Designing industrial facilities requires diligence, long hours, close adherence to the National Electrical Code (NEC), and often, coordination with the authority having jurisdiction (AHJ). For industrial facility electrical designs to be safe and economical, the engineer plays a critical role in designing a properly sized system that avoids waste and oversizing.
An approach CH2M is taking on a recent industrial project includes steps to gather necessary client data, size the service in coordination with the AHJ, and ensure that the 2014 NEC is properly applied. A few key updates to the NEC in 2017 and their potential impacts to future designs are also highlighted.
Gather facility design basis info
The first step in the process is to gather the fundamental design information from the client so that a functional and code-compliant design could be achieved. After several client meetings and extensive data gathering, the client provided the following major design criteria:
When the design boundary conditions have been determined, the next critical step is to determine the service size. The NEC has prescriptive rules for calculating the size of new services. In practice, it is rare to calculate the service size for an industrial facility using the standard methods shown in the NEC. These would more typically be used for commercial applications. For most industrial processes, sizing the overall service for anything close to the connected load would result in a system that is much larger than required, costly, and wasteful. Chapter 2 of IEEE 141: IEEE Recommended Practice for Electric Power Distribution for Industrial Plants discusses this topic in more detail and is an excellent reference.
There are several sections of the NEC that can be used to justify a smaller service size. The most important section is Article 90.4, which states, "By special permission, the AHJ may waive specific requirements in this code or permit alternative methods where it is assured that the equivalent objectives can be achieved by establishing and maintaining effective safety." If this section is used, it is incumbent upon the engineer to provide concrete evidence that the system he or she has designed is safe and the size can be justified. In the case of our plant, we are fortunate that there is an existing plant that runs a nearly identical process. Using 12 months of utility data, existing client metering, and collected field data, we've determined the loading on the existing facility to be:
However, there are several key differences existing between the two sites that prevent us from using the existing facility information without modification:
The projected load data for the new facility is expected to be:
At this point, it should be obvious that the connected load has nearly tripled, yet all of our demand numbers have increased; i.e., we do not appear to be taking credit for additional load diversity. After closely examining the loads in the new facility, we determined that the majority of the equipment would operate coincidently and would not result in an increase in diversity. Based on the collected data and allowing for the additional 2 MVA for future load growth, our service is sized for 12 MVA.
Most AHJs will allow industrial services to be sized under engineering judgment using similar methods, as described above. In the event that you do not have an AHJ that is willing to approve an approach similar to this, there are several code sections that may allow you to reduce the size of both services and feeders.
NEC Article 220.60 allows you to use the largest load of two or more noncoincident loads for both feeder and service calculations. In addition, NEC Article 430.26 allows " ... [for] motors operating on duty cycle, intermittently, or from all motors not operating at one time, the authority having jurisdiction may grant permission for feeder conductors to have an ampacity less than specified in 430.24 ... " Lastly, for existing services, the engineer can use the provisions of Article 220.87 and calculate the service load by multiplying the maximum measured demand over the last year by 125% and then adding the new load.
Initial design decisions
Based on the client's desire for reliability, reduced maintenance intervals, and arc flash reduction, several initial design decisions were made. These decisions (in bold), along with the key reasoning behind them, are briefly explained below:
Differential relaying and ZSI have already been applied as previously discussed. In addition to those methods, the electronic trip units specified also include energy-reducing maintenance switching. The application of energy-reducing maintenance switching may impact selective coordination as required in Articles 700, 701, and 517. Use of other methods, such as ZSI, may be more appropriate on those systems. At the time of specification, breakers and trip unit manufacturers were not known, so the actual incident energy value that can be achieved is also unknown. The energy-reducing maintenance switching can be used to further reduce incident energy if the ZSI and differential protection do not achieve a sufficiently low value.
In addition to the typical process loads connected to the system in Figure 1, the new facility also contains emergency (NEC Article 700), legally required standby (NEC Article 701), and optional standby (NEC Article 702) loads. Article 700.10(B)(5)(a) requires that wiring from an emergency source to supply emergency and other loads is in, "Separate vertical switchgear sections or separate vertical switchboard sections, with or without a common bus, or individual disconnects mounted in separate enclosures shall be used to separate emergency loads from all other loads." This segregation is shown in Figure 2. The 2014 NEC introduced Article 700.8, which requires surge-protective devices on or in all emergency system switchgear, switchboards, and panelboards, so these were incorporated into the emergency system.
As what sometimes happens for these projects, the initial estimate came back with a higher cost than the client could support. To align the project budget with the client's business case, several critical modifications were made: