Modeling Electrical Distribution Systems with 3-D CAD

Electrical distribution systems for commercial buildings can be complex and sophisticated. Two-dimensional construction drawings of a complicated electrical distribution system may not be the best method of clearly defining the electrical scope to the other design team members, the owner and the field electrician.

By Keith Lane, P.E., RCDD/NTS Specialist, TPM, LC, LEED AP Vice President - Engineering. SASCO, Seattle February 1, 2007

Electrical distribution systems for commercial buildings can be complex and sophisticated. Two-dimensional construction drawings of a complicated electrical distribution system may not be the best method of clearly defining the electrical scope to the other design team members, the owner and the field electrician. But 3-D computer-aided design (CAD) drawings can be an excellent communication tool and may be advantageous in these circumstances.

Design and construction professionals typically spend years fine-tuning their abilities to imagine a 3-D world from 2-D construction drawings. Not so for the layperson, which is why 3-D CAD tools provides a method of clearly defining construction scope to this group.

In addition, 3-D CAD presents a significant advantage when coordinating the mechanical, electrical and plumbing services during construction administration. Complex team design issues may be explored and coordinated more easily during the design phase, prior to construction. Problems encountered during design are typically much cheaper to fix than problems that don’t surface until the construction phase of a project.

The 3-D drawing also adds value through the identification then of system routing conflicts during the design process, and allow for a simple means of explaining value engineering design options. The net result has value to the owner with fewer requests for information (RFI) and change orders during the construction phase.

If the entire M/E/P design team provides coordinated 3-D CAD drawings, conflicts that normally are identified only during construction may be more readily identified during the design phase of a project. Routing conflicts between HVAC ducts, sprinkler heads and electrical conduits or bus duct are more readily apparent in 3-D renderings. Light fixtures are more easily coordinated with sprinkler heads and air diffusers. And 3-D drawings are rotated and observed at different angles to improve this system coordination function.

Moreover, 3-D electrical drawings help clearly define that all of the code-required clearances and locations are adhered during the plan review process.

Clearances and locations

Code clearance and location issues include the following:

National Electrical Code (NEC), Section 110.26 table (A) (1) dictates the working clearances in front of electrical equipment. These clearances range from 3 ft. to 4 ft., based on the voltage level and condition. A 120/208-volt panel will require 3 ft. under all conditions. A 480/277-volt panel will require 3 ft. under condition 1 (see below), 3.5 ft. under condition 2 and 4 ft. under condition 3. For higher voltages, above 600 volts (nominal to ground), see NEC Table 110.34:

Condition 1: Exposed live parts on one side and no live or grounded parts on the other side of the working space. Or exposed live parts on both sides that are guarded by insulating material.

Condition 2: Exposed live parts on one side and grounded parts on the other side of the working space.

Condition 3: Exposed live parts on both sides of the working space.

NEC Section 110.26 (C) (2) dictates that for equipment rated 1,200 amps or greater and with overcurrent protective devices or control devices, there shall be one entrance a minimum of 2 ft. wide and 6.5 ft. high at each end of the work space. There are two exceptions to this rule. The first is when there exists an unobstructed and continuous exit. In this case, a single exit may be permitted. Second, if the depth of the work space required in 110.26 as noted above is doubled, a single means of exit can be used. In the second exemption, 6 ft. to 8 ft. of clearance can be required, again depending on the voltage level and the condition.

Note that the 2002 and earlier versions of NEC required that the electrical gear be both 1,200 amps or above, and more than 6 ft. long. The 2005 edition removed the “over 6 ft. in length” rule. This will subject more electrical equipment in commercial buildings to this clearance requirement or to the requirement for two exits.

NEC Section 110.26 (E) indicates that there is a minimum amount of headroom required above equipment, switchboards, panel boards and motor control centers (MCCs). This headroom shall be a minimum of 6.5 ft. Part (F) of this section, for dedicated equipment space, indicates that all switchboards, panel boards and MCCs shall be located in dedicated space protected from damage. Indoors this dedicated spaced is defined as the space equal to the width and depth of the equipment extending from the floor to 6 ft. above the equipment or to the structural ceiling, whichever is lower. A drop ceiling must add strength to the building to be considered a structural ceiling. The typical t-bar ceiling is not considered a structural ceiling.

The intention is to keep this space for the routing of electrical conduit and bus duct and to keep it clear of piping, HVAC ducting, leak detection and other foreign equipment.

NFPA 110, Standard for Emergency and Standby Power Systems, Section , dictates that automatic transfer switches shall be permitted to be installed in the normal electrical service room where twice the clearance required by NEC 110.26 is achieved. This would require 6 ft. to 8 ft. of clearance, depending on the voltage level and the condition.

The location of motor disconnects must be coordinated to ensure compliance with the NEC Section 430.102, which indicates that the individual disconnecting means for a motor shall be located within sight of the motor controller.

Moreover, when all M/E/P and sprinkler designers are using 3-D CAD modeling, the task of maintaining these code-required clearances and locations are performed easily. The 3-D models are rendered and rotated with multiple views to demonstrate all of the building systems with relation to one another and with the architectural background.

Another advantage of 3-D modeling is that it helps determine the actual distance of a feeder or a branch circuit and the total number of bends in the length of a conduit run. Two-dimensional layouts identify a conduit run in the “x” and “y” coordinates only; 3-D also illustrates the “z” coordinate—the up and down movement of a conduit run from point A to point B.

Determining the accurate length of a conduit run helps develope an accurate cost estimate. Additionally, accurate fault current calculations and voltage drop calculations are developed with this information. These calculations are dependant on accurate estimates of the total length of the conductors.

