Fault current calculation

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Peter Furrow

Peter Furrow

We’re not born humble, we’re born to be humbled
Location
Cape canaveral Fl
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Electrical contractor
Submitted a permit for a 200 amp meter main combination panel replacement today in the city of Titusville Florida. This is the only city that has ever requested a fault current calculation for a service upgrade. If I don’t provide the fault current calculation then I’m told I have to use 22KA rated breakers at the service equipment and indoor panel. This is a small residential neighborhood and the house is a 1700 square-foot home...
highly unlikely the transformer from utility is more than 10 KA.
This request for a fault current calculation would seem reasonable on a commercial building . The building official is requesting that I get A letter from utility identifying the transformer Ampere rating. This seems to be an unreasonable request for a residential application. Correct me if I’m wrong.


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Technically on a dwelling it doesn’t have to be posted, but it is still your responsibility to know the fault current value so that you can properly size your equipment.
 
Peter Furrow said:
Submitted a permit for a 200 amp meter main combination panel replacement today in the city of Titusville Florida. This is the only city that has ever requested a fault current calculation for a service upgrade. If I don’t provide the fault current calculation then I’m told I have to use 22KA rated breakers at the service equipment and indoor panel. This is a small residential neighborhood and the house is a 1700 square-foot home...
highly unlikely the transformer from utility is more than 10 KA.
This request for a fault current calculation would seem reasonable on a commercial building . The building official is requesting that I get A letter from utility identifying the transformer Ampere rating. This seems to be an unreasonable request for a residential application. Correct me if I’m wrong.


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Most panelboards will have a 22k minimum main that series rates with 10k branches so its sorta a moot point. Some of the very cheapest Siemens ES series panelboards and I think a few of the cheapest Eaton BR panelboards might only have 10K mains. I believe all SQ D, all CH, and all siemens Pl have (at least) 22k mains.
 
I see four choices.

1. Ask the utility for the fault current.
2. Go over the inspectors head to determine if he's off base.
3. Talk to the inspector to see what his reasoning is for the request.
4. Install a 22k rated loadcenter.

I'd probably start with 3, and then use that to determine if I go to 2 or 1 next based on the conversation I had.
 
Cow said:
I see four choices.

1. Ask the utility for the fault current.
2. Go over the inspectors head to determine if he's off base.
3. Talk to the inspector to see what his reasoning is for the request.
4. Install a 22k rated loadcenter.

I'd probably start with 3, and then use that to determine if I go to 2 or 1 next based on the conversation I had.
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I would go with 4 because the stuff I install is always 22k anyway. I should mention though, regarding my previous post, I was talking about 150-225 amp equipment. If you get into 100 or 125 A you are probably quite a bit more likely to run into 10k stuff because the cheaper stuff will use a regular 2 pole 10k MCCB as a backfed main.
 
electrofelon said:
I would go with 4 because the stuff I install is always 22k anyway. I should mention though, regarding my previous post, I was talking about 150-225 amp equipment. If you get into 100 or 125 A you are probably quite a bit more likely to run into 10k stuff because the cheaper stuff will use a regular 2 pole 10k MCCB as a backfed main.
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We use Siemens gear mostly. I always assumed it was all 10k for the resi loadcenters. It's got me curious, so I'll have to look when I think about it.

If they required fault currents for resi services over here, I'd probably already know. I think his inspector is getting into the weeds with this one.
 
Cow said:
We use Siemens gear mostly. I always assumed it was all 10k for the resi loadcenters. It's got me curious, so I'll have to look when I think about it.

If they required fault currents for resi services over here, I'd probably already know. I think his inspector is getting into the weeds with this one.
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The city of Titusville of Florida has had this request for a few years now. I’m baffled. I think they are getting into the weeds as you say. I really would like to know what initiated this. Something must’ve happened or prompted them to implement this into their permit application for panel changes & service upgrades. I will have answers this week because I am speaking with utility project manager and I will be speaking with the building official as well..
I sincerely want to know their line of reasoning.
I will install GE meter main combination with a 22K a main breaker. But all my OCPD for each circuit will be 10 K. See what happens


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Peter Furrow said:
The city of Titusville of Florida has had this request for a few years now. I’m baffled. I think they are getting into the weeds as you say. I really would like to know what initiated this. Something must’ve happened or prompted them to implement this into their permit application for panel changes & service upgrades. I will have answers this week because I am speaking with utility project manager and I will be speaking with the building official as well..
I sincerely want to know their line of reasoning.
I will install GE meter main combination with a 22K a main breaker. But all my OCPD for each circuit will be 10 K. See what happens


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Our POCO here intentionaly installs transformers for resi overhead that limit the fault current to 5K, but they tell you the fault current is 10K
 
I have seen clusters of duplexes served with 75 and 100 kVA transformers. Not what I would do but stranger things have happened.

