You may NOT use only primary side protection for a 3-phase set-up.
Primary side only is permitted only by the exceptions which focus on buck-boost and single phase set-ups.
Further study will reveal that UNDER CERTAIN CONDITIONS the collapsing field of the secondary can generate staggering voltages which the OCPD is there to drain. Otherwise you get the exact same type of physics that causes your spark-plug to fire off at 35,000 Volts even though the automobile system voltage is only 12 to 14 VDC.
IF THE WIRE SIZE meets the tap rules you might be looking at a transformer set up for isolated power for Point Of Sale or the back-office. In such situations it may occur to the EC to use TWIN single phase panels working off of alternate windings of a WYE secondary to produce plenty of cheap 1-phase slots in an otherwise 3-phase installation.
FYI, when you thumb through Ugly's -- or any manufacturer's XFMR line you will note that a 40kVA or 50kVA is NOT ON OFFER.
The industry standard sizes are listed in Ugly's:
3
6
9 Code provision cut-offs at this size...
15
30
45 sized for 100A/125A situations ... special Code provision permits this popular size to hang above grid ceilings...
75 suitable for 200A/225 situations ... the largest commonly occurring for just that reason -- sometimes feeds mother-daughter panels when the isolated circuits are many but draw low power... this size is also a favorite with Poco's all over for small establishments...
100
150 More Code provision cut-offs inre wire-type/insulation
225
300
500
750 The largest my Poco will permit in a vault -- their vault that is... enough to supply a substantial hotel, etc... any loads bigger than this must use twined XFMRs.
So as to why 45kVA = 100A or 125A set-ups ... talk to the NEMA panel.
Last edited by Tesla; 02/19/10 11:17 PM.
