fault current calculation

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mannyb

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Florida
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Electrician
When A and E drawings dont have fault current calculations it makes me wonder why they dont perform this at pre con stage. Do EC perform this study themselves or hire out engineer.
1.CONTRACTOR SHALL CONDUCT A FAULT CURRENT CALCULATION ON ALL
EQUIPMENT AND MARK AS REQUIRED PER THE N.E.C.
2.CONTRACTOR SHALL PROVIDE AN ARC-FLASH STUDY AND LABEL ALL
EQUIPMENT AS REQUIRED PER THE N.E.C.
 
mannyb said:
When A and E drawings dont have fault current calculations it makes me wonder why they dont perform this at pre con stage. Do EC perform this study themselves or hire out engineer.
1.CONTRACTOR SHALL CONDUCT A FAULT CURRENT CALCULATION ON ALL
EQUIPMENT AND MARK AS REQUIRED PER THE N.E.C.
2.CONTRACTOR SHALL PROVIDE AN ARC-FLASH STUDY AND LABEL ALL
EQUIPMENT AS REQUIRED PER THE N.E.C.
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Personally I try to show on all sets of plans. I always leave a disclaimer for EC to verify specifically cause installed feeder lengths can easily change vs what we anticipate during design. I'm usually a little on the conservative side of things just in case, but the real pain is when you have 4 story multifamily building. If we calculate an available fault current to be 19,500... Id say the OCPD needs to be 22kAIC. But in reality if that feeder was reduced by even 20' it could bump it past 22k.

Usually EC has been doing arc flash studies for our projects. I've done them in the past but currently we dont find enough work in that field to pay for the $10k annual software licensing. If we had a steady stream of projects requiring them it would be a different story. I've never actually been asked to provide one in addition to building design.
 
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I personally like to see all feeder sizes shown on the one line to include the length. Other wise an accurate Short Circuit Study is impossible. And on all projects this is most essential. I am here to learn and share experience . I like your question. And have an interested in answers that follow.
 
JEFF MILLAR said:
I personally like to see all feeder sizes shown on the one line to include the length. Other wise an accurate Short Circuit Study is impossible. And on all projects this is most essential. I am here to learn and share experience . I like your question. And have an interested in answers that follow.
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While job is it to update the online with the actually installed data?

Most MEP engineers finished with the online months or even years before 'as installed' data would be available.

If the MEP source drawings are not provided to the contractor how arevthey supposed to be updated by anyone else?
 
ANSI does not require feeder sizes to be included. They want the worst case values calculated in order to select all equipment Short circuit rated values. That is why Short circuit calculations are based on Impedance values. I can suggest the best reference " BUSSMANN. EDP - 1 "
 
jim dungar said:
In my experience about 40% hire it out, 40% do it themselves, and 20% ignore it.
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I have not seen anything close to the 40% doing it themselves.
What I would suggest is ask your local CBO…I would place a large bet on 95%++ are not “hiring it out”, but passing it off to the EC.
This far in with all the NEC requirements…the ones ignoring are Architects trying to develop EE plans.
 
The maximum available short circuit value is calculated as a 3phase line to line bolted fault. This includes as a general rule the Transformer full load current X 100 / Transformer Impedance e.g. 5.75 %. + 4 x. the transformer FLA for motor contribution. This is a simple explanation for details see " Bussmass EDP - 1 ".
 
A/E often do pre-con calculations for short circuit, but do not have access to as-built feeder lengths or selected OCPD trip unit / fuse selections to do final short circuit or arc flash that the EC would have during / after construction.
Each project is different, as to whether the client pays the A/E to do it themselves OR to review the EC's provided study that is done either by themselves or by the vendor that provides the panels / SWBDs / SWGR. For critical facilities, there is an art to the coordination study and a sweet spot to the balance between selectivity vs arc flash reduction, so often I ask for the A/E (me) to do the studies.
 
mannyb said:
When A and E drawings dont have fault current calculations it makes me wonder why they dont perform this at pre con stage. Do EC perform this study themselves or hire out engineer.
1.CONTRACTOR SHALL CONDUCT A FAULT CURRENT CALCULATION ON ALL
EQUIPMENT AND MARK AS REQUIRED PER THE N.E.C.
2.CONTRACTOR SHALL PROVIDE AN ARC-FLASH STUDY AND LABEL ALL
EQUIPMENT AS REQUIRED PER THE N.E.C.
Click to expand...
I have no problem with a final verification of the actual values. I do have a problem if the designer hasn't done some sort of calculation for expected values, hard to specify equipment to use if you don't know what they need to withstand.

