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Electrical, Power

How to design for transformers, switchgear and UPS

It is critical for electrical engineers to be involved early in the programming of a facility to meet end users’ needs and cost constraints in facility construction

By Mario Vecchiarello, PE, CEM and Timothy King, AIA, LEED AP May 14, 2020
Courtesy: CDM Smith

Learning objectives

  • Grasp multidiscipline design coordination awareness.
  • Understand that electrical design decisions may initiate a building code requirement for other disciplines.
  • Provide awareness of the size of electrical equipment and the spatial requirements around them.

Electrical engineers should be involved early in the programming of a building to meet occupants’ needs and to understand the cost constraints of construction. There are multifaceted interdisciplinary coordination issues that need to be considered in the early planning stages.

Although the architect typically leads the development of the new building’s design, the elecrical and mechanical engineers need to provide the architect with early input on the facility programming to ensure that the electrical and mechanical spaces are not only sufficiently sized to accommodate the electrical and mechanical equipment, but also consider certain code provisions related to large electrical equipment.

Design coordination with the architect

The design of any building requires close coordination between all disciplines; electrical equipment space needs to play an important role in that design. All buildings, especially those with large electrical loads and demand require well-thought-out space needs that can often be overlooked during the early design phase, resulting in inadequate space allocation for the electrical equipment and equipment of other trades.

During the early design, the architect will balance many owner-driven requirements for space needs — or programming of spaces — critical space functions and adjacencies, the building and life safety codes, adequate clearance and equipment access and long-term operations of the facility. While the architect is developing the building layout, the electrical and mechanical designers are progressing through their system and equipment selections, approach and evaluation.

This early phase is challenging and critical to developing a facility and that meets the owners’ budget and functional requirements. At this point in the design, involving the client in electrical equipment selection and getting the client’s sign-off on the type of equipment to be used will help to expedite the facility programming and coordination with the architect.

Figure 1: This graphic illustrates how the single-line electrical diagram translates into equipment space needs, location and interconnection. Courtesy: CDM Smith

Figure 1: This graphic illustrates how the single-line electrical diagram translates into equipment space needs, location and interconnection. Courtesy: CDM Smith

The architect will require input from the other disciplines, including equipment layout for the electrical room, type of equipment, equipment clearances, access requirements for the equipment and other information. Factors that typically need to be coordinated among design disciplines include several items.

Room location:

  • Electrical room location within the overall building footprint and other critical room adjacencies; consider vertical and horizontal conduit runs in and out of the electrical rooms.
  • Number of rooms needed for electrical equipment and the proximity of the electrical rooms to each other.
  • Utility power supply and relationship to utility-provided switch, transformers and possible generators.

Layout, space needs and access:

  • Electrical room equipment layout including equipment size, configuration, access width and depth.
  • Provide adequate access aisle width for electrical equipment installation and replacement.
  • Heating, ventilation and air conditioning exhaust and/or cooling needs including ductwork, dampers and louvers.
  • Electrical conduit and wiring from and between equipment to reduce wiring cost.

Egress width and number of exits:

Fire-rated walls and construction materials:

  • Fire ratings and separation requirements per the building code.
  • Wall materials, gypsum board or concrete masonry block.
  • Wall support capability for large electrical panels or equipment support from the floor.

Planning-level code considerations around certain large electrical equipment will guide some of the planning decisions in the programming of the facility. Below we will discuss various code requirements that need to be considered when applying the above factors during facility programming.

Electrical equipment selection

During the conceptual and preliminary design phases of the project, when the facility programming is being developed, it is critical that the electrical engineer identify the type of electrical equipment that will be specified for the project. Transformers, switchgear and uninterruptible power supplies are typically the three major pieces of electrical equipment that drive the size, location and the adjacencies of the electrical spaces that need to be considered during programming.

However, sometimes the electrical engineers overlook the unique electrical code provisions applicable to the specific configurations in which these pieces of equipment can be specified. Each of these pieces of equipment is available in different variations and ratings, which may drive the facility’s size and configuration, the materials of construction, separation requirements and the fire protection methods.

Because it is typically the NEC that identifies these specific provisions, it is imperative that the electrical engineer work closely with the architect and building mechanical engineer to ensure the electrical code-driven requirements are understood and incorporated into the architectural and mechanical designs.

Figure 2: This shows a 3D model of a transformer room and switchgear room. Courtesy: CDM Smith

Figure 2: This shows a 3D model of a transformer room and switchgear room. Courtesy: CDM Smith

During the conceptual phase of a project, it is important for the electrical engineer to develop a preliminary single-line diagram depicting the anticipated electrical distribution equipment needed to supply the facility’s loads, identifying the type of equipment to be specified and their corresponding ratings. The equipment rating and number of loads to be supplied by the equipment will have a direct impact on its physical size.

