3 PH Transformer Loading

[ElectDesigner]

I was told recently by a manufacturer rep. that most 3ph transformers (excluding power transformers) are only really loaded to 35%? I found this to be a very interesting statistic, he also told me that this is kind of the reason why most manufacturers are trying to maximise transformer efficiency at 35%.

I understand that most of the time the load calc is done based on connected load-and obviously the demand factor is applied, but this statistic seem to suggest that most transformers are oversized?

Another interesting thing I was told was that to obtain UL listing for his manufacturer (they are just starting out in the industry), one of the tests they did was run 200% load for 30mins and if it doesnt blow up - its all good.

 

[templdl]

I was told recently by a manufacturer rep. that most 3ph transformers (excluding power transformers) are only really loaded to 35%? I found this to be a very interesting statistic, he also told me that this is kind of the reason why most manufacturers are trying to maximise transformer efficiency at 35%.

I understand that most of the time the load calc is done based on connected load-and obviously the demand factor is applied, but this statistic seem to suggest that most transformers are oversized?

Another interesting thing I was told was that to obtain UL listing for his manufacturer (they are just starting out in the industry), one of the tests they did was run 200% load for 30mins and if it doesnt blow up - its all good.

We had a very lengthy discussion about this on one of the forums a month or two ago. What you were being told is not true. A transformer's life expectancy may be based upon an expected average loading of 35% but certainly not it's efficiency.
Do a search on this subject which should lead you to a very long string of discussions.

 

[mike_kilroy]

run, find another transformer rep....

run, find another transformer rep....

Not true statements and very misleading. If I were you I would find another transformer rep to work with who has more of a clue.

(I have sold/speced/designed transformers for over 27 years so this is not an off the wall baseless comment)

 

[ElectDesigner]

Not true statements and very misleading. If I were you I would find another transformer rep to work with who has more of a clue.

(I have sold/speced/designed transformers for over 27 years so this is not an off the wall baseless comment)

I guess he was just sharing what he read somewhere - he was referring to general purpose dry type transformers found in commercial buildings. Obviously no ones going to design to below 50% of demand load.

 

[jim dungar]

All general purpose transformers are now built to TP-1 standards (per US Energy Act). Effectively, the TP-1 standards require transformer to be designed to be 'most efficient' at 35% loading.

 

[broadgage]

Desiging transformers for maximum efficiency at 35% load seems reasonable for general use.

The load on most installations is very variable, and often far below the maximum, therefore the transformer will spend most of it working life only partialy loaded.
The transformer needs to be sized for the maximum sustained load, and perhaps for a bit more in case the load grows.

Consider an office building with the following load

When empty at night or at weekends--------------------20KVA
On a typical working day--------------------------------100KVA
On a working day in the hottest month of the year------130KVA

That suggests perhaps a 150KVA transformer in order to allow for full load, plus about 15% for changing needs, yet the average load is probably about 50KVA

If it is known that the load will be constant for most of the year, then a special transformer designed for greatest efficiency at full load might be worth considering.