240volt to 240 volt transformer over current protection

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BSick

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Hi All
I am trying to properly size a 120/240 secondary transformer fuses... The current fuses are un readable due to age... I have a 240 open delta system with center tap grounded at the pole, A 3 kva 240 volt primary to 240 volt with neutral secondary transformer was installed to keep the single phase panel volts from floating all over the place... Particularly line to neutral
Obviously this is an old (1949) system. I want to make sure the fuses are properly sized for safety sake and piece of mind. As we all know, if a 15 amp fuse blows...obviously we need a 30... :) so who knows what some good intentioned person put in circuit...

Question 1. Does it matter what 2 phases the single phase transformer is connected to? (it has a wild leg, which is not meaningful line to line... But will the center tap to ground effect anything?)

Question 2, Fuse sizing... it is a 3 kva single phase transformer... I'm thinking 30 amps per phase on the primary and 15 amps per phase on the secondary.. am I correct?
Thanks for any input
 
Q1: Does not matter.

Q2:
3kva/240V = 12.5 FLA.

In general:

The primary fuses can be upto 250% of the FLA. So a 30 Amp fuse makes sense.
The secondary fuses can be upto 125%, subject to the rule allowing next size up. So could be upto 20 Amp fuses.
 
Thanks for the reply... I will throw out all my old ones and replace with the appropriate fuses...
 
Q0: If the L-N voltages on the single phase panel are floating all over the place, then something is broken on the high leg open delta system.

Q1: If you have an open delta system, you should probably place any single phase loads across the 'base' phases (the ones that are 120V L-N).

The single phase transformer will work just fine across any of the phases, and the secondary neutral grounding won't introduce any problems, but the L-L voltage regulation of the source is poorer across the open jaw, and generally the 'stinger' transformer of an open delta is lower KVA than the 'base' transformer.

-Jon
 
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