Making Room

01/01/2006


Congratulations! You just won the electrical design project you worked so hard to land. But there's no time to rest on your laurels in this "get it done yesterday" environment. It's time to roll up your sleeves and get to work! It won't be long before you get that telephone call from your architect asking you the dreaded question, "So, how much space do you need for the electrical gear?"

There is actually much more to this question than meets the eye. All too often, electrical engineers develop a rough idea of the square footage required for the electrical gear based on a building's total square footage and equipment to be served, but fail to coordinate with the architect to locate the electrical spaces for optimal efficiency. Quite often this makes it difficult or impossible for the electrical gear to effectively serve the load or be expanded in the future.

So, here is some food for thought when locating and coordinating electrical spaces in a typical new building. I'm not addressing any particular building type. Instead, I'll try to keep the focus as generic as possible. And a good place to start is the electrical service entrance room.

Service entrance

For the electrical service entrance room, the actual square footage required is naturally going to be project-specific and depend on many factors. However, the following guidelines should apply to any project and be discussed with the architect early in the project.

The main electrical room should be located on the ground floor along an exterior wall as close as possible to the building transformer. In fact, it would be wise to contact the local utility to confirm the location of a new building transformer. Don't assume anything. Keeping the main electrical gear close to the transformer will result in shorter secondary conductors and yield the most economical feeder, as well as avoid problems with voltage drop and difficult cable pulls.

The location along an exterior wall is often mandatory if the main electrical room has a basement below it. In accordance with National Electrical Code (NEC) 230.70(A)(1), service entrance conductors must terminate on the service disconnect switch immediately after entering the building. Most authorities having jurisdiction (AHJs) will allow only 10 ft. or so of unprotected service entrance conduits within a building. For electrical gear mounted on a slab on grade, this is not a problem because the service entrance conductors can enter the gear from below.

A problem arises, however, when the main electrical room is in a central part of the building and is also located over a basement. The AHJ is likely to insist on installing the service entrance conduits in a rated chase or concrete encasement where the conductors run through the building. Quite often, the main electrical gear is located in a basement, a location that exposes the electrical gear to potential flooding from rainstorms or burst pipes, and which should be avoided if possible. The electrical room should be located near loading docks or other similar areas to facilitate future gear expansion or replacement.

Other items also need to be discussed early on with the utility representative. Parties should agree on a location for the meter socket. The utility will want to make sure it can be easily read. Another issue that should be addressed early is whether the building will require more than one electrical service. This often occurs for calculated service capacity greater than 3,000 amps. If the building requires a high degree of service continuity, then a main-tie-main switchgear arrangement is often specified. Either of these scenarios will drastically affect the required size of the main electrical equipment room and transformer yard.

Moreover, utilities often have their own unique requirements that impact electrical spaces. For instance, here in the Denver metropolitan area, our local utility may require a secondary connection cabinet to be pad-mounted next to the building transformer if the secondary conductors are large enough. Again, this can drastically affect the transformer yard, as additional space will be required for the cabinet and working clearance.

Another important consideration is locating the main electrical room as close as possible to large HVAC equipment. In a typical modern building, the mechanical equipment can be 40% or more of the total electrical load. The most economical installation is to place the main electrical gear adjacent to the largest mechanical equipment. This also avoids the constructability issues associated with routing multiple large conduits through the building ceiling space.

But be aware that locating the electrical room close to the main mechanical room can cause problems as well. For instance, locating the electrical room between a main mechanical room and HVAC chases often results in conflicts between ductwork and electrical gear. The shortest distance for the ductwork to travel is often right over the electrical gear, a violation of the required dedicated space per NEC 110.26(F). It is advisable to study the floor plans and note the locations of HVAC chases. Large vertical ductwork chases are often good places to avoid. Typically, large amounts of horizontal branch ducts converge on the chases making mechanical/electrical conflicts probable.

Generators pose additional problems. If the building will be provided with a generator, the situation becomes more complicated. If the generator, transfer switch and distribution system must satisfy the requirements for a Level 1 Emergency Power System, as defined by NFPA 110.7.2.2, then the transfer switch must be housed in a room separate from the normal service gear. Previous NFPA 110 editions made an exception as long as the space between emergency transfer switch and normal electrical service gear was twice the access distance required by the code. However, the 2005 edition no longer allows this. Ideally, the transfer switch location should be as close as possible to both the main electrical gear and generator.

