Take precautions, such as using a straight edge, to make sure you are correctly reading the tables in the NEC.
Have you ever made a boneheaded mistake that left you embarrassed? Maybe coworkers got a real hoot out of it and teased you for days afterward. Such mistakes may have the upside of providing entertainment, but they also may create serious problems.
Anyone who’s worked in this business for very long knows that applying the rules of the National Electrical Code (NEC) is one place where silly stupid mistakes occur. One reason is that the NEC is full of tables. Some of these, although well laid-out, may be visually confusing just because of their size. The large ampacity tables are a prime example. You could easily let your eye glide across the wrong line.
What’s the solution? Use a straight edge whenever working with such a table.
With smaller tables, you can leave the straight edge in the drawer. Right? It may seem so, but think about how you are physically using reference material such as the NEC.
Maybe the center space of your work area has your drawings or project specs or whatever main document you’re working on. You have your NEC off to one side, so you often read it at an angle. This can easily lead to a “visual misalignment,” and you end up reading the wrong line.
For example, look at Table 220.12. This is a clean, straightforward table. How can you possibly misread it? Angle the page and then look up the Volt-Amperes per Square Foot for Office Buildings. Odds are that you correctly read the value “2,” but the odds are not 100 percent. Given enough times using this table, you will at some point read above or below the correct line in the table. Unless, of course, you use a straight edge.
Line skipping is just one silly stupid mistake you can make with tables. A common reason for other mistakes is a failure to fully read the tables. Using Table 220.12 as an example again, you’ll notice the Type of Occupancy listings are in alphabetical order.
Let’s say you need to determine the Volt-Amperes per Square Foot for a hospital hallway. So you scan down the list, and you come to Hospitals. The correct answer is 2, right? Nope.
That is the answer you will get if you just skim to quickly find an answer, instead of reading and understanding the table. Just below Warehouses is some text preceding another list of occupancies. From this, you can see that the Volt-Amperes per Square Foot of a hallway in a hospital is 1/2. That fourfold error is clearly significant.
Now look at the ampacity tables in Annex B. See in the titles, where these begin with “B.310.15(B)(2)”?
At one time, these were found in Chapter 3 along with the other “B.310.15(B)” ampacity tables. The problem was too many people got confused when trying to select the correct table. That happened because they just wanted to quickly look up the needed value, instead of taking the time to understand the meaning of the table titles and thus look up the value in the correct table. When using any table in the NEC, always take the time to read the title, notes in the table, notes below the table, and anything else that will help you understand the table.
Thus far, two tips: Use a straight edge, and fully understand the table. There are two other things that can prove very helpful. First, nothing prevents you from making notes in the margins of your NEC. But do so judiciously, so that whatever notes you write don’t get lost in a clutter of words scrawled on the page.
Finally, nothing stops you from using different colored highlighters to make unmistakable rows in those tables. Just make sure you use a straight edge when doing so.