Evaluating software tools for electrical system design
Software tools and online resources are available for designing electrical systems.
Many tools are available to the electrical and power professional to assist in the design of electrical systems for buildings and facilities. This article focuses on specific proprietary software tools for unique design applications.
To help illustrate and demonstrate the available tools and online resources for electrical systems design, this article presents a hypothetical example project with various challenges, and surveys the tools that would be most appropriate to address them.
Problem design approach
Project X is a 400,000-sq-ft facility consisting of multiple buildings to be constructed in the heart of an urban college campus. The project will require a new electric utility service routed to a new central utility building and backup generator plant. There is significant distance between the buildings; therefore, multiple distribution points are needed, and voltage drop must be considered. The client has requested that a short circuit/arc flash hazard analysis be completed for the entire electrical system. Selective coordination will need to be considered as part of this study. The local authority having jurisdiction (AHJ) will require plan review documentation, specifically as it pertains to lighting (see Figure 1).
Cable pulling: The site electrical infrastructure distribution for Project X presents existing utility conflicts. The result is a service and feeder duct bank system with many bends, elevation changes, and offsets. This presents a challenge in terms of cable pulling and determining the proper location for pulling points along the system.
For complex cable pulls or simply to document a cable pull run, the software tool Cable 3D from SKM System Analysis models complex three-dimensional cable-pulling tension and sidewall pressure calculations, allowing rapid and accurate design decisions. The Cable 3D program provides both forward and reverse pull results for each cable profile modeled. For Project X, the Cable 3D program is used to document sections of the utility service entrance duct bank, and to adjust bends of cable profiles between sections of medium-voltage cable runs. It also helps demonstrate that a manhole is not necessary as a pulling point for the duct bank routing from the central utility building and the main electrical room, thus informing the designer so that the proper installation is constructed (see Figure 2).
SIMpull Cable Pull Calculator from Southwire is a convenient online tool for cable pulling calculations and estimating support in a spreadsheet format. It is an Excel-based calculator specific to Southwire products.
Pull Planner 3000 from American Polywater Corp. is a software tool for performing cable pulling tension calculations and conduit system design. The Windows-based software is available online for purchase and download.
Cable ampacity: For the same service and feeder duct bank sections modeled for cable pulling at Project X, the burial depths of these conductors are continually adjusted due to structural and other utility coordination. As of the 2011 version, the National Electrical Code (NEC) publishes ampacity tables in Article 310.60 for medium-voltage (2,001 V to 35,000 V) cable installations for a few directly buried cables and several duct bank profiles. The NEC does not publish similar ampacity tables for buried low-voltage (0 V to 600 V) conduit. These tables provide basic current-carrying limitations of conductors at assumed earth temperatures, and thermal resistances for conductor temperatures of 90 C and 105 C, all at assumed burial depth of 30 in. to top of duct bank, and 36 in. to the top of directly buried cables. AmpCalc (cable ampacity software for underground systems) from CalcWare Company is a computer program that calculates power cable ampacity ratings and/or cable operating temperatures for user-defined underground cable installations (see Figure 3). The latest version has also added a feature for aerial calculations. For underground systems, the effect of burial depth, type of burial (direct or concrete encased), and soil thermal resistivity (RHO) will determine the cable temperatures and ampacities for a given scenario. An effort should be made to confirm the RHO value of the soil. Lower RHO values indicate a low thermal resistivity. Therefore, the cable ampacity is optimized. A conservative value of 90 RHO should be used if actual data are not available.
The load factor is another major component to the calculation. The load factor is the average load in kW supplied during a designated period divided by the peak load in kW taking place in that period. It is the engineer’s responsibility to determine if a load factor less than 1.0 is appropriate for the cable design. The program uses the Neher-McGrath calculation procedure to determine underground cable temperatures or cable ampacity ratings for virtually any duct bank or direct burial cable configuration.
At Project X, a concrete-encased medium-voltage duct bank is designed to enter the building and snake under some water pipes, and enter the electrical room just above a structural footing. This scenario has a worst-case burial depth of 19 ft as it enters the building and traverses through some site grade elevation changes. As discussed, the tables in NEC Article 310.60(C)(77) through (80) list ampacities for burial depths of up to 30 in. For example, for a 500 MCM 5 kV cable, NEC Table 310.60(C)(78) lists 295 A for three aluminum conductors in a 27 in. by 11.5 in. cross-section duct bank (30 in. burial depth) . For a similar duct bank at 19 ft deep, the AmpCalc result was 225 A, a 24% reduction of current carrying capacity from the NEC value based solely on burial depth. For this scenario, a second run of conductors is implemented to form an additional parallel run of conductors in the duct bank to accommodate the considerable cable current-carrying capacity reduction due to burial depth.
