Aldav53,
The way it was explained to me when I was a "young feller" went something like this.
In the 30's and 40's the standard voltage for 3-phase equipment in factories, mostly motors, was 240 volts. In those days transformers were much less reliable than today. The organic insulations in use then had a shorter life, and would last somewhat longer at lower voltages.
Delta-delta connections were almost always used because if one transformer of a 3-phase bank failed, the bank could be converted to open-delta by disconnecting the defective transformer, permitting service to the customer to be maintained (with reduced load capacity) until the new transformer could be installed.
The most practical way to supply the 120 volt lighting and receptacle loads in the building was to center-tap one of the transformers.
This one normally had a higher kva rating than the other two, as it supplied all of the single phase loads, plus one third of the three phase loads.
Electricians all were familiar with the "high leg" and were careful to not connect any 120 volt loads to it. The high leg buss was always colored orange to identify it.
There were very few 208 volt loads in existance at that time.
Ed
[This message has been edited by electric-ed (edited 08-04-2002).]
