Sizing your OCPDs, Part 1

Once you've done your load calculations, the next step is to size your overcurrent protection device (OCPD). Annex D3(a) provides a good example of how to do this. Recall that the first step in load calculations is to characterize your loads. You should have a spreadsheet that groups loads as continuous or noncontinuous, with the total VA for each group. As noted in this example, "The overcurrent protection device must accommodate 125% of the continuous load, plus the noncontinuous load."

When you multiply your continuous load by 125% and then add it to the total of the noncontinuous load, you get your total VA. We calculate loads as VA, not as amps, but we size OCPDs in amps. So your next step is to calculate the amps.

If you had a DC circuit, you would simply divide your VA by the nominal voltage. But this is a three-phase AC circuit, so you must multiply the voltage by the square root of 3 first. You can use 1.732 to simplify the math. Multiply 480V by 1.732, and you get 831.36.

Divide the total VA by 831.36 and you have your minimum size OCPD rating. In this example, it turns out to be 136A. No 136A breaker in stock? In our next issue, we'll see what to do about that.

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