Fault Current Calculator Help Please

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Jason F

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Lakeland FL
I am using the Fault Current Calculator available from Mikeholt.com and my system voltage is 120/240 delta three phase which i do not see a drop down menu for. Can somebody help me with this please.. Thank You!!! This is my first post so forgive me if i did something wrong.
 
75 KVA , 1.5% impedence, 20,833 short circuit amps at XFMR. service (800 amp disconnect) is fed with parallel 500 mcm copper in PVC 35' away. I am looking for short circuit current downstream at a 100 amp 1 phase 240v disconnect and panelboard. i can provide all the legnth and wires sizes i just thought the calculator would be easy.
 
Jason F said:
I am using the Fault Current Calculator available from Mikeholt.com and my system voltage is 120/240 delta three phase which i do not see a drop down menu for. ...
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Jason F said:
75 KVA , 1.5% impedence, 20,833 short circuit amps at XFMR. service (800 amp disconnect) is fed with parallel 500 mcm copper in PVC 35' away. I am looking for short circuit current downstream at a 100 amp 1 phase 240v disconnect and panelboard. i can provide all the legnth and wires sizes i just thought the calculator would be easy.
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JF -
Is this something you will use to spec out equipment, or to check if existing equipment meets requirements?

ice
 
1.5% impedance? You sure about that? Sounds low. Most standard dry types are around 5.75%.
 
Pharon said:
1.5% impedance? You sure about that? Sounds low. Most standard dry types are around 5.75%.
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Who says it is a dry type. I think it is probably a utility co transformer as he says it is supplying the service.



That said there is a voltage selection in the spreadsheet for 220/127. That would be a wye system to come up with that voltage, but it may get you close enough for what you want to do.

Should you have an open delta system with a large 120/240 transformer and a smaller high leg transformer, you may just want to base things off the large single phase transformer as being the worst case scenario.

If any calculations put you on the border of selecting higher AIC rated equipment you may either want more help with calculations at that point or assume the higher to CYA.
 
ice - i am checking existing equipment per local AHJ

Pharon - it is a utility transformer and i they provided me the specs

kwired - you are correct it is a 120/240v open delta system with a high leg. that is very common with our local utilty company (i think because its less expensive for them to install) i will run the numbers as you suggested and see what that looks like. i guess i could get a pencil and paper out and figure it that way, its been quite a while so i will need to look over formulas and verify my calcs are done correctly.
 
Jason F said:
ice - i am checking existing equipment per local AHJ

I guess i could get a pencil and paper out and figure it that way, its been quite a while so i will need to look over formulas and verify my calcs are done correctly.
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I would recomend doing that. Bussmann has a paper that details a point-to-point SCC calc. That is a good one to follow.

The crew here is knowledgable, but I would be uncomfortable citing "DIY Engineering by MH Forum" to an AHJ.

Perhaps you could make a stab at it, post your calcs and ask for a critique. Should be a good learning experience.

ice
 
ice - Thanks, I will take a look at the bussman P2P paper. They also have a "FC2" fault current calculator that may be helpful. I have done manual calcs in the past but it can be rather time consuming to relearn because I dont do them very often. Either way i will post my calcs for critique.
 
Jason F said:
ice - i am checking existing equipment per local AHJ

Pharon - it is a utility transformer and i they provided me the specs

kwired - you are correct it is a 120/240v open delta system with a high leg. that is very common with our local utilty company (i think because its less expensive for them to install) i will run the numbers as you suggested and see what that looks like. i guess i could get a pencil and paper out and figure it that way, its been quite a while so i will need to look over formulas and verify my calcs are done correctly.
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I'm guessing your single phase values for the 120/240 portion of the supply will be more then you would ever get from the high leg transformer, especially if it is smaller. You may get a more out of a phase to phase fault involving one phase from the large unit and the high leg. High leg to neutral, should have more impedance as there is more secondary windings in that path
 
I spoke with a local Electrical Engineer we are working with on a project and he told me when calculating a 120/240 open delta system you figure the AIC based the single phase transformer method because the phases are not a true 120 degrees out of phase like a 3 transformer 3 phase system. In a open delta you only use 2 transformers (in my case it is a 75kw and 25kw) and the thrid leg is derived between the two which i was aware of but I did not know to use the single phase process. The local power company provided me with the availble fault current for a open delta system using the 75/25 transformer combination.
 
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