Overall Load Analysis 1

[starkda]

I am responsible for a Load Analysis Study for Construction Power. I was given the Single Line drawing of the overall construction power and the Size of the Main Construction Transformer.

The Mian construction Transformwer is 18MVA and when I do the calcuations respective 480V Distribution Panels (kVA, FLA) I get a total kVA Value (~33MVA) almost 2 times greater than what the Main Construction Transformer supplies. This is all three phase 480V Power and I am not sure at to what I am doing wrong???

Can anyone possibly shed some light as to what I am doing wrong for I am using the MCB size (ex 1200A) of 480V Panel that is fed by a 750kVA Transfomer. I was first calculating the FLA being that we know the overall kVA Rating and the design is NOT to exceed 80% FLA, thus using that value I would then calculate the overall panel kVA Value, then add each of the indiviudal distribution panel kVA Values together to get a total and that is the total value that is exceeding the Main Tranfomer sixe by ~ double.

What am I doing wrong, can anyone help this newbie out?

 

[Skogsgurra]

Did you assume that all branches are fully loaded all the time? That is seldom the case. I cannot for my life remember the English word for the factor used to calculate the reduced total power. It is 'sammanlagring' in Swedish. If that helps.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[RAMConsult]

The word is diversity. OP, you're assuming that every circuit is loaded to the rating of each breaker. It's time to speak to someone with experience doing this type of calculation.

 

[waross]

Lighting will probably be at 100%. Motors at the most that may be running at one time plus 25% of the largest. Other equipment at a reasonable diversity or demand factor.
Use this for each panel. Then add you totals for the main supply. Lighting, motors that will run simultaneously and other continuous loads at 100% and a further diversity may be added to other loads.
If you are so close to the limit of the transformer as to be nervous, you may designate some special equipment such as elevators as "critical" and justify adding a second dedicated transformer to reduce the load on the main transformer.

Bill
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"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

What type of construction site is this? Is it a refinery or a power plant with lots of welders, construction trailers, heat treating, electric powered tower cranes, sump pumps, plus all of the site lighting and construction tools? Do you know what the loads are, or do you just have a one line with some distribution transformers and power panels? Is this in a location where the construction crews' living quarters need power? Are you runnign a lot of air conditioners or electric heat? (I'm working at a site where our construction camp and trailers draw 3 MW.) As mentioned above, lighting will be continuous, but welders especially are very intermittent, as are mmost construction loads.

Are there any large motors or process loads that need to be started and run using the construction power source temporarily until the permanent power source is ready? That can affect sizing of the main transformer to handle voltage drop.

Not all of the loads will be operating at full power or even be on at the same time. Diversity factors for construction loads can vary from 0.2 to 0.8. (A 100 A panel will have either 20 amps or 80 amps of load). You have to know what the load types are to make an educated guess. I add the loads and size for 75% of my guess. Usually the transformers end up being loaded at 75% which means our total diversity factor is about 50% of connected load.

By adding up all the transformer ratings at nameplate value, you are assuming 100% loading which is possible but not likely. An overall load factor of 50-75% would be more in line with my experience. But there have been some sites where we had to bring in diesels to augment the construction power, so not all is perfect.

Another thing to check is if the panels and transformers are new, bought for this purpose, or just happened to be in the storage yard with the temporary construction panels from the last project. The transformer size may not be related to the construction load; it was just what was already available.

Try to talk to a construction electrical superintendant. They are usually responsible for temporary construction power and could give some good information on typical loads.

 

[starkda]

All -

Yall have been more than helpful and yes in my calculations I made the assumption that everything is loaded ~100%. From reading the responses I think all would fall coorect w/in the 50-75% range. Thus the Main Construction Transformer would be within it's maximum MVA Rating.

Thanks for the help gentlemen...