Installing a bigger tranformer than needed

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de2 said:
David, can you explain, why code do not require bigger conductor for bigger xfmr, I want to say because code allows conductor based on the load connected?
Can you point out the article # is possible?
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There isnt an article that says you can, but there isnt an article that says you cant ;) There is 215.(2)(B)(1) which requires feeders supplying transformers to be sized at no less the transformer's nameplate(s), and I could have sworn there was something about transformers over 1000 volts required to be supplied with an ampacity of at least their nameplate but I cant find it.
 
jim dungar said:
However, I would not be surprise to find there is a point where 75kVA has fewer losses than the 45kVA, even though its % efficiency may be lower.
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I just looked up some loading values, from a post 2016 transformer design. These units likely have lower core losses than previous designs.
For a 45kVA 480:208Y/120 the core losses are about 180W and at 100% load the conductor losses are about 1770W, so for a 45kVA load its losses are roughly 1,950W.
For an equivalent 75kVA the core losses are about 253W and at 100% load the conductor losses are about 2518W, so for a 45kVA (60%) load its losses are roughly 1,160W.

All things equal, for a continuous load (which is rare), it may make sense to purchase an oversized transformer just looking at operating expense.
 
jim dungar said:
I just looked up some loading values, from a post 2016 transformer design. These units likely have lower core losses than previous designs.
For a 45kVA 480:208Y/120 the core losses are about 180W and at 100% load the conductor losses are about 1770W, so for a 45kVA load its losses are roughly 1,950W.
For an equivalent 75kVA the core losses are about 253W and at 100% load the conductor losses are about 2518W, so for a 45kVA (60%) load its losses are roughly 1,160W.

All things equal, for a continuous load (which is rare), it may make sense to purchase an oversized transformer just looking at operating expense.
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DOE 2016 requirements are in effect.

the difference in core losses alone are 73 watts. that's 1752 wh per day, 63.94 kwh per year. around here expect to pay about $0.14 per kwh. result for one year.... $89.52 additional cost for the 75kva.

however it is as you say, there are two components to the heat losses. core losses (above) and copper losses. 45kva will dissipate more (copper loss) heat in a 45 kva transformer than 45 kva through a 75 kva. this is because the 45kva will get hotter-as it does the equivalent impedance rises.

the resulting additional energy cost of the 45 kva operated at 45 kva and assuming $0.14 per kwh, is $748.10 above the cost of 75 kva operated at 45. that additional cost to operate the 45 will slide toward the $89.52 as the continuous load decreases.

if im the owner, and there is space, i want the 75 kva.

energy costs vary widely by state. hawaii average cost per kwh is $0.34. an owner there might be more sensitive to the subject.
 
drktmplr12 said:
DOE 2016 requirements are in effect.

the difference in core losses alone are 73 watts. that's 1752 wh per day, 63.94 kwh per year. around here expect to pay about $0.14 per kwh. result for one year.... $89.52 additional cost for the 75kva.

however it is as you say, there are two components to the heat losses. core losses (above) and copper losses. 45kva will dissipate more (copper loss) heat in a 45 kva transformer than 45 kva through a 75 kva. this is because the 45kva will get hotter-as it does the equivalent impedance rises.

the resulting additional energy cost of the 45 kva operated at 45 kva and assuming $0.14 per kwh, is $748.10 above the cost of 75 kva operated at 45. that additional cost to operate the 45 will slide toward the $89.52 as the continuous load decreases.

if im the owner, and there is space, i want the 75 kva.

energy costs vary widely by state. hawaii average cost per kwh is $0.34. an owner there might be more sensitive to the subject.
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when I was posting this I only thought about NEC code sections regarding OCPD and conductor sizing. Losses never came to my mind! I am glad all of you putting your input to this subject. I convince my client not to buy 75kva based on what I read here. thanks for all.
 
de2 said:
when I was posting this I only thought about NEC code sections regarding OCPD and conductor sizing. Losses never came to my mind! I am glad all of you putting your input to this subject.
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That's why simple questions never receive simple answers here.
 
LarryFine said:
That's why simple questions never receive simple answers here.
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How true. But in fairness, and as Mr Dungar points out, transformer losses can be complex depending on many factors. I seem to recall a Square D white paper on this subject. Have to see if I can locate it.
 
texie said:
How true. But in fairness, and as Mr Dungar points out, transformer losses can be complex depending on many factors. I seem to recall a Square D white paper on this subject. Have to see if I can locate it.
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Eaton has a paper on DOE 2016 implications.

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=3558670

first paragraph, fifth page
...it is possible to deduce that because theload losses are factorized by the per unit load, the most importantlosses on the efficiency equation are the no-load losses or corelosses. For this reason, the main focus of the manufacturers isto improve the core performance in order to meet the requiredefficiency levels.
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jim dungar said:
I just looked up some loading values, from a post 2016 transformer design. These units likely have lower core losses than previous designs.
For a 45kVA 480:208Y/120 the core losses are about 180W and at 100% load the conductor losses are about 1770W, so for a 45kVA load its losses are roughly 1,950W.
For an equivalent 75kVA the core losses are about 253W and at 100% load the conductor losses are about 2518W, so for a 45kVA (60%) load its losses are roughly 1,160W.

All things equal, for a continuous load (which is rare), it may make sense to purchase an oversized transformer just looking at operating expense.
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The extra purchase cost of 75KVA transformer and the interest to be paid for that amount loaned from a bank may also be taken into account. And I believe that 75KVA transformer is still more economical to use.
 
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Sahib said:
For a transformer switched on continuously such as a utility distribution transformer, core loss is an important loss and should be minimized by design.
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Arguably core loss is only one term of the total loss, and it is the total loss which should be minimized.

A transformer which is switched on all the time, but which never has any connected load, should be disconnected :)

As has been pointed out above, a transformer may have both larger core loss and lower conduction losses, so the total loss over the course of time will depend on the actual connected load...and the transformer with the greater core loss might be the winner in terms of total loss.

-Jon
 
winnie said:
the total loss over the course of time will depend on the actual connected load...and the transformer with the greater core loss might be the winner in terms of total loss.

-Jon
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With same load cycle, 75KVA transformer may have lower total loss than 45KVA transformer ie its all day efficiency may be higher.
 
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