22 kA on residential is definitely a stretch in most places. Who normally puts over 50 kVA transformers on residential anyway and why would you do that?

50000 ÷ 240 ÷ 1.8% = 11.6 kA and would most likely be less than 10 kA at the main.

Sounds like a strange requirement but who knows the story behind it. Could be legit but probably more like an uninformed decision or over-reaction.
 
mivey said:
I have seen clusters of duplexes served with 75 and 100 kVA transformers. Not what I would do but stranger things have happened.

22 kA on residential is definitely a stretch in most places. Who normally puts over 50 kVA transformers on residential anyway and why would you do that?

50000 ÷ 240 ÷ 1.8% = 11.6 kA and would most likely be less than 10 kA at the main.

Sounds like a strange requirement but who knows the story behind it. Could be legit but probably more like an uninformed decision or over-reaction.
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I Agree with a 50KVA tranny it is not likely an issue, however get into a 100KVA and it is. That isnt that uncommon around here where the POCO has a large tranny serving a secondary that spans multiple poles and many houses, or is in a mixed use area where its serving some businesses too.

I should share a piece on the 10k vs 22k equipment thing. Just recently I needed a 200A 3R MB loadcenter. Due to equipment shortages, I couldnt get my usual square D preference, but I ended up finding a junk BR - hate BR - but I grabbed it - yup 10k main :mad:
 
I submitted a permit for a 200 amp meter main combo, new feeders, & 200 amp indoor panel (residential). The city of Titusville requires a fault current calculation in their permit application to determine if the O.C.P.D need to be rated 22KA as opposed to the common 10 KA rating.
I contacted utility regarding the specific transformer on that property and was told in the letter “The maximum Available Fault Current at the transformer secondary terminals is estimated to be 21,110 symmetrical amperes at 120/240 volts ”.

50 kVA transformer with 21,110 Fault current rating.
Overhead utility 2/0 AL service drop from transformer 60 feet to the weatherhead.
200 amp meter main combination panel, back to back, with the indoor 200 amp panel.
(4/0AL back to back)
Without the computer program or spreadsheet how would an EC perform a fault current calculation in this situation?
What is the best way to determine the approximate fault current values to submit to the building department?
 
Peter Furrow said:
I submitted a permit for a 200 amp meter main combo, new feeders, & 200 amp indoor panel (residential). The city of Titusville requires a fault current calculation in their permit application to determine if the O.C.P.D need to be rated 22KA as opposed to the common 10 KA rating.
I contacted utility regarding the specific transformer on that property and was told in the letter “The maximum Available Fault Current at the transformer secondary terminals is estimated to be 21,110 symmetrical amperes at 120/240 volts ”.

50 kVA transformer with 21,110 Fault current rating.
Overhead utility 2/0 AL service drop from transformer 60 feet to the weatherhead.
200 amp meter main combination panel, back to back, with the indoor 200 amp panel.
(4/0AL back to back)
Without the computer program or spreadsheet how would an EC perform a fault current calculation in this situation?
What is the best way to determine the approximate fault current values to submit to the building department?
Click to expand...

Why don't you just use main rated 22k? Most are, I would say the part I quoted in red is not accurate. Only the very cheapest of a couple brands will have a 10k MB in 200 A equipment.
 
electrofelon said:
Why don't you just use main rated 22k? Most are, I would say the part I quoted in red is not accurate. Only the very cheapest of a couple brands will have a 10k MB in 200 A equipment.
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Yes. The GE meter / main & the indoor panel have 22ka rated main breakers each.
However , the brach circuit OCPDevices located in the indoor panel are going to be 10ka rated..


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electrofelon said:
Very likely, almost certainly, the branch breakers will have a series rating with the main so they can still be 10k
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Are you saying that if I have a 22KA main breaker than it’s OK to put 10 KA branch circuit breakers?
If my fault current rating is above 10 KA then I would think I would have to put 22K brach circuit breakers in. Correct?
I’m not understanding what you mean by series rating with the main.


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