1) Since the load is fixed at the installation time, isn't comparing two different transformers kind of the whole point?
2) Not that one transformer is the issue, but how about the 50 kVA one I mentioned? It has a 1.2% drop from 25% to 100% loading.
1) The OP had a specific question: Should I load my transformer to 100 or less? Nothing to do with multiple transformers or buying a second transformer. The thread did go on to encompass specs and whether to oversize or not, and how much to oversize a new transformer, but in each case my understanding was it was clear ONE transformer was being discussed.
2) Again it was my understanding this was about a large 3 phase xfmr, not a single phase aluminum one like you pointed to. If you want to find a really bad apple, pick the 167kva in the same first chart you looked at: 68% eff @ 25% & 98% at 100%.... There will always be wierd ones, and single phase xfmrs will often exhibit worse efficiency numbers from no load to full load. If you go down the charts and pick 3ph alum you will see more typical is:
50kva 914D 531 270 1316 eff: 97.7 98.1 97.9 97.5
Anyway, I think what this thread has shown, at least to me, contrary to statements from TM, is:
1) Oversizing a transformer will generally cause more watts loss for a given load (since the larger transformer will have higher core losses) so will inherently be less efficient,
2) The small (ok call it upto 1.5%) typical efficiency change from say 1/2 load to full load on a transformer is basically insignificant to use as a design point,
3) Transformers can be designed for extra high efficiency and also for reasonable ratio of core vs i2r losses
4) Per the Eaton efficiency chart sited, trading core losses for i2r losses has little bearing on efficiency at different loads as would appear to 'make sense' - there are just too many intertwined design relationships to it.
5) When designing a new system requiring a transformer, it makes sense to provide more capacity for future growth and the amount of extra is a personal engineering decision at this point and not a required amount by code.