NEC in the Facility: Load Calculations, Part 4

To calculate feeder or service circuit loads, the minimum you can use is found in 220.40 (the first subsection of Art. 220, Part III).

When calculating feeder or service circuit loads, what's the minimum you can use? The answer is in 220.40 (the first subsection of Art. 220, Part III).

The total calculated load on a given feeder or branch circuit must be at least the sum of the loads on the branch circuits this circuit supplies (as determined by 220 Part II). But you must adjust this sum by any applicable demand factors required by 220 Part V and permitted by 220 Part III. If you're using the "Optional Calculations" of 220 Part IV, use the applicable demand factors contained in Part IV rather than those in Part III. You'll find five demand factor tables in Part III, but only two such tables in Part IV.

The Informational Note in 220.40 refers us to examples D1(a) through D10. These examples are immensely educational. However, this numbering can be a bit disconcerting. For example, there is no Example D7. Among other anomalies, you'll find:

  • D3 and D3(a) but no D3(b).
  • No D2, but D2(a), D2(b), and D2(c).

While this numbering system appears to defy logic, a closer look helps reveal why it's that way. D1(a) and D1(b) both address one-family dwellings. So do D2(a) through (c), but with a specific variation. In our next issue, we'll look at D3.

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