Load calculations
Load calculations
(OP)
I am currently in Iraq doing some QA/QC on some electrical work being done for the Iraqi army. While determining the total load of the camp, the contractor is utilizing what's known as a diversity factor (DF) for each building and secondary distribution panel. I did some research and kind of got an idea of what it is. However, they referenced the IEC as the source for the DF being used for each particular type of load and building. I was trying to use the NEC as a reference to get clarity on the subject but it only mentions demand factor. From what I've read about DF and demand factor, those are 2 separate types of calculations correct? My second question is would you use a DF when trying to size a generator to a particular load? I had always just used the NEC's demand factor based on the building/room use, and type of appliance being used. Any help on this topic would really be great. Thanks in advance.






RE: Load calculations
NEC uses demand. At times, the connected load may exceed the calculated demand. No problem on the grid but possibly an issue with a gen set. For gen set sizing, step back and look at the whole picture. Is it expected that the load may exceed the calculated demand? Is the gen set prime rated (10% overload allowed) or standby rated (no overload allowed)?
Unless you have a lot of experience sizing gen sets, stay safe and go 10% or 25% over sized.
Bill
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"Why not the best?"
Jimmy Carter
RE: Load calculations
The generators are supposed to be used as standby generators, but the grid is so unreliable, they are planned to be used as prime power.
I do agree that the generators should be oversized, but the contractor is trying to use the diversity factor all the way to the LTP to keep the load low enough to allow the generators to me the specifications of the contract. He's supposed to be able to provide enough generation to keep the generators at 90% capacity when the full load is applied. I know we could just let him screw himself but the contracting officer would prefer if the we just ensured the contractor got it right from the get-go instead of going through the hassle of getting more generators later after the initial can't hold the load to spec's.
RE: Load calculations
Article 220.60 in the NEC discusses "non-coincident" loads. This comes closer to the definition of diversity. So, for example, if you have two pumps but one is a backup, you only need to consider one pump running for feeder load calcs.
HOWEVER - Waross is 100% correct: sizing a generator is a little different. In the case above, it is likely that it may be desirable to be able to start the second pump while the first one is running, then shut off the first pump. When running only on local generators, this kVA demand must be met, somehow. The standard NEC load calcs methods do NOT address these types of issues and assume a standard utility source.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Load calculations
RE: Load calculations
RE: Load calculations
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Load calculations
That is informative, but gives no basis for his definition of 'Diversity Factor'. How does one come up with this majical 1.5 or whatever that may be? What is it based upon? "Use a diversity factor of 1.5"? What part of the code allows one to do this arbitrarily?
"Diversity factor is the ratio of the sum of the individual
maximum demands of the various subdivisions of a system,
or part of a system, to the maximum demand of the
whole system, or part of the system,..."
This ratio IS unity. In the example, 225+160+112.5+340=837.5,(the sum of the demands of each part of the system). 837.5 is "the maximum demand of the whole system". Therefore the Diversity by the definition above is unity. What elevates it to 1.5. I must be missing something.
RE: Load calculations
RE: Load calculations
For type of loads not covered by NEC, relies on professional judgment of the design engineers.
RE: Load calculations
If by analysis you have found out that the loads do not go "ON" at the same time, you can divide the total demand by the diversity factor that you have computed and size your supply transformer according to new demand.
Just keep in mind that we can only be as good as the data we have. Missing some important data and your installation is toast!
RE: Load calculations
I would discourage using diversity factor. When all the soldiers get up and turn on all the lights and start taking showers and all the pumps and all the hot water tanks come on together, the contractor won't get very far yelling
"The diversity factor, you're not using the diversity factor."
What is the expected load? Are you planning on more than one generator? Will you install a load control panel?
Remember that while utilities use diversity factors, when some event such as extremely hot weather drives their system load past the parameters of the diversity factor, they often purchase more power from an outside source.
That option may not be readily available when the generators are overloaded and can't support an abnormal load that exceeds the diversity predictions.
Bill
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"Why not the best?"
Jimmy Carter
RE: Load calculations
Have a max demand study of the actual load over a time period that would include the building(s) at maximum usage.
Use that data to size the generator.
"Power is nothing without control"
RE: Load calculations
RE: Load calculations
Electrical Engineering Preliminary Considerations
Military Handbook.
RE: Load calculations
RE: Load calculations