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Commercial Load Calculations, Part 3

Continuing with our examination of Example D3 in the NEC, we've determined what the calculated load is. Now we must determine the minimum number of branch circuits required.

As noted in Part 2, these steps follow in a logical sequence. To determine anything, you must first determine the calculated load. Once you've done that, you use the results in subsequent steps.

In this step, D3 illustrates how to find the minimum number of branch circuits required. You could do this step last, as it doesn't depend on any step other than determining the calculated load. However, that would put it out of sequence with the arrangement of the NEC. Always do your branch circuit calculations first, then your feeder calculations.

Under “Minimum Number of Branch Circuits Required,” Annex D says you need to install only enough branch circuits to supply the actual load. It refers us to 210.11, then almost explains how to determine the number of branch circuits needed to supply the general lighting.

What 210.11 tells us is the number of circuits depends on the size of circuits you’re using and how much load they must carry. For illustration purposes, let’s say you decided to use a single 100A breaker for this 44A lighting load. You would need only that branch circuit. Realistically, you would supply this load from some number of either 15A breakers or 20A breakers.

What number? Using three 15A breakers is the wrong solution. In our next issue, we’ll look at why.

« Part 2Part 4 » | Source: Mark Lamendola | Mindconnection