Integrated design of electrical rooms

Electrical engineers should coordinate with mechanical engineers, architects, structural engineers, and others involved in the design of electrical rooms.

03/08/2011


The purpose of an electrical room is to house electrical equipment, providing a space that is both safe and secure for the operation and maintenance of the electrical equipment. Electrical equipment ratings and types can significantly affect the room requirements. Switchboards, switchgear, transformers, generators, UPS, and low- and medium-voltage ratings all impact the requirements for an electrical room.

NEC article 110 Part II (600 V and below) and Part III (more than 600 V) is the primary source of these requirements. The focus of this article will be on electrical equipment 600 V or less. However, the design of these rooms goes beyond electrical codes, requiring an integrated approach among numerous disciplines including architectural, structural, mechanical, and fire protection.

Architectural requirements

Coordination with architectural requirements centers on electrical room space needs including working space around and above the equipment, and access to and from the electrical room. Working space is based on NEC table 110.26 from the 2008 NEC. Table 1 breaks down the clearance requirements based on voltage level and conditions.

Table 1. This breaks down the clearance requirements based on voltage level and conditions. Courtesy: NEC 2008 Handbook

Table 110-16 has three conditions. These conditions take into account the distance from the accessible side of the electrical equipment enclosure to various wall construction types, or other electrical equipment. Note that the voltages referenced in the table are from any single “hot”/“leg” as referenced to ground. So, a 208 V, 3-phase panel board is actually 120 V from any single leg referenced to the ground conductor... The three conditions are illustrated in Figure 2. Protect - Figure 2

Note that the working space discussed is the depth in front of the equipment. The working space has both width and height. The width must be 30 in. or the width of the equipment, whichever is greater. The vertical space (height) required by 110.26 is 6.5 ft or the height of the equipment, whichever is greater. This height requirement means that you cannot install the associated gear in question in a sublevel space (crawl space) or even allow pipes, conduits, or other accessory equipment to be installed in this imaginary “box” created by the requirement(s) as we will review later.

The electrical engineer also should carefully review the type of electrical equipment to be placed in an electrical room. Non-electrical engineers commonly use the terms “switchboard” and “switchgear” interchangeably, but these two types of equipment types have very different requirements for access and clearance. Because switchboards are built to the UL 891 standard and typically come with fixed breakers, they may require only front access for cable terminations. Larger or more specialized switchboards may require rear access for cable terminations and have optional draw-out insulated case devices. Switchgear is built to the ANSI C 37 standard, has draw-out front power breakers, and requires rear access. Draw-out breakers will extend into the pathway in front of the gear and also require space from either overhead hoists or portable lifts to remove the breakers. A good recommendation is to show these additional space needs on the electrical drawings with dashed lines extending from the equipment footprint.

Another important requirement in Article 110 Part II addresses the means of entrance and egress for an electrical room. In electrical rooms more than 6 ft wide with equipment rated 1,200 amps and above, there must be one means of egress at each end of the room.Protect - Figure 5 The doors must be a minimum of 2 ft wide and 6.5 ft high. The doors should swing out of the room and have panic hardware.

Other codes and good practice may point to larger doors for egress. Beyond the code requirements for doors, the engineer and architect should consider real working space needs. For instance, the primary intent of the NEC minimum door size noted above is for egress purposes, not equipment entry or removal. Some types of electrical equipment can be 7.5 ft high and 30 in. wide or larger. Code sized doors at 2 x 6.5 ft would not permit move-in or replacement of that equipment. A good suggestion, therefore, might be 9-ft-high double doors that would accommodate these types of equipment.

Most basic switchboards require only front access and have fixed group-mounted breakers. However, some sophisticated switchboards or standard switchgear require additional front access space for draw-out breakers, and also rear access to pull and terminate cables on the bus. The NEC mandated 3 or 4 ft may not suffice to support these needs. Further, the height of the electrical equipment and the clearance to the ceiling above should be evaluated where top entry conduit bending space may be needed; 3 ft or more may be needed above the top of the equipment. A suggested minimum height for some electrical rooms therefore might be 12 ft or more.

Lighting is another area that engineers should coordinate with the architect. The NEC does not mandate lighting levels, but a good recommendation for lighting is 30 to 50 foot candles (fc). It is good standard practice to consider putting a portion of the lighting in a main electrical room on battery or generator power. Another recommended practice is to provide electrical outlets (duplex/quadruple receptacle) at or near the equipment in question to allow maintenance crews to have task lighting. These outlets should, of course, be provided by a panel that is not downstream of the switchgear or panel board to be shut off for maintenance. This allows maintenance crews to increase the level of lighting, as the 30 fc recommendation is rather dark for electrical maintenance. It is a good idea to consider putting a portion of the lighting in a main electrical room on battery or generator power.

Lastly, arc flash protection has become an electrical room consideration over the past few years under NFPA 70E. Personal protective equipment (PPE) is required for those working on energized equipment within the flash protection boundary. The flash protection boundary is defined as the distance from an arc flash source within which an unprotected worker would receive a second-degree burn. IEEE Standard 1584 provides a guide to calculate specific arc flash levels and determine what PPE should be worn within the flash protection boundary. These boundaries or distances in front of equipment and the PPE categories in electrical rooms should be reviewed with the owner and architect. It may be possible to provide enough space in front of the equipment to have an area outside the flash protection boundary.

