Temporary Power, Sealing Ceiling Tiles and More

0519 Code FAQs
Image credit: Shutterstock / Jamesbin
Published On
May 15, 2019

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. Send questions to codefaqs@gmail.com. Answers are based on the 2017 NEC.

Temporary power compliance

We have an issue with temporary power for a large project on an existing campus. The temporary consists of a 15-kilovolt (kV) feeder from the existing campus, supplying a transformer to step down to 480/277 volts (V). We had specified a 1,600-ampere (A), 480/277V main disconnect with overcurrent protection that would supply a switchboard. The installer supplied the switchboard directly from the secondary terminals of the transformer. An engineer with the electrical contractor informed us that NEC table 450.3(A) Note 2 permits six individual circuit breakers with an ampacity more than 250 percent of the full load amperes for transformer secondary protection. He also told us this installation complies with 240.21(C)(6) because it is shorter than 25 feet and that sections 215.10 and 230.95 do not apply because there are no disconnects rated at 1,000A or more. We disagree. Can you tell us what is required?

The installation you have described is not compliant with the NEC. There are multiple violations and serious safety concerns.

Section 450.3 provides requirements for transformer overcurrent protection. Without additional information, we cannot consider or discuss compliance with 450.3. It is extremely important to note that Section 450.3 does not provide requirements for the protection of transformer secondary conductors. Those requirements are in 240.21(C). In the installation you have described, the transformer primary overcurrent protective device (OCPD) in the 15-kV feeder is the only protection for the transformer secondary conductors. That is an NEC violation.

Transformer secondary conductors may be protected only by the transformer primary OCPD if the requirements of 240.21(C)(1) are met. The provisions of 240.21(C)(1) apply only to single-phase transformers with a two-wire (single voltage) secondary or a three-phase, delta-delta connected transformer having a three-wire (single voltage) secondary. The secondary in this question is wye connected at 480/277V, so 240.21(C)(1) does not apply.

The transformer secondary conductors you describe must comply with 240.21(C)(6), which requires the transformer secondary conductors not to exceed 25 feet in length and that they must meet all of the provided requirements. This includes having the secondary conductors terminate in a single circuit breaker or single set of fuses that limit the load current to not more than the conductor ampacity. This would require a 1,600A disconnect and an OCPD. Ground fault protection of equipment (GFPE) is required for 480/277V disconnects rated at 1,000A or more. These requirements exist to prevent the complete burn down of electrical equipment.

If an arcing fault were to occur in the switchboard installation you describe, it would very likely result in the complete destruction of the switchboard and more. The GFPE requirements of 215.10 (which requires compliance with 230.95) would apply. Additionally, the requirements of 240.87 for arc-energy reduction would apply if a 1,600A circuit breaker is used. Section 240.67, which contains arc-energy requirements for fuses rated 1,200A or more, takes effect on Jan. 1, 2020.

Where arc-energy reduction is required, multiple permitted methods and technologies are available. For example, an energy-reduction maintenance switch can be used. Where a means of arc-energy reduction is used, it will significantly shorten the circuit breaker’s clearing time, resulting in significantly lower values of energy and potential damage if an arc flash were to occur.

Sealing ceiling tiles

I work at a hospital with a clean room used for sterile preparations. The pharmacy organization that provides guidelines on safe and sterile preparation requires the lay-in ceiling tiles to be caulked or sealed to the ceiling frame so the tiles do not pop out when cleaning and mopping the ceiling. When the tiles pop out, they spread particles and bacteria that can be harmful to patients. Mechanical and electrical components are above the ceiling. I am told caulking the ceiling tiles to the frame would be against NEC (2017) Section 100 on “accessible.” Would caulking the ceiling tiles in place make it so the electrical components above were considered not accessible?

To be considered accessible, the space above the lay-in ceiling panels must be capable of being accessed without damaging the building structure or finish. In this case, where lay-in ceiling panels are adhered to the ceiling grid, we would need to cut the lay-in ceiling panels from the ceiling framing members to gain access. There is no doubt that using a utility knife, for example, to cut into the ceiling would damage the ceiling tiles. Once the caulk is applied, the space above the lay-in ceiling panels is no longer considered accessible. See the definition of accessible in Article 100.

