Transformer Secondary OCPD's

[fifty60]

I am going to Fuse the secondary of a 7.5KVA transformer. My primary Voltage is 460V, and my secondary voltage is 230V. There is a center tap option on this transformer that would allow me to derive 115V on the secondary. I am trying to understand the phase relationships between the transformer's secondary taps.

I have 230V between phase terminals X1 and X4. What is the phase relationship between these two points? The lower voltage 115V is derived by taking either X1 or X4 to the center tap.

I know that if I were using the 115V as the secondary voltage, I would only need to fuse 1 secondary conductor because one of the secondary phases would be referenced to ground. This is not the case with the 230V taps on the secondary right? If I am using the 230V I definitely need to fuse both secondary conductors? Thanks in advance for the help.

 

[augie47]

My take is that you need to look at 250.20(B) and 250.21(A).
If the transformer does not qualify under 250.21(A) to not have a secondary ground, then 250.20(B) would require the center-tap ground since it is available and both phases of the
240 would need to be fused.

 

[templdl]

I am going to Fuse the secondary of a 7.5KVA transformer. My primary Voltage is 460V, and my secondary voltage is 230V. There is a center tap option on this transformer that would allow me to derive 115V on the secondary. I am trying to understand the phase relationships between the transformer's secondary taps.

I have 230V between phase terminals X1 and X4. What is the phase relationship between these two points? The lower voltage 115V is derived by taking either X1 or X4 to the center tap.


I know that if I were using the 115V as the secondary voltage, I would only need to fuse 1 secondary conductor because one of the secondary phases would be referenced to ground. This is not the case with the 230V taps on the secondary right? If I am using the 230V I definitely need to fuse both secondary conductors? Thanks in advance for the help.

"Phase relationship" doesn't have anything to so with it as as I'm to understand that yo have a single phase transformer. What you should be considering is the simple ending ratio between the primary and secondary.
Again, considering that you anve a 460v PRI and a 230v secondary you have a 2:1 ratio. The secondary may consist of a single secondary winding with a center tap with connections of X1 X2 and XO (the center tap) or 2 identical secondary with connection of X1, X2, X3 and X4 where X1 and X2 is one 115v winding and X3 and X4 is the second 115v winding. By connection X2 and X3 together you place the (2) 115v windings in series providing 230v between X1 and X4. And 115 between X1 and X2/X3 and X4 and X2/X3.
The OCPD would be provided by a 2p beaker connected to X1 and X4 or by providing fused protecting at those points with X2/X3 tied together as a neutral which is then the grounding point.