Additionally, the type of transformer, switchgear or UPS specified will also impact the physical size and working space required around the equipment as required by the NEC. The single-line diagram should be shared with the architect with an explanation from the electrical engineer describing the types of large equipment, space needed and how the equipment is interconnected. The coordination between the two designers will provide a high-level understanding and reduce the chance to undersize the needed space to meet code and operational needs.

There are several code-driven requirements that can impact the size, layout and construction methods required by the facility to accommodate transformers, switchgear and UPS equipment.

Transformers

A facility’s electrical distribution system will typically include multiple transformers to step utility voltage down to more useful voltage levels. The main transformer(s) within a facility may step the primary utility voltage down from 23 kilovolts to 4,160 volts to power large mechanical equipment such as chillers and large ventilation fans.

A second tier of transformers may again step the voltage down further from 4,160 volts to 480 volts to power intermediate sized mechanical loads and a third to step from 480 volts to 208/120 volts to provide power to use equipment. These transformers come in a wide variety of types and sizes. As such, each has very specific code provisions that may affect building construction materials and space requirements. Part II of NEC Article 450 identifies these “Specific Provisions Applicable to Different Types of Transformers.” We will discuss each type (dry and liquid) and how they may affect the building size and the fire ratings of the rooms.

NEC sections 450.21(A) through (C) define the provisions applicable to dry-type transformers installed indoors. The transformer’s kilovolt-ampere rating, voltage rating, insulation class and construction dictate which provisions apply. Table 1 summarizes some of the code provisions that drive spatial requirements and fire ratings.

Table 1: Several NFPA 70: National Electrical Code provisions drive spatial requirements and fire ratings. The transformer’s kilovolt-ampere rating, voltage rating, insulation class and construction dictate which code provisions apply. Courtesy: CDM Smith

Table 1: Several NFPA 70: National Electrical Code provisions drive spatial requirements and fire ratings. The transformer’s kilovolt-ampere rating, voltage rating, insulation class and construction dictate which code provisions apply. Courtesy: CDM Smith

Although dry-type transformers are the most common type of transformer installed indoors, there may be instances where a liquid insulated transformer is installed within a building or on the roof. These types of transformers have much more stringent code requirements than that of dry-type transformers.

Sections 450.23 through 450.27 of the NEC address the code requirements for the various types of liquid-insulated transformers. Except for nonflammable fluid-insulated and askarel-insulated transformers rated 35 kilovolts and less, the code generally requires liquid-insulated transformers to be installed within a transformer vault in accordance with NEC Article 450 Part III “Transformer Vaults.”

Even when a nonflammable liquid-insulated transformer is not required to be installed within a vault, it is still required to have liquid containment and pressure relief venting that carries the gases to an environmentally safe area. Below is a summary of some of the code provisions for transformer vaults:

  • The vault must be located with access to outside ventilation air.
  • Walls, roof and floor to be constructed for:
    • Structural integrity.
    • Minimum fire resistance rating of three hours (except for a one-hour rating where protected by an automatic fire protection system).
    • Drives the types of construction materials permitted.
  • Doorway construction requirements include:
    • The type of door construction and its fire rating.
    • The sill design must contain a spill of transformer insulating oil.
    • Personnel doors shall open in direction of egress and be equipped with listed fire exit hardware.
    • Locking hardware required to prevent unauthorized access.
  • Vault ventilation openings with respect to:
    • Location from other building features and combustible materials.
    • Arrangement of the opening to allow adequate natural ventilation.
    • Size of ventilation openings.
    • Ventilation opening coverings.
    • Fire-rated dampers that respond to a vault fire.
    • Fire-resistant ventilating ducts.
    • HVAC equipment duct work and associated piping.

It is important for the electrical engineer in conjunction with the architect and the structural, mechanical and fire protection engineers to evaluate the code requirements based on the specific transformer(s) being specified.

Switchgear

Like transformers, switchgear and switchboards come in a variety of configurations and ratings. The specific provisions applicable to different types of switchgear have a significant impact on the size of the electrical space and materials of construction. NEC Article 408 addresses the specific provisions for switchgear installation.

  • Section 408.18(A) stipulates that for other than totally enclosed switchgear, a space of not less than 3 feet shall be provided between the top of the switchgear and any combustible ceiling unless a noncombustible shield is provided between the switchboard and ceiling. This could affect building height and the types of materials of construction to be specified.
  • Section 408.18 (B) addresses the broader requirements of clearances around the switchgear with compliance in accordance with the provisions of NEC Section 110.26. Section 110.26 addresses the clearances required for equipment operating at 1,000 volts nominal or less to ground, if medium voltage equipment operating above 1,000 volts nominal to ground is being considered, sections 110.32 through 110.34 shall apply.
  • The general rules of section 110.26(A)(3) address the minimum height requirements of the working space about electrical equipment, which requires the height of the work space to extend from grade, floor, etc. to a height of 6 ½ feet or to the height of the equipment, whichever is greater.
  • For switchgear operating 1,000 volts nominal or less, section 110.26 (A) requires the depth of the working space, in the direction of live parts, to be not less than that specified in NEC Table 110.26(A)(1).
  • For switchgear operating above 1,000 volts nominal, section 110.34 (A) requires the depth of the working space, in the direction of live parts, to be not less than that specified in NEC Table 110.34(A).

