DEMAND FACTORS IN HOSPITALS

[oneelectricalengineer]

Does anyone know where Stallcup came up with the demand factors for feeders shown on page 26 of the Fall 2004 NECDIGEST?

Are there other demand factors that can apply to a hospital?

 

[steve66]

Re: DEMAND FACTORS IN HOSPITALS

I don't have that book. Could you post some examples??

Steve

 

[oneelectricalengineer]

Re: DEMAND FACTORS IN HOSPITALS

IN the article it shows:

Solution:
Calcualting demad for feeder -circuits
* 250 kva x 90% = 225 kva
* 200 kva X 80% = 160 kva
* 150 kva x 75% = 112.5 kva
* 400 kva x 85% = 340 kva
_____
837.5 kva

further it is indicated a diveristy factor of 1.5 being used for the main service

837.5 / 1.5 = 558 kva for the main service feeder. thus allowing a 600 kva transformer to be used.

I have not found these factors in the NEC, where are they coming from? engineering experience? IEEE ?

 

[charlie b]

Re: DEMAND FACTORS IN HOSPITALS

Without seeing the entire article, without actually seeing their calculation in context, I cannot explain their numbers. But you are right in saying the NEC does not provide those demand factors for a hospital (nor any other demand factors, I believe).

I can say, however, that their use of the concept of ?diversity factor? is valid. To make sure, I just looked up that term in an old college textbook. But I believe (and this is why I would need to read the article) that their selection of a factor of 1.5 was merely an illustration. That is, IF you knew, from other sources, that the diversity factor of this facility was 1.5, THEN you could use that factor in the manner shown. But that does not help us in any real world situation. If you are designing a hospital (or any other facility), and are performing the initial service calculation, you are not given the privilege of randomly selecting a diversity factor, and using it to reduce the size of your service.

 

[steve66]

Re: DEMAND FACTORS IN HOSPITALS

Table 220.11 does provide demand factors (diversity for Charlie) for lighting and receptacle loads in hospitals. 40% of the total load up to 50KVA and 20% over 50KVA. But I'm not sure if that relates to the article you mentioned.

Steve

 

[rbalex]

Re: DEMAND FACTORS IN HOSPITALS

Originally posted by steve66:
Table 220.11 does provide demand factors (diversity for Charlie) for lighting and receptacle loads in hospitals. 40% of the total load up to 50KVA and 20% over 50KVA. But I'm not sure if that relates to the article you mentioned.

Steve

"Demand Factor," "Diversity Factor," and "Diversity" are not quite the same.

"Demand Factor" is a defined term in the NEC; "Diversity Factor" and "Diversity" are not - possibly because they are used with some ambiguity. (Stallcup used the IEEE definition correctly, although I don't know the source of the factor either)

From the NEC:

Demand Factor. The ratio of the maximum demand of a system, or part of a system, to the total connected load of a system or the part of the system under consideration.

From IEEE 100

Diversity Factor The ratio of the sum of the individual non-coincident maximum demands of various subdivisions of the system to the maximum demand of the complete system. The diversity factor is always 1 or greater. The (unofficial) term diversity , as distinguished from diversity factor refers to the percent of time available that a machine, piece of equipment, or facility has its maximum or nominal load or demand (i.e., a 70% diversity means that the device in question operates at its nominal or maximum load level 70% of the time that it is connected and turned on.)

 

[charlie b]

Re: DEMAND FACTORS IN HOSPITALS

Thanks for the IEEE definition, Bob. My reference book, an 18 year old text book on Power Distribution System Engineering, has a very similar definition.

I have grown a bit sensitive to the potential confusion and failure to communicate that arise when the term ?diversity factor? is improperly used in the place of ?demand factor.? For the purposes of most members of this Forum, we could easily spend a long and happy career without ever encountering, let alone using, the phase ?diversity factor.?

 

[oneelectricalengineer]

Re: DEMAND FACTORS IN HOSPITALS

WE HAVE DISCUSSED DEMAND FACTORS AND DIVERSITY FACTORS AND UNDERSTAND THEIR MEANINGS. BUT, WE HAVE NOT FIGURED OUT THE NUMBERS USED IN THE ARTICLE AND THE BACK UP TO THEM. IT IS REAL HARD TO ACCEPT AND USE THESE FACTORS WITH OUT CLEARLY UNDERSTANDING THEM INSIDE AND OUT. THE CORP OF ENGINEERS DESIGN GUIDLEINES TECNICAL MANUALS USE TO HAVE SOME TABLES ON DIVERISTY FACTORS BASED ON OCUUPANCY AND THE NUMBERS OF PANELBOARDS CONNECTED.

