Tip of the Week: Calculating Loads for a Store Building Using Annex D in NFPA 70 (NEC)

What can you learn from the NEC’s Annex D, Example D3? The title of the example is “Store Building,” and that means it’s for a commercial installation (rather than industrial or residential).

You may use this example as a guide for just about any commercial installation. Your application may not have this example’s show window lighting load or outside sign circuit, but it may have other loads you can replace those with. Or take your typical office load; it’s basically lights and receptacles, and you can use this example as a guide (of course, you don’t need the show window lighting).

The first step shown in all of the Annex D examples is to characterize the loads. In this example, we see two simple lists:

1. Noncontinuous loads. These are classified as loads that run three hours or less.

2. Continuous loads. These are classified as loads that run more than three hours.

In each list, the load name and value are shown as a line entry. Adding those loads gives you the total for each list.

Article 220 provides the requirements for calculating branch-circuit, feeder, and service loads. Part II of that Article is for branch-circuits. In Part II, Sec. 220.12 refers us to Table 220.12. That Table lists general lighting loads by occupancy. This occupancy is a store, and thus is listed in this Table. The unit load is 3VA per square foot.

Going back to our example, there’s an asterisked note below the two lists of loads. That note explains why, with these load values, you must use the value from Table 220.12.

Now if you had just sat down to do this job and you hadn’t done one like it previously, you might have missed that requirement to use the value from Table 220.12. For at least the first few times you do a particular application, walking through the Annex D example is a good way to avoid making mistakes.