A common mistake is serving non-emergency equipment from the emergency power distribution system. Per NEC 700.6(D), the emergency distribution system is exclusively for emergency equipment. Often, this error is discovered after enough building space is allocated for emergency system gear only. Re-design to create space for additional gear for legally required or optional standby power—as defined by NEC 701.1 and 702.1—is often extremely difficult and wastes precious time. Refer to IBC 2702 for guidance on which loads should be classified as true emergency and which loads should be classified as standby. Typically Level 1 emergency power systems are used to supply egress lighting or other equipment that would put life in jeopardy if it were to cease operating. Emergency systems must be capable of serving the critical loads within 10 seconds of an interruption of the normal power source. Legally required standby systems, on the other hand, can be allowed up to 60 seconds.

The bottom line is to clearly define which building loads need backup power, then determine whether they are "Emergency," "Legally Required" or "Optional Standby" loads, and allocate space for gear accordingly. If there is any question, the AHJ should be contacted to give their interpretation. Note that standby system transfer switches may be housed in the same room as the normal main electrical gear. One exception is transfer gear for smoke-control equipment. Per 2003 IBC Section 909.11, ATS for smoke-control equipment must be housed separately from the normal source main gear. It is highly recommended to house all of the transfer switches and associated distribution panels in the same room, as it makes routine on-load generator testing easier for maintenance staff. It also allows maintenance staff to easily monitor the status of the switches and distribution panels during a prolonged utility outage.

Distribution

Now that we have looked at the main service requirements for a typical building, let's discuss considerations for power distribution throughout the building. Typically modern buildings distribute 480/277-volt, three-phase, four-wire electric power. Step-down transformers are located close to the major loads to be served. The step-down transformers are typically located in a dedicated electrical closet. Again, the actual closet space required will vary from project to project, but many general considerations can be widely applied to nearly all projects.

Per NEC 450.21(B), dry-type transformers rated above 112.5 kVA require installation in a room with one-hour fire-rated construction. This usually results in fire/smoke dampers being required on HVAC ductwork serving the room. Inform the mechanical engineer of the total installed transformer kVA in each electrical room so that adequate ventilation can be provided. Remember also to specify firestopping for all conduit penetrations. A 120/208-volt branch circuit distribution typically originates in panelboards fed from the secondary of the transformers. Every effort should be made to locate the panels as close as possible to the transformers that serve them. A recommended maximum should be 10 ft. This keeps larger 120/208-volt feeders short and allows the panelboard main circuit breaker to protect the feeder, as allowed by NEC 240.21(C)(2). And remember to provide enough space in the room for transformers and panels serving emergency and standby loads.

Where space is at a premium—Where isn't it at a premium?—transformers can often be installed overhead on trapeze supports or wall-mount brackets. It is important to discuss this with the project's structural engineer to verify the building structure can carry the load and design proper supports. Be sure also to allow enough room for overhead conduits in the electrical room. Be aware that substantial quantities of computer equipment in a building will result in large quantities of branch circuiting and conduit in electrical rooms. Remember also that conduits will undoubtedly be added over the lifetime of the building.

The size of the electrical room will be a function of the required number of circuits and physical area to be covered. A good rule of thumb is four standard office computers on one 120-volt, 20-amp branch circuit. Examination of the floor plan should reveal an approximate number of office workstations. If the project is a core/shell type, assume one work area per 100 sq. ft. Also, the designer should provide additional circuits for fax and copy machines, break rooms, restrooms and HVAC equipment. Once an accurate estimate on the required number of circuits has been developed, add an additional 20% or so for future circuits. As far as area to be covered is concerned, one electrical room should serve an area with a radius stretching about 100 ft. from the room. If a larger area is to be served, branch circuit conductors will probably need to be upsized to overcome voltage drop.

Finding space

The above factors represent some of the major issues that should be considered when defining the requirements for the electrical distribution systems within a typical building. Other issues and challenges will undoubtedly need to be overcome in the design process. It should be emphasized that it is not enough to merely request a set amount of space for electrical gear from the architect and hope for the best. An effort must be made early on to locate the spaces where they can serve the building electrical systems in the most efficient manner.





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