The locations of pull boxes also are determined more easily using a 3-D modeling tool. NEC Section 358.26 requires that there be no more than a total of four 90the conductors through the conduit during construction. The pulling tension is developed through a combination of distance and total number of bends and other factors including type of grip and lubrication utilized. Real field conditions can limit the actual number of 90º bends to two or three bends depending on the total length of pull.

The computer system requirements to run 3-D CAD programs can be quite intense, typically requiring a Pentium 4 processor, 1.4 GHZ or higher. In addition, 1 GB of RAM and a 1024 x 768 VGA display are required. The user may even want to consider dual processors or a separate computer dedicated to running the 3-D CAD iterations, as these computations can be processor intensive.

Without a doubt, 3-D CAD can greatly enhance the electrical engineering process for commercial buildings. NEC-required clearances and M/E/P design issues and conflicts can more easily be coordinated. Additionally, the total conductor lengths (required for accurate fault current and voltage drop calculations) and required locations of pull boxes can be identified. Value engineering ideas can be more easily explained to the owner and other design team members. The electrician in the field can more effectively understand the intent of the electrical design. In the end, RFIs and change orders can be reduced, providing a better, more efficient product to the building owner.

Building information modeling

Hand-in-hand with the use of 3-D design is building information modeling (BIM). Used in conjunction with 3-D CAD design, this technology is completely changing the way buildings are designed. This technology goes further than CAD. The 3-D CAD applications help all members of the design and construction team visualize how the project goes together—and helps prevent what used to be the inevitable “collision” of engineered systems in a building.

With BIM, the structure is digitally built, with opportunity to digitally build a structure from the start. Using BIM, all participants in a project—regardless of their level of CAD design expertise—can share virtual information from design phase to project completion.

The building design is created as a 3-D model that is stored as data in an object-oriented database. Drawings in 2-D and 3-D are generated from the BIM database. As data changes or as work progresses, the building model is automatically modified.

With object-oriented BIM, a part of the building entered into the system is not merely a graphical representation, but behaves like an actual window or wall or door in relationship to other building components. When an object is changed, the other objects adjust in real time, reducing design contradictions. Architects and engineers can work on the design at the same time because they can have access to a central model.

With more accurate information, and building elements that interact with each other, BIM offers engineers and consultants a better way to validate design assumptions in a virtual environment. This method of virtual simulations is more efficient than conducting them in the field.

The Construction Specifications Institute, Alexandria, Va., is involved with BIM because, for more than five decades, the organization has been driven by its mission to organize and streamline storage and retrieval of information necessary for defining building projects. The need for efficient exchange of information contained in building models is why CSI is participating in standards initiatives like the National Institute of Building Sciences’ (NIBS) process to develop a National BIM Standard.

The next step in the evaluation of this enhanced design and construction process is the development of 4-D drawings that introduce time and project scheduling into the mix.

A CAD Case Study: Freedom Tower

On a project as complex as the Freedom Tower, the first building to rise on the site of the former World Trade Center, Skidmore, Owings & Merrill (SOM), Chicago, knew that in order to meet the requirements of a fast-track schedule, high security, and the involvement of many stakeholders who lack access to CAD software, it would need the right design tools. SOM and New York-based engineering firms WSP Cantor Seinuk and Jaros Baum & Bolles (JB&B) turned to Autodesk Consulting to implement the company’s Revit building information modeling (BIM) platform.

Designers, architects, engineers and production teams all use industry-specific tools while working on the model. As they design, Revit automatically creates all other corresponding project information, including accurate floor plans, elevations, sections, quantity takeoffs, area calculations, schedules, and more. And once they update the model, all the other disciplines have access to accurate and complete information about the entire project.

“We started off cautiously,” says SOM parner Carl Galioto. “Our initial plan was to use Revit Building—complemented by AutoCAD—to model only the building’s complex subgrade levels. But because Revit performed so well, we quickly expanded its use to the entire project.” Currently, the Revit-trained team consists of 30 architects from SOM, 10 M/E/P engineers from JB&B and a team of structural engineers from Cantor Seinuk.

As a result, construction is on track for the 2011 completion goal. Galioto credits Autodesk’s expertise as a significant factor in helping SOM and its engineering partners reach this point. “The Revit platform helps us visualize the project as it will really be constructed,” says Galioto.

A CAD Case Study: In the Manufacturing Sector

One A/E firm that saw the light early in making use of computer-aided design with building information modeling (BIM) and 3-D capabilities to foster an integrated approach is GHAFARI Associates, Dearborn, Mich. In fact, the firm always has been an early proponent of utilizing new CAD technologies.

The firm, which has always been heavily involved in the industrial market—with several automotive clients in particular—received Eaton, Pa.-based software developer Bentley Systems’ BE Award of Excellence in the “BIM for Multiple Disciplines” category jointly with General Motors for its work on the latter’s Lansing Delta Township Assembly Plant.

GHAFARI doesn’t only use Bentley System’s software, but also applications from Autodesk. The firm’s designers have found that in implementing 3-D/4-D and building imformation modelling, no single platform fits all of its needs. The choice of platform is based on the individual project.

But whatever application is used, the firm has found that while seasoned design professionals are able to look at a 2-D drawing and visualize 3-D, there are many members of the project team who rely on the newest capabilities of 3-D and BIM in order to clearly understand the construction scope. The 3-D CAD applications presents a particular advantage when coordinating the mechanical, electrical and plumbing services during construction administration.

GHAFARI also has developed its BIM expertise into consulting services outside of its own projects, assisting clients with pre-qualification for BIM suppliers, and BIM enabled project management and deployment. Customers include industrial and governmental agencies, contractors and architects.