AIC Calculations & It's Significance

[Ingenieur]

It is certainly possible the 54 K is legit, but IMO not likely. Very roughly and generally, you will see utility transformer KVA 40-50% of NEC service KVA, so 3x112.5's is quite unlikely. Getting the actual Z can give you a better deal. As I said thats what I did. I told the utility engineer/designer that number seemed way high could he get me actual transformer data. He said sure and went out to the yard and snapped a pic of the actual unit they were going to use.

they will not use the actual s
they use the lowest in their inventory that may be used as a replacement

if I supply the xfmr I would use its pu z

if the util supplies it and states in their terms of service (as pg&e does) they will provide the sc value, that is what I would use

I will not assume the liability of reducing it to save the client some $$ up front and expose him to a possible issue in the future

I would explore the options of mitigating it/reducing magnitude

 

[electrofelon]

they will not use the actual s
they use the lowest in their inventory that may be used as a replacement

if I supply the xfmr I would use its pu z

if the util supplies it and states in their terms of service (as pg&e does) they will provide the sc value, that is what I would use

I will not assume the liability of reducing it to save the client some $$ up front and expose him to a possible issue in the future

I would explore the options of mitigating it/reducing magnitude

I consider things like the utility changing out the transformer to a higher KVA/lower impedance due to failure/miscalculation extremely unlikely and just another "what if". I think there is a balance however and consider it analogous to sizing a service: There is no NEC requirement to base fault current calculations On a value provided by the utility nor for any future what if, just like there is no requirement to size a service that way. But on the other hand, if there is a reasonable possibility the building will be adding enough load in the future to require a larger transformer, it may be worth discussing this and the cost difference with the client - just like you might do when sizing a service.

 

[Ingenieur]

I consider things like the utility changing out the transformer to a higher KVA/lower impedance due to failure/miscalculation extremely unlikely and just another "what if". I think there is a balance however and consider it analogous to sizing a service: There is no NEC requirement to base fault current calculations On a value provided by the utility nor for any future what if, just like there is no requirement to size a service that way. But on the other hand, if there is a reasonable possibility the building will be adding enough load in the future to require a larger transformer, it may be worth discussing this and the cost difference with the client - just like you might do when sizing a service.

does not matter what you or I consider likely
using their number is required by the util per terms of service
if you decide to use 22k vs 56k that is on you
in fact they may not connect if they catch it

discuss it with your e&o provider, see what they say
why would anyone make a change that benefits others (likely costs you due to lower equip cost) and assume liability for it? you are PAYING to incur liability
you have a free pass: the util gave you the number and requires you to use it
not me

ref item 16 http://www.ci.east-palo-alto.ca.us/Archive/ViewFile/Item/351

Submit a letter from PG&E informing us of the available fault current at the main switchboard. Any approval is contingent to the maximum AFC as determined by PGE and that the switchboard and down stream panels match or exceed the AIC ratings shown on the plan. Contact PG&E at 494-1700. the letter must be on the plan.

 

[MyCleveland]

"There is no NEC requirement to base fault current calculations On a value provided by the utility"

We need some basis of design do we not ?

I asked a local utility for a service upgrade SCC, xfmr size, Z, X/R a few weeks back. received a SCC number in a day and was told I might get the other data....maybe in 8-12 weeks.

Gave the same person some meter numbers outside a vault for a local high-rise office building, looking for a SCC value to work up into the building.
They came back and said the vault 9assoicated with those meters) is on the 17th floor of a different high-rise a few blocks away.

Not sure if any data coming from utility is valid...beyond frustrating.

 

[electrofelon]

does not matter what you or I consider likely

There is no gambling going on. It is based on the equipment installed. Should I install a larger service because of something that might happen in the future? Sometimes, sometimes not. IT depends on a variety of factors just like how much extra fault current room I will design with.

why would anyone make a change that benefits others

Nice. If I hire someone for something, I trust that they have my interests and finances in mind. Also sometimes it does save me money. It depends on the pricing structure of the job and other variables. One job I saved $800 because I was able to use a different manufacturer for a sub panel board (I didnt need a series rating after recomputing the fault current).

ref item 16 http://www.ci.east-palo-alto.ca.us/Archive/ViewFile/Item/351

They better have a city or state NEC amendment stating utility value must be used and actual equipment data cannot.

