Demand factors for industrial facilities load calculation

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dahualin

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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.
 
dahualin said:
How do you apply demand factors or diversity factors for industrial facility? Thanks.
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It seems to me that the NEC does not really have any special load calculations for industrial facilities like it does for dwelling units.

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 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."
 
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charlie b SIZE=3 said:
And by the way, "demand factor" and "diversity factor" are vastly different concepts. The term applicable to your question is "demand factor."[/SIZE][/FONT]
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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.
 
dahualin said:
What is the difference between "demand factor" and "diversity factor"?
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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.”
 
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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
 
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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.
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charlie b said:
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.
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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.
 
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.
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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.
 
kingpb said:
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.
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What you are describing is certainly allowed, under 220.60. But it is not strictly accurate to use the phrase ?demand factor? in this case. You are not really starting by counting up all three motors, and then applying a .67 factor to the sum. What 220.60 says you are allowed to do is to disregard the existence of the third motor. You only have to count, in your original ?let?s count the loads? exercise, the largest loads that are going to be run at the same time.

As far at the calculation being discussed, nothing in 220 says that you start by counting up the minimum required for a safe installation, and then take away whatever you want do by applying whatever demand factors appeal to you (or to the owner?s pocketbook). That is why I said that the originator of the calculation made a technical error, and the calculation results were invalid.
 
I find the article "applying demand factor and diversity factor per the NEC" by James Stallcup on line.

http://www.nfpa.org/assets/files//PDF/necdigest/CodeIssues072704.pdf

From the first example he gave, it seems an electrical engineer only can apply demand factor to do load calculation, utility company can apply the diversity factor to size the service transformer. But it doesn't make sense to me. If we size the service cables and main service disconnet based on our load calculation as mentioned in the exampe, and utility company size their transformer with diversity factor applied, the rating of service cable and main service disconnect will be much higher than the rating of transformer. Who is going to pretect the transformer? And it is a waster of cables and super sized disconnect also.

Thanks.
 
ptonsparky said:
Does that mean that every time I add a 5 HP motor thereafter, I must recalculate and update the entire service or do a load study for 30 days? Seems pretty strict.
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A 30 day load measurement is just one way to determine if the system has the capacity to add new loads. There are other ways. I believe that my State has a provision for allowing the addition of small loads (say, perhaps, if you are adding less 5% to the existing load), without any formal calculations or measurements.
 
ptonsparky said:
"...So you must use 100% of such loads."

Does that mean that every time I add a 5 HP motor thereafter, I must recalculate and update the entire service or do a load study for 30 days? Seems pretty strict.
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I suspect the answer is no one actually does this, but they probably should.

Over time, loads tend to be added with little consideration given to whether the electrical system is being overloaded.

A place I used to work at melted a big distribution transformer because over time the loads increased dramatically and they never realized just how bad is was getting.

One would have thought that such a transformer would have had some protection against overheating and/or overloading, but it either failed to work, or someone tweaked it to keep it from tripping, and eventually the transformer failed.

Took almost a whole day to get a new one and get it installed.
 
One would have thought that such a transformer would have had some protection [/QUOTE]Thats just the issue. What it really comes down to is a little common sense. If you own it then protect it. This applies to future additions in the way that if you own the transformer then you better oversize it to allow for it and then monitor it at regular intervals and also spend the money to protect it. If someone else owns it but your livelyhood depends on it not failing, then you better keep an eye on whats going on at least.
 
dahualin said:
From the first example he gave, it seems an electrical engineer only can apply demand factor to do load calculation, utility company can apply the diversity factor to size the service transformer. But it doesn't make sense to me. If we size the service cables and main service disconnect based on our load calculation as mentioned in the example, and utility company size their transformer with diversity factor applied, the rating of service cable and main service disconnect will be much higher than the rating of transformer. Who is going to protect the transformer? And it is a waster of cables and super sized disconnect also.

Thanks.
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That's true in all(maybe not all) cases. The NEC calculations results in service conductors and mains that are much larger than the 3 hour + demand load.
The NEC is a safety manual and the results provide a permanent safe installation.
The utility is responsible for the safety of their transformer. From experience
the utility can make an accurate guess as to what the transformer size should be. If by chance they really make a big mistake on the size, they can just install a bigger one if necessary. The transformer can handle an overload without a problem. You can't do this with the service.
 
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