fault current calculation

[MyCleveland]

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.

drk
Your only difference here is your "design level calc". You still are placing the "DESIGN" on the EC.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?

When your plans went through review, were they approved even though the design was incomplete ?

 

[d0nut]

drk
Your only difference here is your "design level calc". You still are placing the "DESIGN" on the EC.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?

When your plans went through review, were they approved even though the design was incomplete ?

My design fault calcs are going to be fairly conservative with infinite bus, large utility transformer, low impedance, rounding circuit lengths down, etc. I would expect any changes in my equipment ratings to be going down. The breaker coordination portion is what gets pushed more on the EC since I do not know what manufacturer they will be bidding. I will have the system designed in a way that it won't be impossible to coordinate, but it may require some electronic trip breakers in lieu of thermal magnetic breakers and other similar items from the manufacturer.

 

[MyCleveland]

My design fault calcs are going to be fairly conservative with infinite bus, large utility transformer, low impedance, rounding circuit lengths down, etc. I would expect any changes in my equipment ratings to be going down. The breaker coordination portion is what gets pushed more on the EC since I do not know what manufacturer they will be bidding. I will have the system designed in a way that it won't be impossible to coordinate, but it may require some electronic trip breakers in lieu of thermal magnetic breakers and other similar items from the manufacturer.

d0nut
If I am reading this correctly, your fault analysis is final...and the EC is not required to redo ?
If yes, then are your final label numbers (labeling main, motors, etc.) with max AFC numbers way over actual ?

 

[paulengr]

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.

So if the EC does all the engineering studies, exactly what are you doing?

Classic paperwork engineering. Make everyone else do the actual work. You’re fired.

 

[d0nut]

d0nut
If I am reading this correctly, your fault analysis is final...and the EC is not required to redo ?
If yes, then are your final label numbers (labeling main, motors, etc.) with max AFC numbers way over actual ?

I am perfectly fine with using my calculations for smaller projects where it doesn't make sense to require the fault current, coordination, and arc flash studies be performed. In those cases, I will also provide breaker settings once the equipment has been selected as the additional effort is fairly minor. My numbers won't be way over the actual values, but might push into a higher standard rating than would be required using the installed values.

For larger projects, I will require those studies to be performed by the contractor (which actually get performed by a PE hired by the equipment manufacturer and included in their equipment package). This engineer will perform the calculations again based on actual equipment and planned installation, information that I will not have during the design phase. Redoing the studies provides a check on my design calculations, as well as providing more accurate values based on the actual installation. Using installed conditions is important because assumptions that make the fault calculations conservative can have the opposite result on the arc flash studies.

 

[drktmplr12]

So if the EC does all the engineering studies, exactly what are you doing?

Classic paperwork engineering. Make everyone else do the actual work. You’re fired.

No, the EC's Engineer because we have no idea what will be submitted. SQD, Eaton, ABB, AB, Siemens. How can we possibly do the engineering? There's nothing wrong with this approach and it's routine in our industry. They take the liability of what appears on the labels and the settings for system coordination. We make sure it meets the intent of our design. Further, the owner doesn't want their consultant to get the contractor's way. We design buildings, we don't build them. Getting involved only invites contractor's claims for delays. It also delineates clear lines of responsibility directly on the General Contractor.

same can be said for light poles. its routine for a structural engineer hired by the contractor to design and provide wind loading calcs during construction, not the electrical engineer specifying the pole.

 

[kwired]

No, the EC's Engineer because we have no idea what will be submitted. SQD, Eaton, ABB, AB, Siemens. How can we possibly do the engineering? There's nothing wrong with this approach and it's routine in our industry. They take the liability of what appears on the labels and the settings for system coordination. We make sure it meets the intent of our design. Further, the owner doesn't want their consultant to get the contractor's way. We design buildings, we don't build them. Getting involved only invites contractor's claims for delays. It also delineates clear lines of responsibility directly on the General Contractor.

same can be said for light poles. its routine for a structural engineer hired by the contractor to design and provide wind loading calcs during construction, not the electrical engineer specifying the pole.

how can you specify a pole if you don't know what it needs to be able to stand up to?