Look, perhaps you are getting the wrong idea. For one thing, this doesnt happen very often. It is a rare luxury to get a glimpse of the actual transformer data before ordering equipment. I am by no means saying to just lower you AIR arbitrarily because there is plenty of fudge room. Also note that we likely have very different niches; I am a small one man "design build" shop. I dont really do much work off pre-designed plans or bid work. I wouldn't spend much - probably no- time on a job like that trying to wrangle up the chain of command to haggle with fault current.

 

[Ingenieur]

There is no gambling going on. It is based on the equipment installed. Should I install a larger service because of something that might happen in the future? Sometimes, sometimes not. IT depends on a variety of factors just like how much extra fault current room I will design with.


u
Nice. If I hire someone for something, I trust that they have my interests and finances in mind. Also sometimes it does save me money. It depends on the pricing structure of the job and other variables. One job I saved $800 because I was able to use a different manufacturer for a sub panel board (I didnt need a series rating after recomputing the fault current).



They better have a city or state NEC amendment stating utility value must be used and actual equipment data cannot.

Look, perhaps you are getting the wrong idea. For one thing, this doesnt happen very often. It is a rare luxury to get a glimpse of the actual transformer data before ordering equipment. I am by no means saying to just lower you AIR arbitrarily because there is plenty of fudge room. Also note that we likely have very different niches; I am a small one man "design build" shop. I dont really do much work off pre-designed plans or bid work. I wouldn't spend much - probably no- time on a job like that trying to wrangle up the chain of command to haggle with fault current.

we are not talking about a service
you are assuming risk and imposing it on the client
if the current xfmr z is 5% and they replace it with a larger one at 2% now you have an issue
you gotta pick your fights
there is no winning here

save him a buck now and it will be forgotten tomorrow
cause him a problem tomorrow it will not be forgotten

they don't need a law
the muni can appoint a code officer and he can generate approval requirements
the nec is a minimum standard
you can fight it but good luck, you will not prevail

 

[kwired]

This must be first phase of project and more is to come? You have 800 amps @ 240 V three phase service capacity (~333 kVA) yet only shows 2.7 kVA of connected load.

What I see in your one line drawing is an 800 amp main breaker supplying a 200 amp feeder which supplies a panelboard with branch circuits leaving it. I would think most 800 amp breakers you would use for that main would handle 65kA minimum, need for higher would be more of a special condition kind of thing. From there most any 200 amp feeder breaker you put in that is from same manufacturer is likely series rated with that 800 amp main, and "standard 10 kA" branch breakers in the subpanel are likely series rated with the 200 amp feeder.

Check series ratings of what you do have and come back with any questions you have after looking at that information.

 

[mbrooke]

we are not talking about a service
you are assuming risk and imposing it on the client
if the current xfmr z is 5% and they replace it with a larger one at 2% now you have an issue
you gotta pick your fights
there is no winning here

save him a buck now and it will be forgotten tomorrow
cause him a problem tomorrow it will not be forgotten

they don't need a law
the muni can appoint a code officer and he can generate approval requirements
the nec is a minimum standard
you can fight it but good luck, you will not prevail

Whose to say that current padmount is sized by 800 x 240 x 3sqrt? I bet the full load kva for that pad is 1/2 to 1/3 that service, possibly less. Factor in the 12 or 4kv impedance from the line and you are very well under 65ka, even if the unit is replaced with a lower Z.

