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NEC in the Facility: Load Calculations, Part 5

To avoid confusion, follow the NEC sequence when performing your circuit calculations. You'll see this sequence in each of the Annex D examples. You'll also see it in the structure of the Chapter 2 articles.

Getting circuit calculations right is a multi-phase process. In Phase 1, you describe the application. Look at each of the Annex D examples, and you see a short paragraph that does exactly this. Don't mistake this as being purely for the sake of laying out the background for the story problem. It's really an illustration of what information you need to collect on the application before you start calculating loads and sizing conductors. Begin with the one-line diagram.

Think of load calculations as Phase 2 of a multi-phase process. If you look at each of the Annex D examples, you'll notice a recurring methodology. The designer first characterizes each load as either continuous or noncontinuous, then lists it under the appropriate heading. When this is done, the next steps then become simple to execute:

  1. Add up the noncontinuous loads;
  2. Add up the continuous loads, then apply the 125% rule; and
  3. Add that to the first sum.

The caveat on these load calculations is you need to check them against the load tables in Articles 215 or 220 (as appropriate).

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