Can you not specify particular gear or even list of alternate gear that would be acceptable for your design?

At very least design things so that placement of POCO source, length and size of service conductors gives you a certain maximum available fault current and if selected gear is rated no more than that level it is likely acceptable? Of course arc flash studies and marking for required PPE is not necessarily going to be very accurate until all the as-is installed details become available.

 

[MyCleveland]

No, the EC's Engineer because we have no idea what will be submitted. SQD, Eaton, ABB, AB, Siemens. How can we possibly do the engineering? There's nothing wrong with this approach and it's routine in our industry. They take the liability of what appears on the labels and the settings for system coordination. We make sure it meets the intent of our design. Further, the owner doesn't want their consultant to get the contractor's way. We design buildings, we don't build them. Getting involved only invites contractor's claims for delays. It also delineates clear lines of responsibility directly on the General Contractor.

same can be said for light poles. its routine for a structural engineer hired by the contractor to design and provide wind loading calcs during construction, not the electrical engineer specifying the pole.

drk
I guess you can tell not everyone agrees with this method of delivery.
From an earlier post above...I ask again.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?
When your plans went through review, were they approved even though the design was incomplete ?

I would also question whether or not the second engineer (ECs) is taking any liability, is the product sealed and submitted for review ?
When I ask the vendors they have claimed 0 liability. I believe in they are assuming once you reviewed and approved it, it became your work product from an outsourced engineer.

I would also question is a typical OWNER is even aware of what is happening in regard to how the services are delivered. Do you inform each customer of this practice ?

The only comment regarding the light pole base reference is OMG. Never seen that on a set of drawings in 35 years.
What else can we place on the EC in what is intended as Plan/Spec project.

 

[paulengr]

No, the EC's Engineer because we have no idea what will be submitted. SQD, Eaton, ABB, AB, Siemens. How can we possibly do the engineering? There's nothing wrong with this approach and it's routine in our industry. They take the liability of what appears on the labels and the settings for system coordination. We make sure it meets the intent of our design. Further, the owner doesn't want their consultant to get the contractor's way. We design buildings, we don't build them. Getting involved only invites contractor's claims for delays. It also delineates clear lines of responsibility directly on the General Contractor.

same can be said for light poles. its routine for a structural engineer hired by the contractor to design and provide wind loading calcs during construction, not the electrical engineer specifying the pole.

The EC is specifically prohibited from doing engineering. The NCEES model states that the engineer must be independent from the EC. Apparently you don’t understand how it works. The engineering firm is a quality control check on the EC. They provide the design and inspect it to make sure it is done to spec. That is the entire point of having an engineering firm involved! If you aren’t doing this then you are not following NCEES.

In most states that have adopted it design-build is illegal. The engineering firm designs everything based on site data. They specify cable lengths and they specify the equipment specs. As far as equipment goes what spec are you confused about? You specify breaker sizes and AIC. You specify trip curves for distribution breakers (such as very inverse), tap, time dial, and so forth. On UL (molded case) you can specify everything with the amperage, curve type (A, B, C), and minimum AIC. This defines everything except arc flash but the only thing missing is factors like the bus configuration and enclosure size but no matter who does it, this must be determined after installation. No vendor has accurate data but realistically you can design to meet at least 8 cal/cm2. If I can do a 15 MVA servo drive system at 1.2 cal/cm2 then it’s realistic to think you should easily be able to get to 8 cal/cm2.

This is what an engineering firm should be doing. And this is the “primaries” contracting method. It can also be collaborative. Select vendors first. Then engineering firm lays out single line. Contractor supplies raceway lengths, equipment specifications. Engineer checks everything and if needed AIC may need to be raised. This is the essence of an accelerated construction schedule which is highly collaborative because it’s phased construction.

If you don’t do this there is no guarantee equipment won’t be underdutied, arc flash won’t be ridiculous, etc.