This is a building design issue. Where there is a need for a clean room, that area should not contain a lay-in type ceiling.

Fire pumps?

Are foam pumps installed in an airport facility considered fire pumps? The bid drawings include two fire pump controllers supplying two 480/277V, three-phase foam pumps. The fire pump controllers each have a 60A circuit breaker for normal supply coming from a service-supplied switchboard that has a 1,000A main circuit breaker upstream. Each fire pump controller also is supplied at 60A from a standby generator. Do the requirements of Article 695 apply? Is this installation compliant?

While these are foam pumps, not typical water pumps, they do move liquid, so the provisions of Article 695 apply.

A pump is simply a mechanical device using suction or pressure to raise or move liquids. Article 695, “Fire Pumps,” applies to all electrical fire pumps. There is no limitation to water or liquid foam; a fire pump is a fire pump.

The requirements of 695.3 for permitted power sources and 695.4 for continuity of power are extracted and based upon Chapter 9, “Electric Drive for Pumps” in NFPA 20,  the Standard for the Installation of Stationary Pumps for Fire Protection.

These foam pumps are stationary and are installed for fire protection. For additional information, see the scope of NFPA 20 Section 1.1.1, which clarifies that the standard deals with the selection and installation of pumps supplying liquid for private fire protection.

Section 695.3 provides requirements for power sources for electric motor driven fire pumps. Section 695.3(A) permits an individual source where deemed reliable. Utility companies will rarely put in writing that their service conductors are reliable, and for that reason, fire pumps are typically installed in accordance with 695.3(B), Multiple Sources. Section 695.3(B) permits an electric utility service connection as specified in 695.3(A)(1) along with an on-site standby generator in compliance with 695.3(D). The general rule in 695.4(A) is that the supply conductors directly connect the permitted power source to either a listed fire pump controller, a listed combination fire pump controller and power transfer switch, or a listed fire pump power transfer switch. Section 695.4(B) permits a single disconnecting means and associated overcurrent protective device to be installed between the power source and fire pump controller.

This installation is made up of two disconnects between the normal power source and the fire pump controller. They are a 1,000A circuit breaker protecting the switchboard and a downstream 60A circuit breaker supplying the fire pump controller. To be compliant with the NEC, two 60A disconnects and OCPDs supplied by service conductors upstream of the 1,000A circuit breaker supplying the switchboard must be installed for the normal supply of the fire pump controllers.

Concern about selective coordination

The requirements for arc-energy reduction in 240.87 have created some concerns for one of my customers. We installed several 1,200- and 1,600A circuit breakers with energy-reduction maintenance switches. He is concerned that, where energized work is performed in these switchboards and the energy reduction maintenance switches are engaged, he will lose selective coordination. He is concerned there will be an outage. How do the requirements for selective coordination and arc-energy reduction work together?

Selective coordination is simply the localization of an overcurrent condition to restrict outages to the affected circuit or equipment. When engaged, an energy-reduction maintenance switch will ensure the circuit breaker involved will open without any preset time delay. When an owner decides to do energized work on electrical equipment, they have embraced the potential for an outage. If the owner is truly concerned about an outage, they will implement a scheduled shutdown of known duration and will not risk an unscheduled shutdown of unknown duration.

The rules for arc-energy reduction in the NEC do not negatively impact selective coordination in any manner. Where an energy reduction maintenance switch is employed, we have taken steps to reduce the clearing time of the upstream circuit breaker, only for the period of time it takes to perform the task. The clearing time is directly proportional to the energy in an arcing fault. If you cut the clearing time in half, you reduce the arc energy to one-half. Arc-energy reduction methods limit the amount of energy created in an arcing fault and will limit the damage to equipment. Energized work in the same equipment without a means of arc-energy reduction exposes personnel to significantly higher levels of arc energy and results in significantly more damage to the equipment.

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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