Not correctly calculating the Total Electrical Load compromises the electrical design of the Power Distribution System in a facility. Here are a few horror stories illustrating this point.
1. At a peer review of a 60,000 SF clubhouse for a South Florida country club, the electrical engineering design of a prestigious firm specified 10 transformers at 112.5 kVA each. It was a first to review a project with so many generators of the same size. A review of the design using PowerCalc™ resulted in a reduction in the number and size of the transformers for a more reasonable kVA. The owner saved over $2 million by not purchasing unnecessary equipment.
2. A 20 story high rise in South Florida was also designed using a Top-Down approach. The Main Service Conductor was undersized. The solution: the Main Service Conductor had to be re-sized to match the connected/demand load. The consulting firm received a Change Order for $250,000 to correct the design and buy the correctly sized replacement for the Main Service Conductor.What’s causing these issues?
Electrical design from the Top-Down caused these horror stories. The habit of the Top-Down process started in the days before technology was available. As the electrical engineering design involves thousands of calculations, designers had to use “rules of thumb” and “guesstimating”.
But with all the technology available today, this practice continues. This flawed Top-Down process has even been promoted by several well-known software programs for electrical engineering design.
These software programs start a project by having the designer “guestimate” the size of the Total Electrical Load. In the horror stories noted, these software programs were used. The bad results discussed above share a common mistake: the “guesstimate” at project start was never updated with the project’s actual loads.
The Top-Down approach is a bad habit that can result in expense to the consulting firm, safety concerns for the building’s occupants and unnecessary costs to the owner/client.
If you are having expensive issues like these, look no further than a flawed Top-Down procedure implemented by “guestimating” or software that promotes this flawed procedure.
What’s the solution?
In contrast, PowerCalc™ calculates from the Bottom – Up. This process addresses the issue of correctly calculating the Total Electrical Load. But, it is a change in the usual procedure of how engineers and designers approach a project.
Mirrors NEC
As discussed in our last blog, the NEC mirrors this Bottom - Up approach. Article200 moves through the load calculations starting at Article 210 “Branch Circuits”, on to Article 215 “Feeders” and then 220 “Branch Circuits, Feeder and Service Calculations”. Finally at Article 225, the NEC moves to “Outside Branch Circuits and Feeders” and on to Article 230 “Service”. With the Load Calculations completed, the NEC at Article 240 discusses “Overcurrent Protection, Article 250 “Grounding and Bonding”, and then Article 280 Surge Arrestors”.
The overview: the NEC moves from the Branch Circuit out to the Service to calculate the Total Electrical Load and at Chapter 3 moves on to “Wiring Methods and Materials”.
To repeat our last blog, understanding Chapter 2 and its contents establishes the methodology to develop the design of the Power Distribution System. Once the Power Distribution Voltage and Phase are defined, the Circuit Current (Amps) can be calculated by applying the Electrical Theory Equations. Then, from the Calculated Current (Amps) of the Circuit, the designer can determine the Overcurrent Protection Device (OCPD), Conductor Size, Ground Size, Conduit Size and Voltage Drop, etc. for each Branch Circuit, Feeder and Service Conductor.
All good design flows from using actual Electrical Loads to calculate the Total Electrical Load
This progression of the NEC’s process in calculating the Total Electrical Load was also discussed in a good blog in EC&M. Part 1 is linked here EC&M Blog 8_28_2012. This blog likewise outlines the NEC’s specific requirements and Bottom –Up approach. It outlines Article 200’s requirements from Article 210 “Branch Circuit” through to “Service” at Article 230.
Change bad habits to end horror stories
To be clear, the size of the electrical load cannot be known at the start of a project. It can only be added, and adjusted for changes such as the addition or deletion of equipment as the design progresses. Use the Bottom-Up approach of adding the actual electrical loads from the Branch Circuit to the Service Entrance with adjustments for Demand Loads and changes.
Compared to the horror stories above, PowerCalc implements the NEC’s approach of Bottom-Up design. Here is an example of a positive outcome:
A major performing arts center in South Florida uses projection screens. To insure their operation, their facility was mapped as no electrical “as builts” were available. Understanding and correctly mapping the Electrical Loads allowed the identification of a local panel large enough to handle the additional specified electrical loads. A big cost saving for a nonprofit client.