 

[JoeStillman]

Our problem as engineers is that utility data necessary for an accurate SC/AF study is often not available at the time of the design. When this happens, we assume a worst-case utility transformer size/impedance with an infinite source upstream and put that value on the drawings. BUT, with the disclaimer that it's only an estimate and the contractor has to confirm the available fault current BEFORE ordering any equipment. Sometimes it even works. Plan reviewers always spot that note and add pressure to get it resolved with the POCO.

If you've done SC calcs based on your assumptions during design, then it's a simple thing to plug in the more accurate utility data. If you've done it right, the final values will be lower AIC's all around and a win for everybody.

That's for new construction. For renovation work, you've got no excuse for not doing the math up front.

Unless it's a real big design, I prefer not to require the contractor to do the SC/AF study. Why give away all that work to my competitors?

 

[drktmplr12]

drk
I guess you can tell not everyone agrees with this method of delivery.
From an earlier post above...I ask again.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?
When your plans went through review, were they approved even though the design was incomplete ?

I would also question whether or not the second engineer (ECs) is taking any liability, is the product sealed and submitted for review ?
When I ask the vendors they have claimed 0 liability. I believe in they are assuming once you reviewed and approved it, it became your work product from an outsourced engineer.

I would also question is a typical OWNER is even aware of what is happening in regard to how the services are delivered. Do you inform each customer of this practice ?

The only comment regarding the light pole base reference is OMG. Never seen that on a set of drawings in 35 years.
What else can we place on the EC in what is intended as Plan/Spec project.

We do preliminary calculations based on generic equipment. This is "enough" for available fault current and the engineering necessary to ensure we specify gear with ratings that are adequate taking into consideration the variables such as conductor size and length-which shouldn't vary too much if we've done our due diligence. all of this to say, this hasn't been an issue because we do our job and we do it well.

I don't understand at all how the design is incomplete. the documents contain all information required by Florida statutes, and all engineering requirements required for delegated engineers to perform their duties.

No one is asking an EC to take responsibility for an engineering study. it is normally performed, signed and sealed by a PE employed by the gear MFR (SQD, Eaton are most prominent down here). Their liability. the engineering is being delegated to them by the engineer of record.

we certainly do. we work with major counties and large facilities (15 MGD-100MGD wastewater, 10-30 MGD water), and all designs are approached this way, not just by us, but by others as well. delegating certain design elements to engineers other than the engineer of record is a valid approach.

presumably, you are an EE. Can you confidently say you have the expertise to sign and seal wind load calculations for a light pole? If yes, it is your right and prerogative to do so. i won't be affixing my seal to a structural design, and I won't be asking them to bury half the pole just because I know it will be enough. I show a generic detail with enough detail for the contractor to bid, and require they complete a structural analysis of the pole/installation and provide signed and sealed calculations. the end product is signed and sealed by a responsible engineer. this is the "Delegated" engineer's responsibility. FSS 61G15-30.006.

what you deem as "should be included in plan/specs" is subjective. the requirement is that an engineer sign and seal the elements that require it, whether it be the engineer of record or a delegated engineer.

Should I be responsible to sign and seal a generator enclosure for compliance with Florida building code simply because it is being provided under a division that I am specifying? The answer is no. it is my job to ensure the assembly is specified to meet the code and is signed and sealed by a delegated engineer, who has in writing (bid documents) the engineering criteria and requirements. it is then the contractor's responsibility to furnish the generator enclosure as such. this is 100% acceptable and ethical practice.

 

[kwired]

Our problem as engineers is that utility data necessary for an accurate SC/AF study is often not available at the time of the design. When this happens, we assume a worst-case utility transformer size/impedance with an infinite source upstream and put that value on the drawings. BUT, with the disclaimer that it's only an estimate and the contractor has to confirm the available fault current BEFORE ordering any equipment. Sometimes it even works. Plan reviewers always spot that note and add pressure to get it resolved with the POCO.

If you've done SC calcs based on your assumptions during design, then it's a simple thing to plug in the more accurate utility data. If you've done it right, the final values will be lower AIC's all around and a win for everybody.

