NEC Calculation Requirements, Part 3

For any given project, the electrical calculations proceed in a certain logical sequence. You would, for example, have no idea what size service you need if you don’t know the total load it’s expected to serve.

The flow of electricity is from the service to the feeders to the branch circuits to the connected loads. When determining loads, you can think of the “flow of loads” and follow this trail in reverse.

Always start your electrical calculations by determining what the loads are that you’re going to provide power to.

Loads are either continuous or non-continuous. A good practice is to make a two-column table of loads, with continuous in one column and non-continuous in another. You could further enhance this table by adding columns for other characteristics, such as linear and nonlinear.

A load is continuous if you can expect the maximum current to last more than 3 hours [Article 100]. Here is one of many calculation process decision points where you can either do the minimum to “meet Code” or you can do the proper engineering work to ensure an efficient and reliable system.

For example, a warehouse has lights on occupancy sensors that time out after 15 minutes of no activity. You can probably get by with classifying them as noncontinuous, but in so doing you are making many assumptions that must remain true for the life of the facility.

When characterizing loads, you should also consider load diversity. We’ll cover that next.

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