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Sparkeusa (Electrical) (OP)
29 May 10 8:37
I am an engineer in Florida.
NEC, Art. 430.26 direct you to IEEE Std. 141 and IEEE std. 241 to obtain another demand factors to affect classical load calculation.
I read these books and they say about a diversity factor as another factor to apply due to individual non-coincident maximum demand of individual equipments.
Where are listed these types of Florida acepted diversity factor for diferent industries?
Several times I have had to design service for different shops as carpentries where many equipments are installed but only a few are running and I have to install big services.
Plan reviewers say I have to install electrical interlocks amoung equipments to avoid that, but I have never see similar solution.
waross (Electrical)
29 May 10 11:33
Diversity factors are used by utilities for distribution transformer sizing and load predictions.
Demand factors are more conservative and are used by NEC for service and feeder sizing.
On a design for a mill we were faced with a choice between a customer owned transformer or a utility owned transformer.
We would get a better energy rate with a customer owned transformer but we would have to size to Canadian Electrical Code (similar to the NEC).
The utility owned transformer would be sized according to the utilitie's diversity factor.
Our demand factor was 100% and our transformer sizing was almost 1 KVA per horsepower.
The utilities diversity factor allowed them to use 0.5 KVA per horsepower.
That's one of the differences between the application of diversity factors and demand factors.
As for your issues, a lot depends on the UHJ and the type of equipment.
And "It Depends".
Given a machine where one motor loaded a batch and another processed the batch, an inspector may accept that both motors would not run simultaneously and let you size the feeder for the largest motor only without interlocks.
Another UHJ may insist on an interlock scheme.
We had an existing installation of a sewage pump with an identical back-up pump. The owner was required to add a standby generator. He owned a generator that would easily run one pump but not both pumps. The electrical UHJ would not allow an interlock scheme because of the ease of which it may be defeated or removed. The UHJ did accept a transfer switch so that only one pump could be connected to the generator at a time. The environmental UHJ required an alarm system to warn if the selected pump had failed, but accepted the use of the existing high level alarm to imply pump failure.
In a carpenter shop there is always the possibility that a very high work-load will result in more operators being hired and many more machines running simultaneously than the owner will admit to.
If you can'nt find a friendly demand factor in the NEC you can't use it. (mangled syntax but I hope it makes the point)

Bill
--------------------
"Why not the best?"
Jimmy Carter

Sparkeusa (Electrical) (OP)
31 May 10 9:45
Thank you , Bill for your answer.
Some times is easy to know where we have non-coincident loads.
I had a carpenter shop with very reduced square feet, only the owner and may be one more person could be there at the same time, but he had 13 diferent equipments to work the wood in different ways.
Neither technologically nor phisically more than three equipments will be running at the same time, but AHJ requested an electrical or mechanical interlock.
I have never seen interlocks used in that way in a carpentry shop.
Now I have to increase the electrical feeder for one bay in a warehouse. A lot of bays are empty and all of them have  200 A for each.
Utility Co. can give me the maximum demand for a year, but, it is a very old data base because they are been closed since 2008.(Existing data base from 2008 is around 76 A maximum demand for each bay).
If I will take the measure it will be a ridiculous data too due to the same reason, but if I make the calculation, existing service of 1200 A is not enough.
I have seen all kind of factories here, where a lot of motors are working, but feeders are rationally sized.
Bibliography such as Industrial Power System Handbook, by Donald Beeman (1955) talks about these factors for industrial system designs, inside factories, no for utility services.
I don't know how can they be accepted by AHJ but I know they are been accepted before.
Again, thaks a lot for your answer. I think this is a good discussion.
Manny

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