Available Fault Current calculation

[WarrMann]

First time poster, long time reader here. I'm a freshly licensed electrician designing and installing my first solo job. The project is to install a 400amp 208y service for a white box retail space. I need to determine my Available Fault Current so i can label the disconnect equipment as per 110.24. Being that this job is two blocks from the new falcons stadium being built in downtown Atlanta, I'm having a difficult (to use a polite term) time dealing with the poco engineer, I got the impression he had bigger things than my job to deal with from the start.

My question is: what information should I request from the poco engineer to determine the available fault current at my disconnect? Or can someone refer me to a helpful electrician or engineer who can get the calculation done for me? I'm not looking for something for nothing here, and will happily compensate a professional for their time.

Thanks for reading, and I apologize if this is a double post, or under the wrong topic.

 

[kwired]

First time poster, long time reader here. I'm a freshly licensed electrician designing and installing my first solo job. The project is to install a 400amp 208y service for a white box retail space. I need to determine my Available Fault Current so i can label the disconnect equipment as per 110.24. Being that this job is two blocks from the new falcons stadium being built in downtown Atlanta, I'm having a difficult (to use a polite term) time dealing with the poco engineer, I got the impression he had bigger things than my job to deal with from the start.

My question is: what information should I request from the poco engineer to determine the available fault current at my disconnect? Or can someone refer me to a helpful electrician or engineer who can get the calculation done for me? I'm not looking for something for nothing here, and will happily compensate a professional for their time.

Thanks for reading, and I apologize if this is a double post, or under the wrong topic.

Hopefully you can at least get the KVA and impedance of the transformer and from there do your own calculations. You also need size, length and types of conductors, but those are usually easier to get on your own - or you maybe even installed at least part of them.

Then go to Mike Holt's main site and in the free stuff area there is some helpful formulas, charts, spreadsheets, etc., one of them is a fault current calculator spreadsheet. This spreadsheet assumes infinite ability from the primary circuit of the transformer, but still gives you a worst case scenario, and even a fairly cooperative POCO won't always be able to give you much more detail for the primary side.

KVA is often visible on the exterior of the transformer, if you can't get the impedance you could always use the worst average of other units of similar size. I would suggest on what you likely have there to use 2.0% if you can't find out what it actually is and you will likely be close but possibly a little higher then actual fault current available. Round lengths down instead of up, especially if you don't know actual impedance, this will put any margin of error on the safer side.

 

[kwired]

I should also mention the spreadsheet is an excel document, if you don't have excel and don't want to purchase it, it will open with Openoffice, a free application from Openoffice.org. I think Microsoft excel viewer which is free will open it, but you can not change or enter any values making is useless for you on a spreadsheet with no field data already entered.

 

[WarrMann]

Thanks for the response. I've checked out everything Mike had put out on the subject. You've reassured me that I'm on the right track, the piece of advice about measuring the conductors helped out a lot.

As far as putting eyes on the transformer, that's impossible for me. The building is in downtown Atlanta, and all the utilities are underground, transformer is in a vault (grounding is a nightmare. I can't drive rods, so all i have is the city water pipe). My point of attachment to poco wires is inside the building. Im putting my meter and disconnect inside the building. Don't worry, i got permission from the city.

Can you confirm that the info i will need to request from the poco engineer is:

1 - transformer Kva
2 - transformer impedance
3 - approx length of conductor to my point of attachment.
4 - secondary voltage

I can size their wires on my own, and the longest run I have between my equipment is ~10' of conductor. Thanks

 

[kwired]

Thanks for the response. I've checked out everything Mike had put out on the subject. You've reassured me that I'm on the right track, the piece of advice about measuring the conductors helped out a lot.

As far as putting eyes on the transformer, that's impossible for me. The building is in downtown Atlanta, and all the utilities are underground, transformer is in a vault (grounding is a nightmare. I can't drive rods, so all i have is the city water pipe). My point of attachment to poco wires is inside the building. Im putting my meter and disconnect inside the building. Don't worry, i got permission from the city.

Can you confirm that the info i will need to request from the poco engineer is:

1 - transformer Kva
2 - transformer impedance
3 - approx length of conductor to my point of attachment.
4 - secondary voltage

I can size their wires on my own, and the longest run I have between my equipment is ~10' of conductor. Thanks

kVA and impedance is usually the main thing that is harder to obtain on your own, length you usually can get just as good if not better estimate they they may give you on your own. Voltage I hope you already know.

 

[pv_n00b]

I generally get the available fault current from the utility, although getting in touch with the person who can provide that information can be a bit challenging. It is one of the jobs of the utility to provide this information to contractors so they should not completely blow you off. Sometimes they can give you the fault current at your service panel, other times they will give it to you at the distribution transformer feeding the service. You usually need to supply the service address, account number, service panel ampacity, conductor run distance from distribution transformer to service panel, and maybe the service conductor size.

I have found that distribution transformers are rarely marked with the kVA ratings around the areas I work in. If I needed that information I would be back at the utility asking them, and if I have to do that why not just get the available fault current from them. Once you get the available fault current then you have to calculate the value at the equipment in your system. It tends to fall off pretty fast unless you are putting equipment in the same room as the service entrance.

 

[GoldDigger]

POCO may also give you worst case fault current based on their current practices and available equipment rather than just the exact service transformer that is in place at the moment.
They may upsize their equipment at any time or replace a failing transformer with whatever size is in their current inventory.
It would be a shame to have to to upgrade your service equipment or add a series rated device whenever POCO makes a change on their side.

 

[kwired]

POCO may also give you worst case fault current based on their current practices and available equipment rather than just the exact service transformer that is in place at the moment.
They may upsize their equipment at any time or replace a failing transformer with whatever size is in their current inventory.
It would be a shame to have to to upgrade your service equipment or add a series rated device whenever POCO makes a change on their side.

Which is why when I recently installed a 1200 amp 208/120 service that only was supplied with a 225 kVA transformer I still looked into what may be typical for a 500 kVA transformer when making AIC selections on breakers , in case the load someday actually grows to that level and transformer gets changed.

 

[kwired]

I generally get the available fault current from the utility, although getting in touch with the person who can provide that information can be a bit challenging. It is one of the jobs of the utility to provide this information to contractors so they should not completely blow you off. Sometimes they can give you the fault current at your service panel, other times they will give it to you at the distribution transformer feeding the service. You usually need to supply the service address, account number, service panel ampacity, conductor run distance from distribution transformer to service panel, and maybe the service conductor size.

I have found that distribution transformers are rarely marked with the kVA ratings around the areas I work in. If I needed that information I would be back at the utility asking them, and if I have to do that why not just get the available fault current from them. Once you get the available fault current then you have to calculate the value at the equipment in your system. It tends to fall off pretty fast unless you are putting equipment in the same room as the service entrance.

Depends on the utility, some have no idea how to calculate available fault current. All transformers have a nameplate and will be marked with primary and secondary volts, kVA, impedance, and other information that is not as important for fault current calculation. Such nameplate may not be readily accessible however, on padmounts it is usually inside the terminal housing, on pole mounts it is just not able to be read unless you climb the pole or use a bucket truck to get close enough to read it.

 

[Ingenieur]

Many utilities publish a data with std values to use
example http://www.madison-heights.org/depa...ment/docs/Available_Short_Circuit_Current.pdf
if they give a range they probably tell you to use the lowest puZ