When to use computer-based power analysis tools

With the vast amount of power analysis design tools available today, electrical engineers can be overwhelmed when choosing the right tool to use. Learning these tools and knowing when to use them can help an engineer be more efficient with his or her time and results.


Learning Objectives:

  • Understand how electrical calculations were performed during the pre-desktop computer era and the challenges it posed.
  • Differentiate between the two basic types of power analysis tools available for electrical engineers.
  • Learn the challenges an electrical engineer may encounter when choosing a specific type of tool.

One of the biggest challenges for electrical engineers is to find their own path to efficiently design a project’s electrical system. By mixing the best approaches used by other engineers, one can find ways to surpass previous efficiencies. A key to achieving this goal is to understand what tools are available when given a new project.

Knowing how calculations have evolved can really help us appreciate the tools we have today. Electrical engineering was formally considered an occupation in the late half of the 19th century when electrical power distribution was fully commercialized. Electrical engineers of this era performed manual calculations with slide rules, but as the power systems became more sophisticated, the calculations became more complicated. As alternating current (ac) systems dethroned direct current (dc) systems, and transformers and 3-phase systems were introduced to the system, non-manual calculation tools were necessary to eliminate the negatives of manual approaches: human error and demanding work.

They used to do what?

Figure 1: This table shows the basic formulas needed to obtain essential values of a dc or ac power system. These formulas can be easily used in handwritten calculations. Courtesy: Primera Engineers, based on Stallcup’s, Electrical Design, 2008 Edition, aSeasoned engineers often tell stories about how “back in the day, you didn’t have this or that” before the age of digital computers and computer-aided design (CAD) software, and most of the stories are true. Prior to the 1950s with its analog computers and analyzers, and the 1960s with digital computers, engineers did most their calculations by hand with the aid of slide rules. With the introduction of the per-unit system to address the change of voltages on transformers, and the introduction of symmetrical components to address the complication of three-phase systems, manual calculations became more labor intensive.

For larger projects, things can get complicated when engineers calculate all the basic parameters needed for a complete electrical system design using hand calculations. Let’s not forget the challenge of saving copies of the calculations. However, by being up-close and personal with the results, engineers from those times were able to build a natural knowledge of how the systems worked.

The handwritten form is the primal way of performing calculations, and it’s not going away. For the engineer, the key is to know which tasks are better performed using handwritten calculations. One of the good things about this format is that it is free. You only need a pen (or pencil), a piece of paper, and a calculator. This is as simple as it gets. This type of tool is perfect during a project meeting.

For example, conversations during the meeting may involve asking the lead electrical engineer to give an estimate of a circuit breaker size on a 480 V to 208 Y/120 V, 45 kVA, transformer primary side. Knowing the simple formulas to convert Volt-ampere into full-load amps) at a specific voltage (see Figure 1) and any code factors to be applied in the situation (i.e., NFPA 70: National Electrical Code table 450.3(B)), the engineer can calculate that the breaker size will probably be 70 A.

Figure 2: This is an example image of a panelboard schedule imported into AutoCad. Using the third-party software XL2CAD, an engineer can easily import spreadsheets, saving the time it takes to type it into an AutoCAD text table. Source: Primera Engineers Remember, there are additional considerations to be taken into account, such as the type of overcurrent protection device or transformer type, but overall this is a good example of how to quickly and efficiently use handwritten calculations without running into major complications. Today, manual calculations can also be assisted with the use of published tables by manufacturers and vendors. These tables can be great resources when a specific product is being used in a project. For example, Cooper Bussmann’s Selecting Protective Devices Handbook is a must when doing coordination studies using fuses and other types of overcurrent protection devices. Eaton’s Consulting Application Guide Technical Discussion is another good example of a manufacturer’s published guide that has essential information that can help an engineer throughout his or her design.

Computers and software

The desktop computer began to make its appearance in the 1960s and became commonplace in the 1980s. Software products for these computers were created to help users with their daily needs. With this same mentality, programmers have created different tools to analyze electrical systems. With the arrival of computers, storage capability also came into consideration. A key aspect of using software tools is not only the capability of software products to calculate values, but the option to store, print, and share results in a clear and organized way.

Most software products started with a spreadsheet format, which eventually took over as the primary tool for electrical calculations. Most of their evolution was purely driven by user needs. At first, these types of software calculated nonlinear load flow, short-circuit currents, and voltage drop in a quick and simple way. The interface between the software and the users consisted of collecting all the information needed to calculate a value. The software required the user to input these values in a spreadsheet form. Much of the existing software still uses this type of spreadsheet format but has an updated, more user-friendly interface that looks less like a typical spreadsheet.

Figure 3: This shows a Panelboard Schedule using Microsoft Excel. Typical values calculated in this example are connected, demand, and design load for a specific panelboard. Courtesy: Primera Engineers

When Microsoft released the powerful Excel program in early to mid-1990s, companies took it upon themselves to create a large number of templates for calculation purposes. This helped them take control of and analyze the information the way they preferred. Another advantage of using Excel is the option to export spreadsheets into AutoCAD. Using DotSoft’s XL2CAD, a third-party software, an engineer can easily import Microsoft Excel spreadsheets into AutoCAD drawings (see Figure 2), and therefore saving the time it takes to type it into an AutoCAD text table.

In today’s world, spreadsheets are still very useful for tasks such as: panelboard schedules (to calculate connected and demand load; see Figure 3), reviewing metered information (to calculate sum, average, low and maximum values), and for service/feeder calculations. 

The majority of companies own Microsoft Office licenses, which includes Excel. As a result, using this software can be seen as a no-cost option. There are still companies that are hesitant to spend money on new software that promises to facilitate all their needs by a single click of a button. These companies have already spent years of effort creating all these spreadsheet templates and have mastered their use. Alas, most companies eventually encounter the frustrating task of updating calculation values when there is a design change.

For example, most of these spreadsheets are separate, so if the mechanical engineer changes the horsepower size on some of the project’s motors, the electrical engineer has to make these changes in multiple locations. In addition to updating all spreadsheets affected by these changes (panelboard schedules, equipment schedules, riser diagrams, service calculations), the engineer must update all files created and imported into the AutoCAD drawings. This is where sophisticated software can be a game changer.

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