NEC in the Facility, July 15, 2010

Question: What are a dozen things you can do to vastly enrich your understanding of the NEC?

Answer: Work through the 12 examples in Annex D of the NEC.

The answer is a bit misleading, though. For one thing, there are actually more than 12 examples. The D1 through D12 examples include ones such as D2(a), D2(b), and D2(c). Furthermore, most of these are for residential calculations. Don't let the residential aspect throw you, however. Those residential examples provide you with a good way to work through NEC concepts you'll use in the facility.

The two straight industrial examples are:

1. D3(a). Industrial Feeders in a Common Raceway.
2. D10. Feeder Ampacity Determination for Adjustable Speed Drive Control.

Example D3(a) is highly instructive and well-worth serious study. It begins with a description of an industrial multi-building application case, and then poses this problem: "Determine the overcurrent protection and conductor size for the feeders in the common raceway, assuming the use of XXHW-2 insulation (90 DegrC)." There's a specific reason for 90°C insulation in the example, as you'll see in a future issue.

Now that the situation and problem are defined, D3(a) walks you through a method of arriving at the correct answer to the posed problem. And it is a method. Follow it, and NEC-compliant calculations for conductors and overcurrent protection are fairly straightforward.

The first step is to determine your calculated load. This is basically a matter of adding up your noncontinous loads, continuous loads, and motor loads while applying the required demand factors, largest motor 25% rule, and so forth.

We'll examine the other steps, plus an insightful twist, in our next issue.