In our previous two issues, we discussed how studying Example 3(a) in Annex D is beneficial. Example D10 is another NEC resource worth spending time on. Unlike D3(a), it includes a figure that illustrates the application.
The problem is one of determining feeder capacity for six identical elevators. This isn't a typical application in MRO for facilities, but the example provides useful insights to apply to other applications.
The example uses five steps to arrive at the total feeder ampacity.
Since each of the six loads is supplied by its own transformer, your first step in load determination is to calculate the current draw of that transformer. Multiply by the number of transformers, and you have the total conductor ampacity (step two).
For step three, apply the demand factor. For step four, apply the 125% rule to the controller. And, finally add all of these ampacities for your total ampacity.
This example doesn’t take long to walk through, so where does the "spend time on it" part come in? Most industrial and many commercial facilities make extensive use of large motors and large variable speed drives. Instead of elevators, maybe your facility has six 30-ton chillers or six large plant air compressors.
Work through D10 again, but this time use your own actual loads and draw your own application figure instead of using Figure D10. In addition to making you sharper on feeder calculations, doing this exercise means you'll be checking your actual installation against the Code. This could prove especially beneficial if there have been equipment upgrades since those feeders were first installed or if you're planning upgrades.