Sizing your OCPDs, Part 2

Dec. 20, 2012

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.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.