Transformer Available fault Current

[topgone]

It's now my trashed chart but as I recall it only showed L-L

Post 9 & 10 sure confused my issue.

I was happy with Jim's 5820.. now I,m back to 11,000 ????

Not sure if I need to clarify... it's a 12kv pad with a 480Y/277 secondary.

Julius or Topgone, think one of you guys can talk down to my level ???

I am sorry. Been out lately.

Looking at your numbers, it's easy to see that what you got is a transformer secondary that can be wired Y (480V) or YY(277V). Your secondary fault current will now be twice your calculated bolted fault current at 277V than if you wired it at 480V. Either ways, you have the right formula; just make sure you wired the secondaries the way you based your calcs on, i.e. the bolted fault current will be 5820 X 1.1 (6402A) @ 480V or 10,085 X 1.1 (11,093A) @ 277V.

 

[kwired]

I am sorry. Been out lately.

Looking at your numbers, it's easy to see that what you got is a transformer secondary that can be wired Y (480V) or YY(277V). Your secondary fault current will now be twice your calculated bolted fault current at 277V than if you wired it at 480V. Either ways, you have the right formula; just make sure you wired the secondaries the way you based your calcs on, i.e. the bolted fault current will be 5820 X 1.1 (6402A) @ 480V or 10,085 X 1.1 (11,093A) @ 277V.

Can you give us a diagram of this "YY(277V)" connection and a good application of what it may be used for? Maybe I just don't understand what you are getting at? If you have 480 volts from a wye source you still have 277 to the neutral, even if you don't use the neutral as a circuit conductor or even if you don't ground the neutral.

 

[mivey]

I am sorry. Been out lately.

Looking at your numbers, it's easy to see that what you got is a transformer secondary that can be wired Y (480V) or YY(277V). Your secondary fault current will now be twice your calculated bolted fault current at 277V than if you wired it at 480V. Either ways, you have the right formula; just make sure you wired the secondaries the way you based your calcs on, i.e. the bolted fault current will be 5820 X 1.1 (6402A) @ 480V or 10,085 X 1.1 (11,093A) @ 277V.

I think you are mistaken. Single-phase transformers can have 120/240V windings paralleled or 240/480V windings paralled. Not so with 277V transformers. Besides, the transformer is a 3-phase padmount.

Can you give us a diagram of this "YY(277V)" connection and a good application of what it may be used for? Maybe I just don't understand what you are getting at? If you have 480 volts from a wye source you still have 277 to the neutral, even if you don't use the neutral as a circuit conductor or even if you don't ground the neutral.

I think he is talking about paralleling the secondary windings. AFAIK, that is only an option on 120/240 or 240/480 LV secondaries.


PS: FWIW, if you look at the L-N fault for the split-winding transformers, then you have to consider the 1/2 winding impedance and it is not just a straight ratio. Even so, the L-N fault at the terminals is slightly higher than the L-L fault.

I made a quick calc on a 120/240 transformer and the L-N fault current was about 110% of the L-L fault current. However, this quickly changes by the time you get to the main panel and the L-L dominates.