500 KVA outdoor transformer

[lilsparky]

Good morning everyone,
I am upgrading a facility that is changing from using hydraulic motors to electric motors, lighting and instrumentation. The facility is currently fed from an outside padmounted 500 kva 12.47v/480v transformer. In order to save money I have been instructed to try to keep all the loads within the 500 kva range.

Here's my dilemma:
The outdoor transformer also feeds another building (small building) to a 480v/208v 75kva transformer. My total kva load with both buildings is 442 kva. However when I add 1.25% diversity I get 553 kva which is over the 500 kva. Is it unethical to keep this transformer or should I just bite the bullet and add the next higher transformer? I didn't know whether I'm within an operating range of a 500 KVA transformer? I think I'm too far out of the range.

Thanks

 

[Shoe]

Good morning everyone,
I am upgrading a facility that is changing from using hydraulic motors to electric motors, lighting and instrumentation. The facility is currently fed from an outside padmounted 500 kva 12.47v/480v transformer. In order to save money I have been instructed to try to keep all the loads within the 500 kva range.

Here's my dilemma:
The outdoor transformer also feeds another building (small building) to a 480v/208v 75kva transformer. My total kva load with both buildings is 442 kva. However when I add 1.25% diversity I get 553 kva which is over the 500 kva. Is it unethical to keep this transformer or should I just bite the bullet and add the next higher transformer? I didn't know whether I'm within an operating range of a 500 KVA transformer? I think I'm too far out of the range.

Thanks

Is the 442KVA design load or actual load? If design load, what dictates the need for 125% diversity on the entire connected load?

 

[lilsparky]

I would say it's a combination because the existing pieces of equipment that will be staying, the actual load is known. For the new pieces of equipment it's design load. So your saying only add diversity factor to the design load?

 

[Jraef]

Hopefully when you did your load cals, you remembered that if you are REPLACING the hydraulic motors with electric, you no longer count the loading of the hydraulic power units, right?

 

[kingpb]

ok, I'll bite..... where does the 1.25 diversity factor come from?

 

[templdl]

Let's back the truck up a second.

You provided little or no information about the transformer except that it was 500kva.
So that on option can be looked atc an you provide the actual nameplate information? Is it liquid of air cooled?

It would make it easier if you took a picture of it an included it as part of your post.
You have concluded that you are limited to 500kva when there may be a chance that you have some additional headroom based upon the temperature rise and the possibility of the option of adding fans.

 

[Shoe]

I would say it's a combination because the existing pieces of equipment that will be staying, the actual load is known. For the new pieces of equipment it's design load. So your saying only add diversity factor to the design load?

No, I don't know enough information to say. Do you have a break-down of the loads both existing (to remain) and new that you can share?

 

[GeorgeB]

I am upgrading a facility that is changing from using hydraulic motors to electric motors ...

Jraef noted that the hydraulics are being replaced, thus the power units won't be needed ... I'll add that the additional stage of conversion was inefficient, likely (almost certainly) under 90% so you should be good. IF the hydraulics were variable speed and low speed, you MAY have to oversize a drive system for very low speed operation, but even then, you will almost certainly still be ahead.

 

[bob]

Here's my dilemma:
The outdoor transformer also feeds another building (small building) to a 480v/208v 75kva transformer. My total kva load with both buildings is 442 kva. However when I add 1.25% diversity I get 553 kva which is over the 500 kva. Is it unethical to keep this transformer or should I just bite the bullet and add the next higher transformer? I didn't know whether I'm within an operating range of a 500 KVA transformer? I think I'm too far out of the range.

Thanks

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) under consideration. Diversity is usually more than one.
Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.
The diversity factor is meaningless when you are trying to determine the demand on the transformer. Your transformer load is 442 kva. There is no need to increase size based on your information.

 

[lilsparky]

OK guys I have been trying to attach a picture of the nameplate for the 500 kva transformer but for some reason it's not uploading.

So Bob, I may have said diversity factor but meant demand factor. However you say I don't need to add anything? the calculated kva for the transformer is enough?

 

[bob]

How did you determine the 442 kva? Do you have actual reading from the utility?

 

[lilsparky]

No, no actual readings from utility.

I have nameplate data from existing pieces of equipment, and design data for new pieces of equipment.

 

[bob]

Generally a customer like yours will be billed for KWH's and a demand charge. Check the utility bill and and charge.see if there is a demand charge. Otherwise, you will be to determine what the demand factor for the load will be added. If you do have a demand charge you will need to determine what portion of that demand was associated with the load you are removing. I would not expect that all the added load will be running at the same time.

 

[lilsparky]

Unfornately there are no meters for this building. It's on a base in the middle of no where. Is it possible that after I added up all my KVA for all the loads (a mixture of motors, step down transformers, vfds, HVAC) that all is well? Isn't there a factor to include for future loads? Can I go through all the loads and put a factor on them based on usage, i.e. I have a 50 ton crane that will only be used when no other motors (besides hvac) will be used?

 

[kingpb]

Based on what you've told us, I would leave the 500KVA. Even if it was overloaded for short times it won't blow up or anything. It MIGHT simply shorten the life of it, but there are so many factors.

Make note of it, and do a little planning to be able to change it out later.

 

[templdl]

OK guys I have been trying to attach a picture of the nameplate for the 500 kva transformer but for some reason it's not uploading.

So Bob, I may have said diversity factor but meant demand factor. However you say I don't need to add anything? the calculated kva for the transformer is enough?

Thanks for trying. Knowing what the transformer nameplate included could determine if you really had anything to be concerned about.

 

[T.M.Haja Sahib]

I would say it's a combination because the existing pieces of equipment that will be staying, the actual load is known. For the new pieces of equipment it's design load. So your saying only add diversity factor to the design load?

The figure 442 KVA, you arrived, is without dividing by any diversity factor to the loads existing or designed. That is it is the total of installed and to be installed loads. So the actual load on the transformer even after multiplying by 125% factor would, most probably, much lower than 500KVA.

 

[lilsparky]

The figure 442 KVA, you arrived, is without dividing by any diversity factor to the loads existing or designed. That is it is the total of installed and to be installed loads. So the actual load on the transformer even after multiplying by 125% factor would, most probably, much lower than 500KVA.

I'm glad you mentioned that. I have been asked to put in diversity factors for the loads. I have identified when certain pieces of equipment works by themselves and/or with other pieces of equipment to try to come up with a diversity factor. For examplefor some reason I can't attach my spreadsheet or pictures to this forum I'll just give a brief synopis) All loads are 480V


40 Hp Air compressor w/1/20 hp dryer (one unit)
15 Hp VFD for Rotation Dollies
75 KVA Transformer
AHU #1 7.5 Hp
AHU #2 2 Hp


So when work is being done on the Rotation Dolly, the air compressor is being used, and the AHUs are on. Which means the 50T crane and trolley aren't being used, nor will the hydraulic units be used. So I'm trying to come up with some diversity factor for these scenarios. Any suggestions?

 

[lilsparky]

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) under consideration. Diversity is usually more than one.
Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.
The diversity factor is meaningless when you are trying to determine the demand on the transformer. Your transformer load is 442 kva. There is no need to increase size based on your information.

So if I wanted to know the demand on the transformer, by the definition, I find the max demand on the transformer and divide that by the total connected load. Then use that demand factor on each load or what? I believe I am on my way to finding the max demand.

 

[zog]

The cost difference between a 500kVA and say a 750kVA transformer, when you include installation cost is minimal. Upgrading in the future will be a huge cost. IMO you should always go big when there is any doubt, will be cheaper in the long run.