Section 110.10 pertains to circuit impedance, short-circuit current ratings and other characteristics. Some electricians think this is a relatively new specification, but it is not. The wording in 110.10 is almost the same now as it was in the 1965 NEC. In accordance with 110.10, the overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. Selecting the proper rating for the overcurrent protective device is important, but it is not the only critical aspect of designing the electrical system. If the electrical system has not been designed, selected and installed without considering the overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected, conductors and electrical equipment could be damaged or destroyed if a high-level short circuit or ground fault were to occur. The second sentence in 110.10 states that the fault is assumed to be either between two or more of the circuit conductors (short circuit) or between any circuit conductor and the equipment grounding conductor(s) permitted in 250.118 (ground fault).
In Article 100, short-circuit current rating is defined as the prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria. Short-circuit current ratings are not the same as interrupting ratings. Interrupting ratings are marked on overcurrent protective devices such as fuses and circuit breakers. Short-circuit current ratings are marked on equipment such as panelboards, switchboards, switchgear, disconnects (safety switches), motor control centers, motor starters and industrial control panels.
One very important phrase in 110.10 is “selected and coordinated.” This is also referred to as “selective coordination.” Article 100 defines selective coordination as localization of an overcurrent condition to restrict outages to the circuit or affected equipment, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents. In a coordinated system, if there is a fault, such as a short circuit or ground fault, the overcurrent device immediately ahead of the fault opens and is isolated to only that overcurrent device. If the fault is in a branch circuit, the branch-circuit overcurrent device opens, which is what should happen when this occurs. Without selective coordination, the branch-circuit overcurrent device may not be the only overcurrent device that opens. Figure 2 shows an example of an electrical system in a normal condition. The fuse symbol represents overcurrent devices such as fuses or circuit breakers. The top fuse symbol represents the main overcurrent device in the service disconnecting means. The motor symbol represents branch-circuit loads. The fuse symbols at the bottom represent branch-circuit overcurrent devices such as fuses or circuit breakers. The two rows of fuse symbols in the middle represent feeder overcurrent devices. This drawing does not show the entire electrical system; only part is shown to illustrate the term “selective coordination.” Branch-circuit conductors and equipment are protected at their ampacity. Likewise, feeder conductors and equipment are also protected (see Figure 2).
Figure 3 illustrates what could happen when there is a fault in an electrical system without selective coordination. When there is a fault in the branch circuit, the branch-circuit overcurrent device opens. Without selective coordination, the branch-circuit overcurrent device may not be the only overcurrent device that opens. In this illustration, two feeder overcurrent devices and the main service overcurrent device have opened as a direct result of the fault. In Figure 3, all of the feeders and branch circuits lose power unnecessarily because the fault was not limited to the branch circuit (see Figure 3).
Figure 4 illustrates a properly designed and installed electrical system with selective coordination. As in the previous illustration, the fault is in the branch circuit and the branch-circuit overcurrent device opens as a direct result of the fault. The difference between this figure and Figure 3 is the branch-circuit overcurrent device opens before any upstream overcurrent device opens. Since this fault is in the branch circuit, the fault should be contained in it. Because only the branch-circuit overcurrent device opens, there is no unnecessary power loss. If the fault would have been in the feeder, only that feeder overcurrent device should have opened (see Figure 4).
The last sentence in 110.10 states that listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.
Next month’s column continues the discussion of Article 110.
About The Author
Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.