EMT Support, Powering Up A Trailer And More

Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC), Jim will help you solve it. Questions can be sent to codefaqs@gmail.com. Answers are based on the 2014 NEC.


Unsupported raceways


I have a question about support of electrical metallic tubing (EMT) in electrical closets in a commercial occupancy. We roughed-in electrical closets on 15 floors of a commercial building. In each electrical closet, there were multiple short lengths of EMT used to nipple between panelboards and wireways. The smallest trade size was 2-inch EMT, and the nipple sizes were between 8 and 16 inches long. During a discussion on the job, it was pointed out that the NEC required the nipples to be supported. After we discussed the possible methods to support those nipples, we realized they would be supporting the means that we could install to support them. You could climb the nipples like a ladder; they are that strong. Why did the NEC remove the permission for unsupported nipples not longer than 18 inches?


In the 2008 NEC, new subdivision 358.30(C) was added to clearly permit unsupported lengths of EMT. Similar permission was included for intermediate metal conduit (IMC), rigid metal conduit (RMC), polyvinyl chloride (PVC) pipe, and reinforced thermosetting resin conduit (RTRC). Unsupported lengths were permitted where (1) oversized concentric or eccentric knockouts were not encountered; (2) the unsupported raceway was not longer than 18 inches; (3) the raceway is unbroken, meaning no couplings were installed; and (4) each nipple end had to terminate in a box, cabinet or other termination point. In the 2011 NEC revision process, the technical committee reversed their position and deleted the 3XX.30(C) subdivision for many reasons. (This subdivision existed in four articles.)While it seems logical to have such a rule, it can create more problems than it solves. Inspectors do not typically red-tag an installation for short lengths of raceways between boxes, wireways, panelboards, etc. Including prescriptive requirements to address unsupported raceways, as seen in the 2008 NEC, actually created problems. For example, the following short runs would require support: a 4-inch nipple of ¾-inch EMT where an oversized concentric knockout is encountered, two 12-inch nipples of 5-inch RMC installed with a coupling to form a single nipple, and any raceway nipple over 18 inches in length. The technical committee recognized that the requirement for unsupported raceways caused confusion and removed it.


Intersystem bonding


Is the intersystem bonding termination points we install to comply with 250.94 at service equipment permitted for termination of a bonding conductor that is installed for flexible natural gas piping? We recently completed a 60-unit townhome project and were contacted by an engineer asking us how we installed the bonding conductors for the flexible gas piping. The contractor that installed the bonding conductors terminated them on the intersystem bonding termination points at the service equipment that we installed on each unit. We responded to the engineer simply to document that we did not install the bonding conductors and that the contractor that installed them needed to remove them from our termination points and develop their own. Were we correct?


The requirements of 250.94 apply to the systems addressed in the NEC that require such a termination point for bonding. Those systems and requirements include optical fiber in 770.100(B)(1), communications systems in 800.100(B)(1), radio and TV equipment in 810.21(F)(1), cable TV systems in 820.100(B)(1), and premises-powered broadband systems in 830.100(B)(1). The intersystem terminations required in 250.94 are not intended for bonding flexible gas piping. While the current title of Section 250.24 is “Bonding for Other Systems,” a review of existing requirements in the NEC and the exception in 250.94 makes it apparent that this rule applies only to communications systems and not bonding of flexible gas piping. See Figure 800.(a) in the front of Article 800, which provides an example of the intersystem bonding termination for bonding of communications systems. It is interesting to note that the technical committee with purview over 250.94 has changed the title in the public input phase for the 2017 NEC to “Bonding for Communication Systems.” This will provide necessary clarity in prescriptive text to limit the intersystem bonding termination points installed in accordance with Section 250.94 to communication systems addressed in the NEC.


Industrial carts or EVs?


In a large facility used primarily for storage, shipping and receiving, we are installing charging equipment for electric carts that personnel are to use to move around the facility and perhaps to move small boxes and other items. The electrical drawings denote the disconnecting means, charging equipment and branch circuits as “EV” for electric vehicle. The cut sheets provided for the EVs reveal that these are a type of electric golf cart. Are we required to install the branch circuits, disconnecting means and other equipment for these carts in accordance with Article 625?


No, Article 625 will not apply. The answer to your question is found in the definition of “electric vehicle” in Section 625.2, which clarifies that an EV is an automotive-­type vehicle for on-road use and that industrial transports, golf carts and similar equipment are not included. The fact that the drawings note the equipment as EV will not impact the installation. It is prudent, however, to contact the engineer and document that the industrial carts in this installation are not EVs as defined in 625.2 and confirm that Article 625 does not apply.


Temporary trailer


We installed four temporary 100-ampere (A) feeders to job-site trailers on a construction site. The owners insurance company did a job-site audit, and we were told that all four trailers needed ground rods installed. These are temporary and will only be in place for a few months. We have been installing Type SER cable as feeders to temporary trailers for years. No one has ever requested a grounding electrode. Is a ground rod required for a temporary feeder to a trailer?


Yes, a job trailer is a structure and is subject to the requirements of Section 250.32. This requirement mandates that a grounding electrode or grounding electrode system be installed for buildings or structures supplied by feeders or branch circuits. The exception in 250.32 relieves the requirement for buildings or structures that are supplied by a single branch circuit (this includes a multiwire branch circuit). However, in this case, these structures are supplied by 100A feeders, and a grounding electrode is required. All of the rules in Part III of Article 250 apply. If a ground rod is used, Section 250.53(A)(2) requires it be supplemented, which, in most cases, would be a second ground rod. Section 250.53(A)(3) requires a minimum distance between the ground rods of 6 feet. If the trailer is supplied with a temporary water supply via a metal piping system, it would be required to be bonded in accordance with 250.104(A). There are no exceptions in Article 250 (for grounding and bonding)or in Article 590 (for temporary power and lighting) that would eliminate the need for a grounding electrode system on a feeder-supplied temporary trailer.


Battery ASCC


How do I determine the available short-circuit current (ASCC) for a large battery installation that is part of a uninterruptible power supply (UPS) system in a data center? This is an existing installation that had water damage and we are replacing a lot of equipment, including fused disconnects and fuses to protect the conductors supplied by the battery system in accordance with 240.21(H). The damaged original disconnects, fuses and conductors were removed by others. Is there a method available to determine the maximum ASCC from the battery system?


I am not aware of a method or calculation to determine the ASCC of existing battery systems. There are many different battery types and configurations, and the battery manufacturer is the only entity that can provide such data. The NEC requires that a disconnect be installed for the battery system in Section 480.6(A) and also requires in Section 480.6(D) that the disconnecting means be marked with the nominal voltage, the maximum ASCC that can be derived from the battery system and the date the ASCC calculation was performed. The informational note that follows these requirements informs the Code user that battery-equipment suppliers can provide the short-circuit current on any particular model or type of battery. Similar information can be found in NFPA 70E. The ASCC of a battery system must be known to perform an arc flash hazard risk assessment. Informational Note No. 1, following 70E 130.7(C)(15)(B), also sends the reader to the battery manufacturer for ASCC values derived from battery systems.

About the Author

Jim Dollard

Code Columnist

Jim Dollard is the safety coordinator for IBEW Local 98 in Philadelphia. He is a member of the NEC Correlating Committee, NEC CMP-10, NEC CMP-13, NFPA 70E, NFPA 90A/B and the UL Electrical Council. He can be reached at codefaqs@gmail.com.

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