Voltage drop: Voltage drop is an often overlooked requirement for electrical systems. The NEC provides language in informational notes regarding voltage drop criteria for branch circuits and feeders to provide reasonable efficiency of operation. The criteria imposed is to prevent a voltage drop of 3% at the farthest outlet or load, where the maximum total voltage drop on both feeders and branch circuits does not exceed 5%. Voltage drop is also a requirement for buildings seeking U.S. Green Building Council LEED program certification, as it is indirectly dictated as a prerequisite under ASHRAE 90.1-2010: Energy Standard for Buildings Except Low-Rise Residential Buildings as a mandatory provision of the few electrical requirements listed under Section 8: Power. ASHRAE 90.1 lists maximum voltage drop values for feeders at 2%, and branch circuits at 3%, both at design load. To accommodate voltage drop requirements, some electrical designers apply across-the-board rules by dictating in the contract specifications that cable sizes be increased according to linear feet of circuit length. For example, for 208 Y/120 V circuits, increase conductor size by one size for every 100 ft; and for 480 Y/277 V systems, every 150 ft. This conservative design approach may be suitable only for branch circuits and is difficult to present during permit applications to a code official who might be looking at the specific feeder voltage drop language from the NEC. Yes, some AHJs will request that voltage drop calculations be submitted along with a code plan review.
Voltage drop is a function of circuit load, cable size, and circuit length. A software program that contains a load flow design module will have the necessary computational equations to calculate voltage drop. Power Tools for Windows (PTW), from SKM Systems Analysis, is a very powerful program that is used throughout the industry to meet the rigorous demands of electrical system design. The DAPPER module consists of the load flow and voltage drop module. This software requires the proper input of data, including conductor type and length, as well as loads up to the branch circuit level. The program will provide a consolidated report, which can be very useful in documenting the results of the voltage drop calculations. It also supports Crystal Reports, a report-writing application that creates user-friendly and attractive customizable reports. Another usable feature of PTW is the ability to adjust transformer taps to compensate for voltage drop situations. This is very useful because it presents an alternative solution to cable upsizing. The load flow report will include the adjusted voltages, including tap adjustments (see Figure 4).
Voltage Drop Calculator from Southwire provides simple voltage drop calculations for individual cable runs in direct buried conduit and overhead installations. It provides results based on minimum conductor size or maximum conductor distance.
Generator sizing: At Project X, a backup generator plant is required to support all of the emergency and legally required standby loads, as well as a large number of optional standby loads defined by the client. Given the large amount of motor loads connected to the backup system—in addition to the design requirement for redundancy—the challenge is to determine the generator capacity and the optimum number of diesel generators to support the load.
Proper generator sizing depends on understanding steady-state and transient load on engine and alternator, sequence of load changes during backup operation, power quality requirements, and the effect of harmonics induced by nonlinear electronic loads. Generator sizing tools provide the ability to select generators based on voltage dip, frequency dip, and voltage total harmonic distortion. This allows the designer to select the generator that best fits his or her project requirements. The designer can select different types of loads including motors, VFDs, UPSs, battery chargers, office equipment, air conditioning, miscellaneous loads, lighting, and medical imaging equipment, to name a few.
For Project X, a generator sizing tool is used to determine the number of appropriate load steps corresponding to the priority levels for the automatic transfer switches (ATSs) in the system. First, all the loads are grouped by emergency system category (emergency, legally required, and optional standby). Then each group is identified by load category (motor, lighting, air conditioning, UPS). An initial generator software report selects multiple gensets to meet the criteria of supporting the large number of motors in the system. The project requirement is for N + 1 redundancy, so the desire is to keep the required numbers of paralleled generators to a minimum.
By analyzing the effects of across-the-line motor starting vs. soft starting or VFDs for motors, and providing additional load steps for the motor loads, the size and quantity of the generators needed is reduced. Load steps are implemented by adding ATSs and sequentially adding loads via the building automation system. Also relevant is the impact of the VFDs and UPSs in the system. These nonlinear loads have a significant impact on alternator sizing for the gensets due to the detrimental effects of increased power system component losses and heating; objectionable neutral current in 3-phase, 4-wire systems; excessive generator voltage distortion; and control interaction and instability. These programs provide very good options for including available filtering tools for the VFDs and UPS technologies (6 pulse vs. 12 pulse rectifiers and filters), and aiding the consulting engineer in making system-wide decisions based on all of these components.
The larger generator manufacturers provide free generator sizing software, available online or by contacting the local representative for each manufacturer. In addition to generator selection and sizing, these software tools can also provide specification cut sheets, installation drawings, emission information, product specification in text format, and the ability to link directly to a supporting dealer for budgetary quoting and additional support. In alphabetical order, the available generator software packages include:
- Caterpillar: Electric Power SpecSizer
- Cummins: Power Suite
- Generac: Power Design Pro
- Kohler: Quick Size and Quick Spec
- MTU Onsite Energy: PS-SPEC
Lighting calculations: Project X requires unique lighting solutions for the different space types throughout the building. The light levels must meet the specific design criteria as defined by the owner and energy codes to perform the required tasks within the space. The local code authority requires an egress lighting calculation submission to prove the designed lighting levels will meet the code minimum egress light levels.