Arc flash labeling is pretty limited in NEC. Simple signs that state there is a risk of arc flash is about all that is required, some equipment already has labels that meet NEC already on it from the manufacturer. An actual study showing incident energy and what PPE should be used is not covered or requried by NEC.
 
kwired said:
Arc flash labeling is pretty limited in NEC. Simple signs that state there is a risk of arc flash is about all that is required, some equipment already has labels that meet NEC already on it from the manufacturer. An actual study showing incident energy and what PPE should be used is not covered or required by NEC.
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Being in the business of providing arc flash labels, I have to agree with that statement. 110.16(A) says Field or Factory marked.

But for other than dwelling units at 1200 amps and above 110.16(B) there is at least a requirement for verification of the upstream device's operating time with a label, or by using an NFPA 70E-prescribed arc flash label. Responsible companies follow 70E for their Electrical Safety Program, which leads to arc flash labels for everything.
 
mannyb said:
When A and E drawings dont have fault current calculations it makes me wonder why they dont perform this at pre con stage. Do EC perform this study themselves or hire out engineer.
1.CONTRACTOR SHALL CONDUCT A FAULT CURRENT CALCULATION ON ALL
EQUIPMENT AND MARK AS REQUIRED PER THE N.E.C.
2.CONTRACTOR SHALL PROVIDE AN ARC-FLASH STUDY AND LABEL ALL
EQUIPMENT AS REQUIRED PER THE N.E.C.
Click to expand...
I think the issue is largely that the actual gear and service supplied is often not known at the time the other engineering drawings are done so to get something accurate, it is necessary to use what amounts to as builts for this purpose.

Many larger contractors have PEs on staff. Others farm this kind of thing out.
 
petersonra said:
I think the issue is largely that the actual gear and service supplied is often not known at the time the other engineering drawings are done so to get something accurate, it is necessary to use what amounts to as builts for this purpose.

Many larger contractors have PEs on staff. Others farm this kind of thing out.
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Gear is ordered when the job is awarded...who takes the hit when AICs & SCCRs fall short if actual values are not known until as-builts are generated.
How was a permit even issued if the design was incomplete?
 
This is all ridiculous. If not given values then put it in the bid. As in “equipment designed for X AIC” or “assumes X AIC”. As others mentioned you can pretty easily come up with a maximum estimate with minimal data using the infinite bus assumption. Inductive loads aren’t going to affect it that much. No need for feeder lengths.

The big thing is that costs can go up dramatically with AIC. These days up to around 35 kA is fairly cheap. Going to say 65 kA is expensive and over 90 kA is prohibitively expensive. That’s for 600 V class.
 
paulengr said:
This is all ridiculous. If not given values then put it in the bid. As in “equipment designed for X AIC” or “assumes X AIC”. As others mentioned you can pretty easily come up with a maximum estimate with minimal data using the infinite bus assumption. Inductive loads aren’t going to affect it that much. No need for feeder lengths.

The big thing is that costs can go up dramatically with AIC. These days up to around 35 kA is fairly cheap. Going to say 65 kA is expensive and over 90 kA is prohibitively expensive. That’s for 600 V class.
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Rest easy...that is exactly what is happening.
 
JEFF MILLAR said:
I personally like to see all feeder sizes shown on the one line to include the length. Other wise an accurate Short Circuit Study is impossible. And on all projects this is most essential. I am here to learn and share experience . I like your question. And have an interested in answers that follow.
Click to expand...
Yeah agreed, usually I'll include all calculated parameters that lead to my calculations; assumed feeder size, type, length etc.
 
wait engineers are signing and sealing without putting available fault current on the plans? it is required to be listed on the plans. engineers are not supposed to "leave it to the contractor." How else do you demonstrate nothing is over-duty? I suppose you could take utility fault current data and then double it (or 1.5x) to get your interruption rating.

We typically do a design level calculation for available fault current only, based on generic equipment and utility fault current data. Then we require the EC to provide a formal preliminary study in ETAP or SKM with the proposed equipment and demonstrate nothing is over-duty. The preliminary study is to be approved as a condition to approving the gear submittals. Then the study gets updated again to the as-installed condition and expanded to include arc flash labels and selective coordination.
 
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