Support X-Ray Demand Factors

[aaron.jonesSAP83]

According to the 2011 NEC 517.73(2), the demand factor for x-ray equipment is based on the number of pieces of diagnostic equipment on the panel.  While most demand factors like this work with the way Revit is set up, the demand factor for X-ray equipment does not.  For X-ray equipment, the demand factor is either:

• 100% - if there's only one piece of equipment

or, if there is more than one piece of equipment:

• 50% of the first piece
• 25% of the second piece
• 10% of any remaining pieces of equipment

For example, if I have four pieces of equipment, one fed from 1HA, one from 2HA, and two from 3HA, with all of these panels fed from MSA, demand factors at the panel would need to be calculated as follows:

• 1HA - 100% of the equipment rating
• 2HA - 100% of the equipment rating
• 3HA - 50% of the largest equipment, 25% of the next largest equipment rating
• MSA - 50% of the largest equipment (among all 3 panels), 25% of the next largest equipment (among all three panels), 10% of the remaining equipment ratings

With the way Revit supports demand factors, we either have to apply a demand factor of 100% for the first piece of equipment all the way to the top of our system (i.e. MSA), or we don't comply with the NEC in the event that there's only one piece of equipment on the panel.  While it is nice to use Revit to design our power distribution, there are quite a few things missing such as this which prevent us from being able to really take advantage of it.

[sasha.crotty]

Thanks for taking the time to submit your idea. Unfortunately, this idea did not get the support of the community over the last 6 months and as such we will not be pursuing it at this time. Please feel free to rework (titles and clear descriptions are really important) and resubmit this one down the road.

[Martin__Schmid]

Hi @aaron.jonesSAP83

Thanks for raising this issue.

Actually, the code says :

if there's only one piece of equipment

• 100% long time rating or 50% momentary, whichever is greater

or, if there is more than one piece of equipment:

• 50% momentary of the first piece
• 25% momentary of the second piece
• 10% momentary of any remaining pieces of equipment

So, in the cases where 50% momentary is > 100% long standing, the settings below should work, shouldn't they?

What do you typically see as values for momentary vs. long standing?  Is it going to make a significant difference in the size of a feeder?

[aaron.jonesSAP83]

@Martin__Schmid, These loads are quite often some of the largest we have.  We feed a typical X-Ray with a 100-300A circuit breaker.  However, I've done some digging and asking around in the company, and I think the original method I was asking for (what I was always taught) is either a conservative calculation method our firm uses or a misinterpretation of the code (either that or the code used different wording prior to 1999).  I think Revit works just fine the way it is now for X-Rays.