 

[Ingenieur]

Whose to say that current padmount is sized by 800 x 240 x 3sqrt? I bet the full load kva for that pad is 1/2 to 1/3 that service, possibly less. Factor in the 12 or 4kv impedance from the line and you are very well under 65ka, even if the unit is replaced with a lower Z.

did not say that:guessed 225 kva
I doubt it is 100-125 kva

the utility say 56 ka
who has the information, we do not
plus the local code may REQUIRE them to use the util number

no one disputes it may be less
but it may be more
or may be accurate
all moot
likely required to use the utility number

 

[electrofelon]

Whose to say that current padmount is sized by 800 x 240 x 3sqrt? I bet the full load kva for that pad is 1/2 to 1/3 that service, possibly less. Factor in the 12 or 4kv impedance from the line and you are very well under 65ka, even if the unit is replaced with a lower Z.

Exactly. If I am going off actual transformer data, I am just assuming infinite primary. I have seen some figures where taking into account primary impedance cuts the fault current in half.

plus the local code may REQUIRE them to use the util number

they don't need a law
the muni can appoint a code officer and he can generate approval requirements

Yes they do need an legal amendment. There is no language in the NEC stating to use "worst case" "possible" "lowest potential future Z" or " figure provided by the utility". IF they adopt the NEC, and want to rquire you to use the utility figure, they need an amendment adding that language. Thats just the way it works. Maybe they have that where the OP is, I dont know, but that summary from the plan review dept does not technically cut it. Sure maybe its not worth the fight, but they are not supposed to pull stuff out of their ###.

 

[Ingenieur]

Yes they do need an legal amendment. There is no language in the NEC stating to use "worst case" "possible" "lowest potential future Z" or " figure provided by the utility". IF they adopt the NEC, and want to rquire you to use the utility figure, they need an amendment adding that language. Thats just the way it works. Maybe they have that where the OP is, I dont know, but that summary from the plan review dept does not technically cut it. Sure maybe its not worth the fight, but they are not supposed to pull stuff out of their ###.

afraid not
what is a 'legal amendment'?
do you mean enact an ordinance like code adoption? or revise it?
not really: they can use whatever they want that has a reasonable basis
I was the appointed Engineer for a dozen boro's, townships, etc
they all had Solicitors, we could generate code requirements wiithout council action
the code official is the appointed ahj
a code is different than law
you can challenge it in court
good luck getting a judge to allow you to use a value lower than the util who provides the power and xfmr

In fact a law I wrote was enacted at the state level
voted on last Thur, I was there to answer any questions
relates to gf protection in mining: most stringent in the country
MSHA may adopt


the OP knows this
hence asking about mitigation methods rather than lowering the current

 

[Ingenieur]

a service drop that cuts xfmr lug fault i in 1/2?
S kva
v volt
Zpu (xfmr)
1 ph (makes math simpler)

i fault = if = S / (v Zpu)
Zpu (ohms) = v / if
to cut i fault at feeder in half Z feeder must = xfmr Z = v / if
so Zpu = Z feeder

i rated = S / v
feeder v drop = i rated x Z feeder = S / v x v / if = S / if
= S / (S /(v Zpu)) = v Zpu or v Z feeder since Zpu=Z feeder

 

[mbrooke]

did not say that:guessed 225 kva
I doubt it is 100-125 kva

Guessed- yet your are sternly doubting the fact it probably is 100-125kva.

the utility say 56 ka
who has the information, we do not
plus the local code may REQUIRE them to use the util number



no one disputes it may be less
but it may be more
or may be accurate
all moot
likely required to use the utility number

I am skeptical of how that number is derived. R+Jx values of the source and transformer would put me at ease.

Exactly. If I am going off actual transformer data, I am just assuming infinite primary. I have seen some figures where taking into account primary impedance cuts the fault current in half.

Oh yes! You'd be surprised what just a few miles away from a substation can do.

Yes they do need an legal amendment. There is no language in the NEC stating to use "worst case" "possible" "lowest potential future Z" or " figure provided by the utility". IF they adopt the NEC, and want to rquire you to use the utility figure, they need an amendment adding that language. Thats just the way it works. Maybe they have that where the OP is, I dont know, but that summary from the plan review dept does not technically cut it. Sure maybe its not worth the fight, but they are not supposed to pull stuff out of their ###.