The AGi32 software from Lighting Analysts Illumination Engineering Software is a calculation tool for accurate photometric predictions and can assist in the selection and layout of lighting fixtures. The program allows selection of specific IES files that represent the photometric data for the luminaires specified. The software allows users to create a rendering of the exact dimensions and ceiling height of a room, including obstructions such as shelving and furniture. There are default reflectance values for walls and ceilings that are inherent to the software. However, if the room finishes and material types are known, the actual reflectance values can be changed to influence a more accurate calculation.
Lighting fixtures can then be added to the model based on the lighting layout and mounting heights. After all criteria are entered, the software will calculate the point-by-point lighting levels in the space, with an option to create a 3D-rendered image of the model (see Figure 5). The results of the calculation can improve lighting designs by validating design criteria or indicating areas with deficiencies. The software also has a report tool that allows users to format and formalize a point-by-point calculation. The reports can be tailored to include numerous values, such as minimum, average, max/min ratios, and maximum foot-candle levels, fixture type, and Watts/sq ft lighting power density values. There is also a program function that allows users to export the point-by-point calculations to an Autodesk CAD file.
ElumTools is another software tool available from Lighting Analysts Illumination Engineering Software and is a fully integrated lighting calculation tool for Autodesk’s Revit 2012 and newer. ElumTools runs in Revit and provides a toolbar for access to analysis tools for application to any Revit room or space. It is available as an add-on for Revit.
Visual is lighting design software from Acuity Brands Lighting Inc. It is a comprehensive tool for lighting design that brings productivity to the lighting design process. Tools within the software package are tailored for specific aspects of lighting design. There is also an economic tool that can be used to determine the most economically viable lighting strategy.
Arc flash, short circuit, selective coordination: The client has requested that the engineer of record complete short circuit, selective coordination, and arc flash hazard studies for Project X. These studies require extensive, complex calculations and numerous iterations may be required. To complete these calculations for Project X, a system model is created using PTW software from SKM System Analysis. This software is able to perform the aforementioned calculations within the single model. The Project X one-line diagram is modeled within the software including all utilities, generators, cables, transformers, panelboards, switchboards, overcurrent protective devices/settings, motor loads, and branch circuit loads. The software has a large library database, which includes information from multiple manufacturers.
The DAPPER module performs load flow, voltage drop, complete fault analysis, and demand load analysis. The fault analysis provides a network solution of 3-phase, single-line-to-ground, line-to-line, and double-line-to-ground fault currents. It calculates both symmetrical and asymmetrical fault duties. The output gives accurate fault current values for each bus, informing the minimum kAIC rating of the electrical equipment.
According to NEC Article 700.27, emergency system overcurrent protective devices (OCPD) shall be selectively coordinated with all supply side overcurrent protective devices. The CAPTOR module produces time-current curve (TCC) coordination drawings that can inform users of the selection of OCPD to meet selective coordination. The TCC curves are displayed on a screen with interactive graphics that allow users to manipulate the OCPD settings to achieve selective coordination.
After the OCPD setting and selective coordination are set, the arc flash analysis can be performed. The Arc Flash Evaluation module calculates the incident energy and arc flash boundary for each location in the power system. The calculation automatically determines trip times from the protective device settings and arcing fault current values. The incident energy and arc flash boundaries are calculated following the NPFA 70E, IEEE 1584, and NESC standards. The software also has the capability of creating customizable arc flash labels.
ETAP is another electrical power systems design and analysis software tool. It can be used in design, analysis, maintenance, and operation of electrical power systems. There are both ac and dc databases, and it can perform calculations for both types of systems.
Harmonics: Project X has a complex mechanical system that requires a large number of VFDs for variable speed control of motors for pumps and other pieces of equipment. VFDs are nonlinear loads, which cause harmonic distortion on the electrical system of the building. Harmonics can be harmful to sensitive communication and data processing equipment. Therefore, consideration must be given during electrical system design to reduce the effects of harmonics on these systems. IEEE 519 recommends a limit of 3% to 5% total harmonic distortion (THD) at the point of common coupling, and 5% to 20% total demand distortion. To achieve these limits, there are design strategies that can be implemented to mitigate the harmonics imposed by nonlinear loads. The application of harmonic filters at either the VFD or the service entrance is one solution. Another is the use of 12- and/or 18-pulse VFDs. The application of filters and multipulse drives can become costly. It is important to thoroughly analyze the system and select the most appropriate solution. Simply providing all motors with 18-pulse drives may not be the most effective solution and may incur undue cost to a project. DriveSize is a free program from ABB that can be used to compute network harmonics and to create dimensioning documents. The program calculates THD according to IEEE Standard 519 and IEC61800-2 standard. The values indicate the effect the drive load has on the electrical network at the point of common coupling. By analyzing the drives on an individual basis, the high harmonic loads can be identified, allowing for a more custom solution to be implemented for the project.
The Harmonic Calculator from Allen-Bradley (Rockwell Automation) provides an Excel-based calculator for determining how much voltage and current distortion may exist on the electrical system when operating nonlinear loads.
Alberto G. Cordero is a senior electrical engineer with Affiliated Engineers Inc., where he specializes in complex higher education, health care, and research facilities. Candace Dolan is an electrical engineer with Affiliated Engineers Inc., where she specializes in higher education and research facilities.