With review ongoing to implement NEC 2020's changes, as construction professionals we cannot lose sight of the NEC's basics. In our last newsletter, we examined the importance of the Total Electrical Load. Importance of the Total Electrical Load In followup, let's look at a few situations resulting from when the Total Electric Load is miscalculated...one of the most common mistakes in electrical design.
As we all know, crazy things happen at the construction site. At a recent Blueblook Conference, the General Contractors were talking about gas equipment being specified instead of electrical, an undersized main service conductor specified for a 20 story highrise, 10 same sized transformers instead of just 5. These are costly mistakes ranging from an estimated $250,000 to millions to fix.
In a peer review, we looked over the installation of a backup generator at a South Florida country club.The electrical design specified ten (10) 112.5 kVA transformers. That's a lot of transformers easily catching the attention of any seasoned professional. Instead, we advised replacement with ten (10) 30-45 kVA transformers. This change alone resulted in savings of over $2,000,000 for the facility's owner. It is noteworthy that the original design was by a major engineering firm using leading software.
And for all of us involved in construction everyday, we know these types of mistakes are not uncommon. These issues arise from the current electrical design practice of using "rules of thumb", makeshift spreadsheets, and software that "designs" from the top-down (estimating the total electrical load at the start of design), instead of knowing the actual load by adding loads from the circuit to the facility's service entrance (bottom-up).
Why are these types of mistakes happening so frequently?
Because the electrical design process is complicated. A typical 2-story office building of 10,000 SF averages over 2,000 electrical calculations. And then, the results of these calculations must be checked for compliance with the National Electrical Code (NEC). As all of you know, the NEC's Handbook has 1000+ pages with endless look up tables. These calculation and code issues need to be addressed by a tool that automates the electrical design process.
The solution starts with the basics. Always work with the correct Total Electrical Load. As discussed in our last article, PowerCalc's approach calculates loads from the branch circuit to the service entrance (bottom-up).Importance of the Total Electrical Load This approach mirrors the NEC's framework. In Article 2 "Wiring and Protection", the NEC's requirements move from design of the Branch Circuit out to the facility's service entrance reflecting a "bottom / up" approach.
In comparison, guesstimating the size of the electrical load in conjunction with presets of panelboard sizes, transformers, equipment disconnects, feeders and overcurrent protection devices at the start of the project may lead to an inaccurate baseline on which to build the electrical engineering design rather than an accurate Total Electrical Load..
Why is this so important? And what is the outcome of an addition or subtraction mistake? Electrical equipment is either oversized at potentially great cost to the developer or undersized at potentially great risk to safety (fire) for those occupying the building.
So, the Total Electrical Load drives the success of your electrical engineering design.