That's for new construction. For renovation work, you've got no excuse for not doing the math up front.

Unless it's a real big design, I prefer not to require the contractor to do the SC/AF study. Why give away all that work to my competitors?

I'd guess short length service conductors is where you would find more problems. An additional 25 feet makes a huge difference most the time.

Arc flash and determining proper PPE to post is what needs more details based on as installed conditions that may not be as narrowed down until further into project.

 

[drktmplr12]

The EC is specifically prohibited from doing engineering. The NCEES model states that the engineer must be independent from the EC. Apparently you don’t understand how it works. The engineering firm is a quality control check on the EC. They provide the design and inspect it to make sure it is done to spec. That is the entire point of having an engineering firm involved! If you aren’t doing this then you are not following NCEES.

In most states that have adopted it design-build is illegal. The engineering firm designs everything based on site data. They specify cable lengths and they specify the equipment specs. As far as equipment goes what spec are you confused about? You specify breaker sizes and AIC. You specify trip curves for distribution breakers (such as very inverse), tap, time dial, and so forth. On UL (molded case) you can specify everything with the amperage, curve type (A, B, C), and minimum AIC. This defines everything except arc flash but the only thing missing is factors like the bus configuration and enclosure size but no matter who does it, this must be determined after installation. No vendor has accurate data but realistically you can design to meet at least 8 cal/cm2. If I can do a 15 MVA servo drive system at 1.2 cal/cm2 then it’s realistic to think you should easily be able to get to 8 cal/cm2.

This is what an engineering firm should be doing. And this is the “primaries” contracting method. It can also be collaborative. Select vendors first. Then engineering firm lays out single line. Contractor supplies raceway lengths, equipment specifications. Engineer checks everything and if needed AIC may need to be raised. This is the essence of an accelerated construction schedule which is highly collaborative because it’s phased construction.

If you don’t do this there is no guarantee equipment won’t be underdutied, arc flash won’t be ridiculous, etc.

you misunderstand. no one is asking the EC to do engineering analysis. I take exception to the suggestion that I am not doing my due diligence or working within code of ethics and in compliance with state statutes. note that NCEES doesn't regulate engineering practice, they administer examinations and provide results to state boards. oversight of engineering practice and licensure is done by each state board and codified in respective state statutes.

you are right that the engineering firm is a quality check on the EC to ensure it's done per plan and spec. this is true in 99% of circumstances.

design-build is legal in Florida, but this isn't design-build where the contractor hires an engineering firm. it's design-bid-build. owner hires a consultant to provide a constructible design that is placed out to bid by general contractors. all relevant engineering criteria are in the documents.

We size cables, breakers, show distances, make sure adjustable trip breakers are provided where needed. we don't specify exactly what the trip curves must look like. that is delegated to the engineer performing the coordination and arc flash study. 100% acceptable practice to delegate engineering duties. that said, if I did my job, there won't be over-duty equipment, any variable that is within and outside my control will have already been considered.

what should be done is subjective and depends on what services the Owner wishes to pay for. if we offer those services, we must perform them with all due consideration. what must be done is objective, codified, and subject to state board discipline.

all due respect, sir. if you are designing 15 MVA servo drive systems, we operate in very different capacities for very different owners.

 

[drktmplr12]

how can you specify a pole if you don't know what it needs to be able to stand up to?

Can you not specify particular gear or even list of alternate gear that would be acceptable for your design?

At very least design things so that placement of POCO source, length and size of service conductors gives you a certain maximum available fault current and if selected gear is rated no more than that level it is likely acceptable? Of course arc flash studies and marking for required PPE is not necessarily going to be very accurate until all the as-is installed details become available.

we know exactly what it needs to withstand, Florida building code applicable wind zone. we identify the criteria, and the structual analysis is delegated to a structural engineer. there are dozens, if not hundreds, of structural engineers that do these routinely down here.

with respect to the second comment, we do base our design on the two major MFRs down here (SQD + Eaton). I'm not sure where this idea came about that I am advocating not calculating available fault current or required AIC for equipment. both are required to be on engineering plans.