 

[charlie b]

Re: DEMAND FACTORS IN HOSPITALS

Thanks for the comments, oneelectricalengineer. However, let me request that you please turn off the CAPS LOCK feature. It makes your post harder to read, and is considered by some to be SHOUTING.

 

[oneelectricalengineer]

Re: DEMAND FACTORS IN HOSPITALS

sorry about the caps, i work in caps all the time in putting contract documents together.

 

[sandsnow]

Re: DEMAND FACTORS IN HOSPITALS

I thought that article was confusing too. There was only one example of diversity in use, but the stated purpose of the article was to show the difference of the two. The state inspects hospitals here, so the in that respect the article was not pertinent to me.

Charlie B
based on what you said, I don't need to try to understand it and also the article was just a bit of minutae. Did I get that right?

 

[oneelectricalengineer]

Re: DEMAND FACTORS IN HOSPITALS

The aricle conclusion reads as if designers can use the demands factors ( note diversity factor was not mentioned) to reduce component sizes in the electrical system and save costs. so for a hospital, only table 220.42 (2005) applies.

From the note at the bottom of the table it appears that operating rooms and dining rooms lighting can not be included because potrntially all the lighting in theses areas will be used at one time.

What is the length of time to be considered? 3hours as to be continuous? some dining areas may not used for three hours per meal? are lights assumed to be turned on in the morning and run all day?

 

[valassi]

Re: DEMAND FACTORS IN HOSPITALS

I am doing a Hospital electrical design and "diversity factor" is not in. (6,000 KVA) That factor is more common with power electrical companies for network design, not for one user's purposes.

 

[oneelectricalengineer]

Re: DEMAND FACTORS IN HOSPITALS

the latest NEC digest just came out. The Stallcup's reference to the Standard Handbook for Electrical Engineers from Mc graw hill happened to be in my library. The only demand factor I found in the handbook was for power distribution diversity factors for the utility companys to apply for different users from the tranformers to the generating equipment( not inside a building system that the article seemed to be indicating ).

 

[charlie b]

Re: DEMAND FACTORS IN HOSPITALS

I have been sent a copy of the article in question (with thanks to ?The Other Charlie?). I have had a chance to read through it, and am now ready to answer the original question:

Originally posted by oneelectricalengineer: Does anyone know where Stallcup came up with the demand factors for feeders shown on page 26 of the Fall 2004 NECDIGEST?

It is clear to me that Mr. Stallcup selected a set of demand factors purely for the sake of illustration. He was simply showing us how to do the math.

He was not saying that those factors (90%, 80%, 75%, and 85%) were taken from any specific NEC article. They were not. He was not saying that they can be applied to any specific situation. They can not.

Here?s how to interpret that article:
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• <font size="2" face="Verdana, Helvetica, sans-serif">If you had information related to a specific project, and</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">If you looked in the NEC and found some demand factors that applied to your project, and</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">If you had a source (I cannot even begin to guess what that source might be) that gave you a ?diversity factor? that applies to your project,</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Then you could substitute those ?actual? numbers into the ?sample? formulas given in the article, and come up with a size for the main service or for a feeder.</font>

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[jtester]

Re: DEMAND FACTORS IN HOSPITALS

I got my Winter 2004 nec digest. Inside is a letter Mike Holt sent to the editor and Stallcup offers an explanation. There Stallcup implies that engineers can develop and use their own diversity factors, with the approval of the AHJ, to determine demand load.

As an engineer, I would be very concerned about using diversity factors that reduce what the NEC specifies as appropriate for load calculations, even if the AHJ said it was OK. And I don't know of many who would let me use my own diversity factors. I hope Mr Stallcup can offer additional information.
Jim T

 

[charlie b]

Re: DEMAND FACTORS IN HOSPITALS

I agree with you here. I?ve not come across a situation in which I felt inclined to apply a diversity factor. I cannot think of a good technical basis to justify its use. I might be able to do something for an upgrade to an existing facility, if I had detailed measurements of the loads on all the feeders and on the main. But it would be a step in a non-conservative direction. I would have to work hard to sell the technical argument to myself, before I could offer it to the AHJ.