Fault current L-N

[ptonsparky]

My POCO supplies me with their information for faults at the transformer. They send it as: 5100 amps L-N, 240v. In this case 120/240 single phase.

Mike Holts fine calculator does not like L-N. A spreadsheet I made 20 some years ago calculates the L-N faults as 1.5 times the L-L fault. Note 2 on page 7 of MH's Short Circuit Calculations Book indicates this is correct even though his spreadsheet never displays that answer for L-N faults.

Should I use 5100/1.5=3400 as my input for MHs calculator?

 

[Sahib]

May be true but try to obtain the percentage impedance value of the POCO transformer also for a crosscheck!

 

[mivey]

Should I use 5100/1.5=3400 as my input for MHs calculator?

If you want L-L that would be a reasonable approximation outside of modeling the transformer. I just ran a model I had handy and it was about 1.34X so keep in mind you need a safety factor to cover the approximation.

 

[mivey]

May be true but try to obtain the percentage impedance value of the POCO transformer also for a crosscheck!

That will not give you a crosscheck for L-L vs L-N.

 

[ptonsparky]

If you want L-L that would be a reasonable approximation outside of modeling the transformer. I just ran a model I had handy and it was about 1.34X so keep in mind you need a safety factor to cover the approximation.

I am taking a SEWAG that Safety Factor = CYA?

 

[mivey]

I am taking a SEWAG that Safety Factor = CYA?

That's another way to put it.

 

[sparks1]

% impedance

% impedance

My POCO supplies me with their information for faults at the transformer. They send it as: 5100 amps L-N, 240v. In this case 120/240 single phase.

Mike Holts fine calculator does not like L-N. A spreadsheet I made 20 some years ago calculates the L-N faults as 1.5 times the L-L fault. Note 2 on page 7 of MH's Short Circuit Calculations Book indicates this is correct even though his spreadsheet never displays that answer for L-N faults.

Should I use 5100/1.5=3400 as my input for MHs calculator?

According to your calcs the poco is allowing you a % Z to determine the available fault current. That = 3400amps. What % do they allow?

 

[ptonsparky]

The POCO makes the initial calculation for the fault current at the transformer terminals. They may use a standard chart that lists currents for typical installations with xxx transformer, or some other method. IDK.

Once they give me the FC, I don't need to know the impedance.

 

[sparks1]

Fc

Fc

The POCO makes the initial calculation for the fault current at the transformer terminals. They may use a standard chart that lists currents for typical installations with xxx transformer, or some other method. IDK.

Once they give me the FC, I don't need to know the impedance.

If the % Z was known?Would not need the poco to determine fault current for 120/240v single phase