How to design electrical rooms

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


This article has been peer-reviewed.Learning objectives:

  • Explain the applicable code requirements including NFPA 70: National Electrical Code.

  • Evaluate the design criteria for appropriate electrical-room size to accommodate present and future needs.

  • Analyze the requirements for coordinating with structural, architectural, fire protection, and HVAC requirements.

Electrical rooms and mechanical, electrical, and plumbing (MEP) spaces are often an afterthought when it comes to building design and planning, either relegated to locations that are left over or deemed undesirable for other planning purposes. This shortsightedness can have unfortunate consequences on the cost, operations, and flexibility of the systems for the future.

Figure 1: The dedicated space (depicted in red) and the working clearances (depicted in blue) are shown in a new emergency distribution room. Courtesy: CannonDesignNFPA 70: National Electrical Code (NEC) dictates the minimum amount of space needed around the equipment for access, operations, safety reasons, and conduit installation. Together, with the actual equipment sizes, this defines the overall minimum dimensional requirements of the room.

There are three types of general interior electrical spaces that factor into new building design: main equipment rooms, distribution pathways, and local/branch equipment rooms. Code-required working space and dedicated space needs must be met. This article will outline important considerations for these spaces in the early stages of building design as they relate to building type, intended occupancy, size, and future expectations of both the building and the electrical systems.

Working vs. dedicated spaces

Let’s first define what differentiates working and dedicated space as stated by the NEC (see Figure 1). The working space helps safeguard a clear working zone around all equipment and ensures protection for any workers or occupants within the room. This includes defining minimum width, depth, and height requirements for the working space, which varies due to voltage and the specific equipment. The higher the voltage of the equipment, the greater the depth of the working space. The width should be equal to the width of the equipment and no less than 30 in., while allowing for opening any doors or hinged panels to a full 90 deg. The height should be 6 ft 6 in. from the floor, or the height of the equipment if greater than 6 ft 6 in.

The style and construction type of the electrical equipment dictates whether only front access is required, or if rear and/or side access also is required. For each point of access to a piece of equipment, the minimum working clearances must be provided.

Dedicated space is a zone above the electrical equipment. It’s reserved to provide future access to the electrical equipment, protection of the electrical equipment from foreign systems, and for installing conduit/other raceways supporting incoming and outgoing circuits. The requirement for dedicated space applies primarily to switchgear, switchboards, panelboards, and motor control centers. The space should be equal in width and depth to the equipment size and extend from the floor to a height of 6 ft above the equipment (or to a structural ceiling, whichever is lower). No equipment or systems foreign to the electrical installation are allowed in this zone by the NEC.

Figure 2: The medium-voltage switchgear sections and unit substation transformers in a large data center installation require additional space and clearances. Courtesy: CannonDesignThe area above the dedicated space may contain foreign systems, provided proper protection prevents damage from drips, leaks, or breaks in these systems. However, it’s good practice to avoid having these systems installed in electrical rooms altogether.

While installations of equipment greater than 1,000 V generally follow the same principles, some of the specifics vary, requiring additional clearance around the equipment due to the increased hazard that these voltages impose (see Figure 2). Access to this equipment is preferably limited to only those deemed qualified to be there. For this reason, electrical equipment should be installed in rooms or spaces that are dedicated for that purpose and have controlled access.

The first room type we’ll explore in this article—a main equipment room—has distinctive needs that separate it from distribution spaces or local/branch equipment rooms.

Main equipment rooms

The main electrical room, or service entrance space, should coordinate with the local electrical utility (refer to NEC Article 230, Services, for additional details pertaining to the installation of service entrance conductors and equipment). For example, main equipment rooms have requirements that dictate access to the space from the exterior for servicing, maintenance, and service feeder installation. The type of equipment installed will also further determine the room requirements. The service entrance room is typically located on an exterior wall for both code and practical reasons; it makes installation easier and minimizes the length of the service entrance conductors. Because the service conductors are usually the largest in the facility, this can have a substantial impact on cost.

Figure 3: At the Coppin State University Science and Technology Center, conduit pathways need to be considered when designing electrical rooms to ensure proper clearances are met and that the distribution is efficient. Courtesy: Mathew Kirschner PhotographUsing arc-resistant switchgear will also impact space needs. This equipment will be taller and may have a larger footprint. Engineers will also need to account for the potential exhaust gases and arc flash energy by providing a pathway to expel them and relieve the pressure buildup from inside the switchgear.

If an exterior transformer is used to provide the service to a building, feeders from the transformer enter the building and transition to the main service entrance disconnect, typically a switchgear, switchboard, or panelboard. These feeders are often routed underground into the building through the exterior foundation wall via a coordinated opening. Additional coordination with the structural engineer is needed to avoid footings.

The elevation of the service entrance conduits many times do not naturally align with the equipment to which it is routed. Additional space in the form of increased height or footprint commonly is required to allow for the successful transition and termination of these conduits and conductors. Service installations that require medium-voltage equipment and/or transformers installed indoors will require additional elements including more space, higher fire ratings of the rooms (per NEC Article 450), and increased ventilation.

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