Load calculations

[ogbuehi]

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.

 

[waross]

Demand and diversity are different ways express the same effect.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ogbuehi]

Thanks waross for that. Is there a reference that has the diversity factor broke down like the NEC does for demand factor?

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.

 

[dpc]

You might check the IEEE Red Book and/or Grey Book for discussion of diversity and demand. The two terms are related, but not quite identical at least in IEEE parlance.

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

 

[ogbuehi]

Thanks for the tip on the IEEE. Well most of these building are just lighting, A/C and outlets. So I'm gathering that the diversity factor is a more accurate way to determine load in respect to determining the generator size. If not, otherwise it seems it just better to just add up all the loads.

 

[jghrist]

I'd be careful using USA derived diversity factors for Iraqi army buildings. There may be no diversity if soldiers come in out of the desert to a warm building and turn the A/C on full blast. It may take quite a while for the building temperature to get down even at night when the all the lights are on. My experience in the middle east would also indicate that the A/C may be on full blast while all the windows are open as well.

 

[VTer]

please see attached article

"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

 

[EEJaime]

VTer,
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.

 

[rbulsara]

Diversity factors are based on historical experience or specific knowledge of the operations. There is no magic formula, just a definition.

 

[rbulsara]

Although NEC provides some load calculation methods for residential and some other type of loads but only applicable in the USA.

For type of loads not covered by NEC, relies on professional judgment of the design engineers.

 

[burnt2x]

To add some more on rbulsara's point/s; using a factor to computed loads is trying to "fit" the source design to the actual load usage. NEC provides calculations applicable only in the US, very true.
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!

 

[waross]

Diversity is the ratio of the maximum loads that may be energized to the maximum loads that are expected to be energized at one time.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[BigIanH]

For sizing a generator, don't risk using theory too much.
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"

 

[ogbuehi]

I think that's what we may wind up doing that. What we're going to do is force the contractor to tell us exactly how he plans to do the load calculations. It makes sense to me that they shouldn't be trying to use the diversity factor to downsize the generator but should be trying to oversize them.

 

[rob46]

Please reviewthe following document.You may find it on the web.

Electrical Engineering Preliminary Considerations
Military Handbook.

 

[marks1080]

It would seem to me that you would want to size your generator to serve the worst case scenario - which would be all equipment is ON - or max possible load. You may want to consider future expansion as well and size accordingly, although contractors probably won't like that.

 

[ogbuehi]

Man I love this forum. You guys are full of great information. That military handbook has great information. To add some more details about the project, the camp is supposed to be powered by the grid. But the grid is so unreliable they are planning to run on prime power most of the time. So we are requiring the contractor to provide enough generation to run the camp as if there was no grid to rely on. As far as expansion goes, it is unlikely the contractor will even go for that. If they do decide to expand, they will have to consider how they will plan to add that additional load. I think we will just go with max load with everything running to size the generator. I was just thinking that maybe using the demand/diversity factor would give me a more accurate determination of the load.