I am reviewing a electrical system design for a industrial facility. The cover sheet has a load summery on it. They have applied very aggressive demand factors to each categories: 75% (lighting), 60% (HVAC equipment), 65% (Refrigeration equipment), 40% (Battery Chargers), 25% (Receptacles), 40% (Truck Shop equipment), 40% (Misc.). NEC doesn't mention any of those demand factors for industrial building.
How do you apply demand factors or diversity factors for industrial facility? Thanks.
David Lin
It seems to me that the NEC does not really have any special load calculations for industrial facilities like it does for dwelling units.Originally Posted by dahualin
I seem to recall it does require a certain allowance for lighting on a SF basis, and receptacles on some number of VA per outlet.
The numbers listed seem pretty reasonable to me though, as a realistic guess as to the actual electrical usage in the plant, if that is what they are trying to get at.
Bob
Bob is right. Table 220.12 gives the minimum allowance for lighting loads in various types of facilities. But for industrial facilities, not being given a separate demand factor in Table 220.42, we have to count 100% of that lighting load. Table 220.44 gives a demand factor of 50% for any receptacle load over 10KVA (and a 100% factor for that first 10KVA). You are right in saying that the NEC does not give demand factors for the other loads in your list. So you must use 100% of such loads.
So the answer is that the calculation is invalid. If I were the one reviewing the calculation, I would send it back as “rejected,” with the simple comment that none of the demand factors comply with NEC requirements.
And by the way, "demand factor" and "diversity factor" are vastly different concepts. The term applicable to your question is "demand factor."
Last edited by charlie b; 03-21-07 at 10:29 AM.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.
What about non-coincident loads? 220.60. I think it would be unlikely in an industrial setting, but it is possible.
What is the difference between "demand factor" and "diversity factor"? I know the NEC call "demand factor" and utility company call "diversity factor". As I understanding, they are the same idea, but the exact percentage is different. NEC is kind of conservative and utility company is very aggressive. This is my opinion. Please correct me if I am wrong. Thanks.
SIZE=3
David Lin
The quick and simple answer is that “Demand Factor” is generally less than one (it can never be higher than one), and that “Diversity Factor” is generally higher than one (it can never be lower than one). The two are not reciprocals of each other. Indeed, they have no relation, as one involves "connected load" and the other does not.
The NEC allows us to use “Demand Factors” in calculating the load on a feeder or service. It essentially acknowledges that not every load is likely to be running at the same time. The result is that we can use smaller conductors than would have been allowable, if we had to size the conductors for the total connected load.
The NEC definition of “Demand Factor” speaks of the ratio of the maximum demand of a system to the total connected load. The NEC does not define “Diversity Factor.”
The definition of “Diversity Factor,” in one of the textbooks from my MSEE program, does not mention “connected load.” It is all about the amount of load that is measured during a period of time. It speaks of a power distribution system being viewed as a collection of sub-systems, in each of which you can measure the maximum load during a period of time.
“Diversity Factor” is defined (in this book, anyway) as, “The ratio of the sum of the individual maximum demands of the various subdivisions of a system to the maximum demand of the whole system.”
Last edited by charlie b; 03-21-07 at 02:19 PM.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.
Charles,
I have just googled on line and found out the term
Diversity Factor - The ratio of the sum of the coincident maximum demands of two or more loads to their non-coincident maximum demand for the same period
Last edited by dahualin; 03-21-07 at 03:00 PM.
David Lin
As defined in IEEE 100, The IEEE Standard Dictionary of Electrical and Electronics Terms, Sixth Edition (as close to "official" as you will likely get):wink: :
diversity factor (1) The ratio of the sum of the individual maximum demands of the subdivisions of the system to the maximum demand of the complete system. Note: Since maximum demand of a system cannot be greater than the sum of the individual demands, the diversity factor will always be equal to or greater than unity.
"Bob"
Robert B. Alexander, P.E.
"I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means.” (Corollary to Charlie’s Rule)
If the code does not mention it, then how is it wrong? Where in the code does it say that if it is not covered, you cannot do it?
If I have three motors, and only two are going to run at any one time, and operationally, the third will not be started until one of the others is shutdown, I will definitely apply a .67 factor to all three or 1.0 to two and a "0" to the third. This is quite common as illustrated in the OP, especially in industrial and process environments. Our clients would think I'm insane if I provided power based on connected load, without using demand factors. Haven't had a design rejected/questioned yet.
"Just because you're paranoid, doesn't mean they're not out to get you"
The scope of Article 220 excludes all alternative methods not otherwise specifically permitted in the Article. (These are the "requirements" for the calculations) However, Sections 220.50 and 220.60 may have relevance as specific permissives to your example.220.1 Scope This article provides requirements for calculating branch-circuit, feeder, and service loads. Part I provides for general requirements for calculation methods. Part II provides calculation methods for branch circuit loads. Parts III and IV provide calculation methods for feeders and services. Part V provides calculation methods for farms.
"Bob"
Robert B. Alexander, P.E.
"I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means.” (Corollary to Charlie’s Rule)
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