"Electrical engineers design to standards".
This was a phrase one of my college professors shared with me in a professional practice course my senior year. In a sense, he was absolutely right. Whether you work in communications, electronics, power, or elsewhere, electrical engineers are designing to standards set primarily by the IEEE (Institute of Electrical and Electronics Engineers) and the NFPA (National Fire Protection Association). These rules help the world stay connected by ensuring that designs are consistent and meet minimum guidelines for quality and safety. Standards are developed by professionals for professionals, and are critical to the world we live in as technical professionals.
Unfortunately, the minimum often isn't enough. The reality is that a factor of safety, otherwise known as design margin, is usually needed to ensure safety and quality of operation. There are a few reasons why this is the case:
Standards are interpreted - Design standards often don't tell engineers everything they need to know to complete a project. For any meaningful design, the standards are subject to interpretation and engineering supervision.
Conditions change - Often, projects are designed based on information gathered at the project onset (e.g. a geotechnical report covering things like soil thermal resistivity). Over the design life of the project, design conditions will change and likely become less favorable at some point. A reasonable design will consider these aspects to ensure longevity and safety.
Construction isn't perfect - As a designer, it's easy to put something onto paper that minimizes project costs. However, the design also needs to be buildable. If engineering drawings require a project to follow unreasonable tolerances, it is unreasonable to expect good results.
Many electrical designers are apprehensive to add factors of safety because they believe that the National Electrical Code is already conservative. In some cases this is correct. For residential calculations of loads, the values used for conductor sizing are often highly generous. In many cases, though, the Code is far from conservative. Medium voltage ampacity tables used in the NEC assume highly favorable soil conditions as a basis for design. Without additional derating and consideration of soil parameters, one may be inclined to undersize cables.
Of course, engineers have to be careful about adding too much design margin. Adding margin beyond the amount necessary to ensure quality and safety just means adding cost; projects get more expensive and little to no benefit. It is the job of power systems engineers to solve this problem and choose the correct solution.
Design margin doesn't come in the form of a simple number that gets added onto everything, it's an entire design philosophy. What is the maximum temperature you are designing to? How heavily will you load you conductors? What is your basis for withstand determination? All of these kinds of questions need to be answered to ensure a good design.
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