Structural considerations

It is important to coordinate with the structural engineer on equipment weights and electrical room floor capacities. Many types of electrical equipment carry significant weight load. Transformers, for example, are a common source of notable weight in an electrical room. Table 3 shows some common weights associated with unit substation transformers.

Table 3. This table shows some common weights associated with unit substation transformers. Batteries associated with UPS systems also present significant structural loading issues. Courtesy: M+W Group

Conduits are often overlooked when considering structural issues. Conduits and cable can present significant weight loads when hung from a structure, so attachment and bracing detail for conduits should be reviewed with the structural engineer. In addition, electrical and structural engineers often fail to coordinate conduit penetrations. An electrical engineer may not always consider the significant space conduits can require for a large feeder. Where these conduits penetrate through the electrical room floor, walls, or ceiling, they may weaken certain structural components. These penetrations also can present a fire stopping issue in and out of an electrical room and should be reviewed with the architect. Conduits that run in or under the slab of an electrical room also need to be coordinated with the structural engineer.

Housekeeping pads should be reviewed with both the structural and architectural teams. The specification division or drawing in which this work is shown is frequently a source of controversy between contractors. The electrical engineer should review equipment pad sizes and requirements with the architect and structural engineer.

Seismic requirements are another issue that electrical and structural engineers must address. The electrical engineer should review with the structural engineer the seismic zone where the building is located. Seismic requirements for bracing of equipment and conduits within the electrical room should be specified.

Mechanical systems

There are several integration issues to be addressed with the mechanical team. Maximum operating temperature is one issue for concern. Most common types of electrical equipment have a maximum operating temperature of 104 F. A suggested specification of 86 F is recommended for most electrical rooms. UPS systems typically have valve-regulated lead acid batteries and an attached warranty that is dependent on temperature. The life of the battery (let alone the warranty) is affected by losing 50% of its life if temperature of 80 F is exceeded (temperature varies by manufacturer).

The electrical equipment itself generates heat in the electrical room. These loads should be reviewed with the mechanical engineer. In most cases the primary source of heat load are transformers; panels, switchboards, and switchgear are not significant sources of heat rejection. There are many rules of thumb for estimating the cubic feet/minute (cfm) for electrical rooms. One is 1 cfm/sq ft for electrical rooms without transformers and 3 cfm per KVA for electrical rooms with transformers. Actual HVAC calculations should always be done (see sidebar) using actual manufacturer data for the proposed transformer types. Representative heat rejection values for some common general-purpose dry type transformers are shown in Table 4.

Table 4. Representative heat rejection values for some common general-purpose dry type transformers are shown. Courtesy: M+W Group

Notes: 1. 3.4 btu/W; 2. Examples only: dimensions used were for 115 C rise; losses were 150 C rise

The location of mechanical systems, such as ducts or pipes, is covered in NEC Article 110 Part II. A dedicated electrical space is established above the electrical equipment extending over the entire equipment footprint up to 6 ft high or to a structural ceiling. This is a foreign system “no fly zone” for mechanical or other nonelectrical systems. Sprinklers may be installed within the dedicated space if they are for that space and protected. Foreign systems may be installed above (but not within) the dedicated space if they are protected from damage due to conditions such as condensation, leaks, or breaks.

Some special-purpose electrical rooms like those for UPSs require even closer coordination with the mechanical engineer and others. Batteries associated with UPS are very temperature sensitive and temperatures above 77 F can affect battery life spans. UPSs also contribute heat to the electrical room. In addition, depending on the type of battery specified and local codes, special hydrogen detection and ventilation systems may be required.

Fire protection requirements are covered in NFPA 13. Normal electrical rooms should either have sprinklers or have a 2-hour construction rating.

Generator electrical rooms also present special challenges to all the disciplines. Structural engineers will be concerned with vibrations and weights. Often an isolated, reinforced pad is provided on the floor of the room. The architect will be concerned with the room’s fire rating, which will be higher than that for a standard electrical room depending on fuel and emergency code classifications. Ventilation and fuel storage issues will be of concern to the mechanical engineer. The electrical engineer also should carefully review where the automatic transfer switches (ATSs) will be located in the generator room. Larger ATSs may require both front and rear access and have draw-out components similar to switchgear. Lastly, whether the generator is classified as a true “emergency” (code mandated) system or just a standby (or optional backup) will affect issues of separation, the fire ratings of the room, and the system therein.

Conclusion

The design of electrical rooms requires an integrated approach among disciplines. Architects and structural, mechanical, and fire protection engineers should work as a team in designing these rooms. The NEC is the main source for electrical room requirements, but other codes, good practices, and recommendations should be considered. The result will be a room that is safe and secure, and provides for the functional operation and maintenance of the specific electrical equipment located within.


Rener is a senior design manager with M+W Group. He has more than 20 years of experience in electrical engineering and multidiscipline design management. His specialization is in advanced technology facilities including data centers, high-performance computing centers, cleanrooms, and laboratories. Rener has published or presented more than a dozen technical papers, is an officer for the Chicago chapter of the IEEE Industry applications society, is a member of the Consulting-Specifying Engineer editorial advisory board, and serves the Village of Lake Bluff as an elected official.



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