517.73(a)(2) - nec 2008

[charlie b]

Article 517, Part V, is called ?X-Ray Installations.? Paragraph 517.73(A) is titled ?Diagnostic Equipment.? The first sentence within 517.73(A)(2) gives a diversity scheme (i.e., 50% of the first, 25% of the second, 10% of the rest), but it speaks of X-ray units, not of a more generic ?diagnostic equipment.?

QUESTION: Can I apply the diversity scheme to the feeder of a panel that powers three X-ray units and two MRI units? That is, can I treat an MRI the same as an X-ray?

 

[masterinbama]

If it's a small imaging center I don't see why not. But I don't think I would in a full bore trauma center IMO they would be more apt to have all of the machines shooting pictures at the same time.

 

[charlie b]

It's one suite of a large hospital building.

My concern is that the diversity scheme I describe above also speaks of the "momentary peak" values. My thinking is that an X-ray exam is a quick thing: a second or two, perhaps, and you no longer see the "momentary peak" current. But an MRI exam lasts much longer, so it's load characteristics will not be the same as those of an X-ray. I infer that we should not be using the diversity scheme.

Has anyone encountered this issue before?

 

[Pierre C Belarge]

This is an interesting question.

X-ray equipment is one type of diagnostic equipment.


517.73 Rating of Supply Conductors and Overcurrent Protection.

(A)Diagnostic Equipment
This tittle directs one to the thought of diagnostic equipment.

(1) & (2)
1 & 2 specifically address X-ray equipment which are one portion of diagnostic equipment.


Part V. X-Ray Equipment
Here we see the entire part specifically addresses X-ray equipment.


I would say that using the specific section number addressed in not the section I would address to size feeders for multiple pieces of equipment other than X-ray equipment.
I would look to other areas of the NEC.

 

[charlie b]

Yes, Pierre, that is the question, and it is an "interesting" one. Isn't there supposed to be an ancient curse to the effect, "may you live in interesting times." This week has been "interesting" in many ways, and I can only hope it gets less interesting by the weekend.

What is at stake here is the size of the branch circuit panel and its feeder, as well as the size of the transfer switch and its feeder.

 

[M. D.]

As in ,..It's been installed already???

 

[charlie b]

I think the building is under construction. I had only had a small involvement in the design efforts. The project lead electrical engineer asked my opinion (as well as a few other opinions) about calculating the load for the panel in question.

 

[cowboyjwc]

I guess I would agree with Masterinbama, again (that's twice today :smile.

I would have to know how they plan to use the area. If there is only one tech and no way that more than one piece of equipment is going to be used at any given time, I might not have a big concern. But things can change and who's to say that they'll never hire a second tech.

As usual it may be best to err on the side of caution.