 

[kwired]

we know exactly what it needs to withstand, Florida building code applicable wind zone. we identify the criteria, and the structual analysis is delegated to a structural engineer. there are dozens, if not hundreds, of structural engineers that do these routinely down here.

with respect to the second comment, we do base our design on the two major MFRs down here (SQD + Eaton). I'm not sure where this idea came about that I am advocating not calculating available fault current or required AIC for equipment. both are required to be on engineering plans.

Well with experience, many can do that sort of thing with almost any product and not need to do calculations or only need limited details to factor in sometimes.

Then there is many that design with boilerplate specs that seldom are under designed for the situation but very likely could easily have been lesser specifications and still been fine. Not saying there is necessarily anything wrong with that.

 

[petersonra]

Well with experience, many can do that sort of thing with almost any product and not need to do calculations or only need limited details to factor in sometimes.

Then there is many that design with boilerplate specs that seldom are under designed for the situation but very likely could easily have been lesser specifications and still been fine. Not saying there is necessarily anything wrong with that.

These days 65k SCCR and 65kAIC is readily available at little to no cost increase over lower ratings at 480 V. It is not worth trying to figure out if a lower rated product is suitable. The time and effort spent on it does not justify the little bit of savings. That is why virtually all of the specs I see these days spec 65k.

 

[MyCleveland]

We do preliminary calculations based on generic equipment. This is "enough" for available fault current and the engineering necessary to ensure we specify gear with ratings that are adequate taking into consideration the variables such as conductor size and length-which shouldn't vary too much if we've done our due diligence. all of this to say, this hasn't been an issue because we do our job and we do it well.

I don't understand at all how the design is incomplete. the documents contain all information required by Florida statutes, and all engineering requirements required for delegated engineers to perform their duties.

No one is asking an EC to take responsibility for an engineering study. it is normally performed, signed and sealed by a PE employed by the gear MFR (SQD, Eaton are most prominent down here). Their liability. the engineering is being delegated to them by the engineer of record.

we certainly do. we work with major counties and large facilities (15 MGD-100MGD wastewater, 10-30 MGD water), and all designs are approached this way, not just by us, but by others as well. delegating certain design elements to engineers other than the engineer of record is a valid approach.

presumably, you are an EE. Can you confidently say you have the expertise to sign and seal wind load calculations for a light pole? If yes, it is your right and prerogative to do so. i won't be affixing my seal to a structural design, and I won't be asking them to bury half the pole just because I know it will be enough. I show a generic detail with enough detail for the contractor to bid, and require they complete a structural analysis of the pole/installation and provide signed and sealed calculations. the end product is signed and sealed by a responsible engineer. this is the "Delegated" engineer's responsibility. FSS 61G15-30.006.

what you deem as "should be included in plan/specs" is subjective. the requirement is that an engineer sign and seal the elements that require it, whether it be the engineer of record or a delegated engineer.

Should I be responsible to sign and seal a generator enclosure for compliance with Florida building code simply because it is being provided under a division that I am specifying? The answer is no. it is my job to ensure the assembly is specified to meet the code and is signed and sealed by a delegated engineer, who has in writing (bid documents) the engineering criteria and requirements. it is then the contractor's responsibility to furnish the generator enclosure as such. this is 100% acceptable and ethical practice.

drk
I know I will never convince you that this method of design should not be allowed, but it is just how I see it.
You....I....or any other EE taking on a project should be the only EE of record unless some special project requires dual engineers with specific scopes. If needed, these scopes and each engineer should be defined upfront with the client and not packaged out in the ECs bid.

Of course I do not design pole bases. I inform my client that a civil or structural engineer will need to generate these base designs as part of the bid package....not passed to the EC to source and hire a DELAGATE.
Providing electrical or any contracting services in hard enough without tossing in numerous engineering assignments.
I would think ever GC would agree with this too. I have spoken with numerous ones who have been stuck with gear packages fall short when the final report shows up.

