Jeff w/Mustang Electric South Florida

[jeffco97@bellsouth.net]

My problem is the plan examiner and engineer are requiring a short circuit fault calculation on even the most basic temp service application. I would like to learn a simple (for some) calculation to do myself. I went to mike holts web site under Free Stuff and downloaded Fault Calculations but still need so
me help.

120/240V 1-phase
25KVA TRANSFORMER
125' length
Conductors: 2-#2 Alum w/ 1-#4 Alum nuetral conductor
Load 100A Temp service Panel

 

[bphgravity]

Hello Jeff. I am a little confused as why YOU are responsiible for this and not the engineer. That is typically their job?

There is a good article on this very issue in the November/December 2006 IAEI News. Here is a link to that article:

http://www.iaei.org/subscriber/magazine/06_f/whitzel.html

 

[don_resqcapt19]

Jeff,
You need one more piece of information...the impedance of the transformer and then you can do a worst case calculation. There is an online calculator at the Bussmann site.
Don

 

[don_resqcapt19]

Bryan,

Hello Jeff. I am a little confused as why YOU are responsiible for this and not the engineer. That is typically their job?

I have never seen engineered drawings for a temporary service, or even a dwelling unit service for that matter.
Don

 

[ryan_618]

This is absurd. Even if the transformer had an amazingly low impedence of 1 percent, the fault current wouldn't be aobve 10,000A!

 

[chris kennedy]

This is absurd. Even if the transformer had an amazingly low impedence of 1 percent, the fault current wouldn't be aobve 10,000A!

What? Then why am I using 65k breakers and 100k fuses?
I get the available fault current from POCO and its usually around 25k. I had one at 40k.

 

[jeffco97@bellsouth.net]

Hello Chris, We are a small electrical company based out of Port Saint Lucie FL. The Building department (plan review) lets the Electrician draw up his own schematic or riser diagram provided the job involved is under 800A and 50,000$ in value.

 

[don_resqcapt19]

Chris,

What? Then why am I using 65k breakers and 100k fuses?
I get the available fault current from POCO and its usually around 25k. I had one at 40k.

Ryan's comment is based on the 25kva transformer in the original post. Using the Bussmann the 25 kva transformer with a 1% impedance and an infinite primary could supply a bit over 10,400 amps to a short circuit at the secondary terminals. Ryan's point would be that there is less than 10,000 amps available at the service equipment and standard equipment has that rating. There would be no need for any engineered drawings.
Don

 

[chris kennedy]

Chris,

Ryan's comment is based on the 25kva transformer in the original post. Using the Bussmann the 25 kva transformer with a 1% impedance and an infinite primary could supply a bit over 10,400 amps to a short circuit at the secondary terminals. Ryan's point would be that there is less than 10,000 amps available at the service equipment and standard equipment has that rating. There would be no need for any engineered drawings.
Don

Thank you. I see Bussman took thier Selective Coordination Guide off. I have this in print. Why do you suppose its gone?

 

[bphgravity]

The Building department (plan review) lets the Electrician draw up his own schematic or riser diagram provided the job involved is under 800A and 50,000$ in value.

This is a provision of the FBC, section 105.3.1.2

In this particular instance, a simple letter from FPL or your local co-op indicating there will be less than 10,000 amperes of available fault current aviable at the service would be sufficient for permit issuance.

Per energy regulations in Florida, fault currents are NOT permitted to exceed 10KA when supplying residential services. Your local plans examiner should already know this.

 

[HighWirey]

"Hello Jeff. I am a little confused as why YOU are responsiible for this and not the engineer. That is typically their job?"

In my orbit it was always incumbent upon the EC to provide the fault calculations. I mostly subbed it out. Cheaper, faster, better. Lucky here, as we had a retired GE engineer available. All the local ECs used him. I believe when the customers engineer saw our engineers name on the submittal, he just stamped the submittal "Approved". Never had a reviewer comment. Our man's calcs were good, as were our installs. Everyone was happy.

PS - I've tried several software programs and the customer did not care for the submittal, plus I could not stamp the calcs. Gave it up.

Best Wishes

 

[kingpb]

Bryan,

I have never seen engineered drawings for a temporary service, or even a dwelling unit service for that matter.
Don

Every power plant I have been invovled in has a temporary construction power design drawing.

IMO, EC's should not be doing fault calcs unless they have a trained power system engineer on staff. In power and heavy industrial this would be unheard of. I know this is more commonly done in the commercial arena, where many of the "engineers" don't know how to do it so they put it in the spec for the EC. Unofrtunately, a lot of breakers show up with default settings that then never get coordinated.

 

[jrannis]

FP&L provides the fault current on their letterhead usually pretty quick about it too!

 

[iwire]

Every power plant I have been invovled in has a temporary construction power design drawing.

OK for the very few ECs that are involved in power plant construction that is good info.....for the other 99.99999% of us there will be no engineer involved in dwelling unit and temporary services.

 

[kingpb]

OK for the very few ECs that are involved in power plant construction that is good info.....for the other 99.99999% of us there will be no engineer involved in dwelling unit and temporary services.

Sorry, didn't know this forum is restricted to Resi' type applications. I thought it was an OPEN forum to ALL types of questions.

 

[Rockyd]

Sorry, didn't know this forum is restricted to Resi' type applications. I thought it was an OPEN forum to ALL types of questions.

Please do clarify. I was under the impression that it doesn't matter that you were union or non union, resi, comm, or industrial, that this is the #1 site in America, and that this was a forum that tackled all scopes of work covered by the NEC, and some not (that are industry related), did I miss something?

I'm a good electrician, even have more than one masters license, but I could never hope to be sharper than the sum total of the forum. For that matter, some days I have tough time following on some very interesting posts above my head, but I get to learn from it.

Edit - spelling

 

[bphgravity]

Relax guys.

Bob's point is that there are an extremely small number of persons active on this forum that would relate to power plant construction and we are pretty certain it is not within the scope of the OP's question.

We certainly encourage the discussion of power plant construction if you so desire, but within context of the thread.

 

[iwire]

Relax guys.

Bob's point is that there are an extremely small number of persons active on this forum that would relate to power plant construction and we are pretty certain it is not within the scope of the OP's question.

We certainly encourage the discussion of power plant construction if you so desire, but within context of the thread.

Thank you Bryan, that is all I meant.

The opening post stated..

My problem is the plan examiner and engineer are requiring a short circuit fault calculation on even the most basic temp service application.

In my opinion we where going a bit off topic with power plant engineering.

Anyone is more then welcome to start a new thread about power plant engineering.