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Sizing your OCPDs, Part 2

Annex D3(a) provides a good example of sizing your overcurrent protection device (OCPD). This example shows a common problem. The size you come up with doesn't fit a standard breaker.

That's where the rule for the next size up comes in handy. But what exactly is this rule, and where does it come from? It's shorthand for the rules found in 240.4(B) and 240.4(C). The essence is that, if you've calculated an OCPD size that doesn't fit the standard sizes [240.6(A)], you can use the standard size that is next on the list of standard sizes (going from smaller to larger). In this example, the calculation is 136A so you can use a 150A breaker.

240.4(B) lets you use the next size up without increasing your conductor size, while 240.4(C) requires you to ensure your conductor ampacity is equal to or greater than that of the OCPD rating.

Doing your calculations in the same sequence shown in all the Annex examples will provide you with several advantages. One of becomes clear in this situation. To comply with 240.4(C), you just recalculate the VA by using the OCPD rating instead of the actual load.

Depending on the application, other Articles may apply [240.4] and those will complicate or even prevent your use of the next size up rule.

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