You addressed the liability question....but when I have asked the gear mfger's this question they have claimed no liability.

From an earlier post above...I ask again.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?
When your plans went through review, were they approved even though the design was incomplete ? Please do not tell me it is complete because you have already stated it is preliminary.

 

[drktmplr12]

drk
I know I will never convince you that this method of design should not be allowed, but it is just how I see it.
You....I....or any other EE taking on a project should be the only EE of record unless some special project requires dual engineers with specific scopes. If needed, these scopes and each engineer should be defined upfront with the client and not packaged out in the ECs bid.

Of course I do not design pole bases. I inform my client that a civil or structural engineer will need to generate these base designs as part of the bid package....not passed to the EC to source and hire a DELAGATE.
Providing electrical or any contracting services in hard enough without tossing in numerous engineering assignments.
I would think ever GC would agree with this too. I have spoken with numerous ones who have been stuck with gear packages fall short when the final report shows up.

You addressed the liability question....but when I have asked the gear mfger's this question they have claimed no liability.

From an earlier post above...I ask again.
Once the ECs PreLim is approved, and then at the time of final update something changes where an item is underrated...who pays to fix this ?
When your plans went through review, were they approved even though the design was incomplete ? Please do not tell me it is complete because you have already stated it is preliminary.

Delegating engineering tasks happens all the time and everywhere. It is specifically addressed in Florida state administrative code.

FAC Rule 61G15-30.005
FAC Rule 61G15-30.006

What you are suggesting shouldn't happen if the engineer of record has already determined, by their own calculation, the fault current available and indicated so on the plans, as well as the required duty rating for equipment, also indicated on the plans. If the engineer of record specified something that created an over-duty situation, it is equitable for the owner to pay the difference to rectify. After they work out the change order, they can chew out the engineer for putting them in that situation and possibly reconsider using them in the future.

 

[MyCleveland]

Delegating engineering tasks happens all the time and everywhere. It is specifically addressed in Florida state administrative code.

FAC Rule 61G15-30.005
FAC Rule 61G15-30.006

What you are suggesting shouldn't happen if the engineer of record has already determined, by their own calculation, the fault current available and indicated so on the plans, as well as the required duty rating for equipment, also indicated on the plans. If the engineer of record specified something that created an over-duty situation, it is equitable for the owner to pay the difference to rectify. After they work out the change order, they can chew out the engineer for putting them in that situation and possibly reconsider using them in the future.

All of my comments were based on your post #20...
"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 in post #38 you state...
"What you are suggesting shouldn't happen if the engineer of record has already determined, by their own calculation, the fault current available and indicated so on the plans,"
If you have already done the work, then why the Pre-Lim calc for ordering gear ?

I never stated what you doing is illegal...its happening everywhere. Since you referenced your state laws, trying "Section 61G15-30.003 - Minimum Requirements for Engineering Documents" and see if you are compliant with all of these line items.

 

[drktmplr12]

All of my comments were based on your post #20...
"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 in post #38 you state...
"What you are suggesting shouldn't happen if the engineer of record has already determined, by their own calculation, the fault current available and indicated so on the plans,"
If you have already done the work, then why the Pre-Lim calc for ordering gear ?

I never stated what you doing is illegal...its happening everywhere. Since you referenced your state laws, trying "Section 61G15-30.003 - Minimum Requirements for Engineering Documents" and see if you are compliant with all of these line items.

it isn't an unreasonable burden-they have to do the study anyway.

my calc is for the building department. their calc is for me to confirm everything meets design criteria.

making a preliminary submission is a precaution-like requiring a conduit plan for electric rooms, instead of just letting the journeyman wing it. it is intended to have them demonstrate they've thought it through from top to bottom and equipment complies with the design requirements. we are all human, mistakes happen-maybe the wrong transformer was submitted, with lower impedance than a generic transformer? Maybe a breaker with the wrong pick-up unit? Maybe wrong duty rating? that said-it is far cheaper to fix a problem that hasn't been installed yet.