Hopefully 2020 will be something productive this time around...

 

[electrofelon]

afraid not
what is a 'legal amendment'?
do you mean enact an ordinance like code adoption? or revise it?
not really: they can use whatever they want that has a reasonable basis
I was the appointed Engineer for a dozen boro's, townships, etc
they all had Solicitors, we could generate code requirements wiithout council action
the code official is the appointed ahj
a code is different than law
you can challenge it in court
good luck getting a judge to allow you to use a value lower than the util who provides the power and xfmr

In fact a law I wrote was enacted at the state level
voted on last Thur, I was there to answer any questions
relates to gf protection in mining: most stringent in the country
MSHA may adopt


the OP knows this
hence asking about mitigation methods rather than lowering the current

For example, in Washington, they adopt the NEC. There are state amendments/additions and they are either in the RCW (revised code of washington) or the WAC (Washington administrative code). There are some local codes too such as the Seattle electrical code which is made law by the city council. If an inspector wants something, he has to give me reference from one of the aformentioned documents.

So you are claiming this?

inspector/plan reviewer: the AIR of your equipment is too low
Sparky: No its fine, here are my calculations. See? Here is a picture of the transformer data plate.
inspector/plan reviewer: You need to use the figure provided by the utility.
Sparky: can you give me a code section requiring that?
inspector/plan reviewer: Its on that checklist on that paper discussing the plan review process.

IMO that is not legally acceptable. It needs to be formally stated in a legal document that has been adopted into law.

FWIW, I would predict they would be fine with an actual calculation. They probably just word it that way because they assume that is the route most people go.

 

[mbrooke]

a service drop that cuts xfmr lug fault i in 1/2?
S kva
v volt
Zpu
1 ph (makes math simpler)

i fault = if = S / (v Zpu)
xfmr Z (ohms) = v / if
to cut i fault at feeder in half Z feeder must = xfmr Z = v / if

i rated = S / v
feeder v drop = i rated x Z feeder = S / v x v / if = S / if
= S / (S /(v Zpu)) = v Zpu

Even for an infinite source, a LOT!

 

[Ingenieur]

Guessed- yet your are sternly doubting the fact it probably is 100-125kva.

I am skeptical of how that number is derived. R+Jx values of the source and transformer would put me at ease.

Oh yes! You'd be surprised what just a few miles away from a substation can do.

Hopefully 2020 will be something productive this time around...

yes

again, does not matter what we think
what matters is the value the util derived with info we do not have AND apparently put in writing

generally system Z matters little since avail fault mva >>>> xfmr kva
not to mention a huge gen supplying a small xfmr
usually uOhm inductive

do not know what the '2020' comment means?

 

[Ingenieur]

Even for an infinite source, a LOT!

what is the v drop at 100' at 830 A? 120/208
at 23' or ~ 1/2 x ifault

 

[electrofelon]

a service drop that cuts xfmr lug fault i in 1/2?

Even for an infinite source, a LOT!

I admit I have seen very few figures that have taken into account primary impedance (the one that was half I mentioned was from a MH thread A while back). What are some reduction values you guys have seen?

 

[mbrooke]

yes

again, does not matter what we think
what matters is the value the util derived with info we do not have AND apparently put in writing

generally system Z matters little since avail fault mva >>>> xfmr kva

Some distribution circuits can significantly lower the fault at the spades- especially a 4160 volt system which has a good chance of being here considering the POCO. Even still, 12 and 16kv system scan still exhibit a good increase in impedance with distance.

not to mention a huge gen supplying a small xfmr
usually uOhm inductive

This is a utility application.

do not know what the '2020' comment means?

The NEC- in so far they have yet to clear up the mud and fog on fault current.

what is the v drop at 100' at 830 A? 120/208

:roll: It is well known that service will not reach 800amps.

 

[mbrooke]

what is the v drop at 100' at 830 A? 120/208
at 23' or ~ 1/2 x ifault

About 7%- but not going to be carrying that most likely. As is 1/0 carrying 830 will get to hot- even for POCO standards.