There are significant variations in the distance requirements for clear working space depending on nominal voltage to ground and the conditions of installation which directly impact the size of the electrical space. Figure 3 shows the comparison between medium-voltage switchgear and low-voltage switchgear/switchboards, which demonstrates that medium-voltage equipment is significantly larger, and the type of low-voltage equipment specified also varies in size significantly.

Figure 3: This displays medium-voltage switchgear, which demonstrates how significantly larger medium-voltage equipment is compared with low-voltage equipment. Courtesy: CDM Smith

Figure 3: This displays medium-voltage switchgear, which demonstrates how significantly larger medium-voltage equipment is compared with low-voltage equipment. Courtesy: CDM Smith

Additional electrical code provisions that will have an impact on the electrical space requirements and the limitations imposed on the installation of equipment by other trades are:

  • Sections 110.26(C) and 110.33: Address the minimum requirements for the entrance to and egress from working space for electrical equipment operating at 1,000 volts nominal or less and greater than 1,000 volts nominal, respectively.
  • Section 110.26(E): Dedicated equipment space to a height of 6 feet above the electrical equipment shall not have equipment foreign to the electrical installation and shall not be installed in a way above the dedicated space that could damage the equipment from condensation, leaks, etc.

For medium-voltage switchgear, it is common to have station batteries to provide control power to operate the electrically operated breakers. These battery systems typically consist of a battery charger, direct current distribution equipment and several battery cells connected in series to make up a battery system rated for 125 or 48 volts DC.

In addition to the spatial requirements associated with the switchgear, the design team also needs to pay attention to the code requirements associated with the installation of batteries per NEC Article 480 “Storage Batteries” and the accompanying codes and requirements with regard to battery storage type UPS systems.

Uninterruptible power supply

UPSs for use in emergency systems, as defined by NEC Article 700, typically consist of an inverter with storage battery. As with the transformers and switchgear, these systems have specific provisions applicable to the type of system selected. When these systems are used as life safety systems, they are required to be located within two-hour fire rated rooms or to be installed in spaces fully protected by approved automatic fire suppression systems per NEC Section 700.12(B).

For battery storage type UPS systems, NEC Article 480 would be the primary code that applies to the battery installation. However, the informational note under NEC Section 480.1 references several IEEE recommended practices and UL standards related to the sizing and installation of the various types of batteries that the facility designers must become familiar with. Also, the battery installation shall comply with Chapter 52 (“Energy Storage Systems”) of NFPA 1: Fire Code. There are various requirements pertaining to location, ventilation, battery type, means of egress, fire rating, etc.

Emergency/standby generators

Although generator-based emergency standby systems are beyond the purview of this article, it is necessary for readers to understand the standards associated with the installation and operation of these systems because of their significant impact on facility programming.

For generator-based emergency standby systems installed indoors, the design team needs to consider supply air for combustion, ventilation systems to remove heat dissipated from the generator, exhaust system installation and the fuel storage requirements for on-site fuel. In applying these factors, the design team must consider additional standards that will influence design decisions. For instance, NFPA 110: Standard for Emergency and Standby Power Systems contains requirements for the performance of the emergency power supply and environmental considerations.

NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines establishes criteria for minimizing fire hazards associated with the installation and operation of stationary combustion engines and gas turbines. Like the NEC, these standards drive clearances around the equipment, materials of construction and fire protection measures that need to be applied.

Figure 4: A battery uninterruptible power supply system is configured in a rack storage unit with appropriate room ventilation rates to meet code. Courtesy: CDM Smith

Figure 4: A battery uninterruptible power supply system is configured in a rack storage unit with appropriate room ventilation rates to meet code. Courtesy: CDM Smith

Coordinating efforts

The numerous code and design considerations that should be evaluated during the early design phase of the project for large electrical equipment including transformers, switchgear and UPS can be summarized with the following:

  • The architect is concentrating on the building program and layout of the entire building and needs help understanding the electrical room requirements.
  • The electrical engineer should explain the single-line electrical diagram to the architect so they can understand the relationship of the large equipment to each other, the site and the electrical room.
  • The electrical engineer should provide simple electrical room layouts with key equipment, clearance and space needs considering HVAC and exhaust needs.
  • The architect and structural engineer should ensure the building walls and floors meet the fire ratings and can support the equipment structural loads, including seismic supports.
  • The electrical engineer should be prepared to study the various code requirements in detail and communicate your findings to the architect and the mechanical leads.

Mario Vecchiarello, PE, CEM and Timothy King, AIA, LEED AP
Author Bio: Mario Vecchiarello is a senior vice president and technical delivery manager at CDM Smith. He is a member of the Consulting-Specifying Engineer editorial advisory board. Timothy King is an architect and vice president at CDM Smith with 28 years of experience in planning and design of buildings.