Annex D of the NEC holds many valuable lessons for anyone needing proficiency in electrical calculations. Let’s see what those are, beginning with Example D3.
Though the example is for a store building, it can be applied to many types of applications. It begins with providing certain details, such as the square footage, number of installed receptacles, and the type of service. That’s the first lesson: assemble the basic details of the installation.
The second lesson is that everything in this example flows in a logical sequence of steps that walk you through determining:
- Calculated load.
- Minimum number of branch circuits required.
- Minimum size of feeder (or service) overcurrent protection.
- Minimum size of feeders (or service) conductors.
The other Annex D examples have different steps, but in every example (and in the real world) you:
- Assemble the basic details of the installation.
- Determine the calculated load.
- Take subsequent steps in the logical sequence.
Example D3 provides a good template for determining the calculated load. The most efficient way to do this is to first characterize each load as either noncontinuous or continuous. Though D3 doesn’t do this in a two-column table, it lists loads under one of two headings: continuous or noncontinuous.
The next steps are:
- Add up the noncontinuous loads.
- Add up the continuous loads, multiply by 125%.
- Add up the two totals.
There’s more to learn, as we’ll see in Part 3.