110.24(A)

[cowboyjwc]

This is mostly for plan checkers and inspectors, but contractors should chime in.

110.24(A) Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current.

So I've been trying to get this at plan check on the plans or a letter from the utility. I have not been very successful, the utility seems to balk at giving this information even though they are the ones that know the specs on the transformer and the size conductors they will be using.

Of course when I don't get it on the plans then come final, when I have asked for it for the third time, I have to fight for it still.

On one project the utility simply gave me a letter with worse case scenario as the number, that ended up throwing everything down stream off and adding considerable cost to the job, when they were having to put in 42K subpanels.

So for plan checkers, are you getting it on the plans?
For inspectors, are you simply waiting and getting it at final?
For contractors or engineers, is anyone even asking you for this?

Thanks

 

[charlie b]

I have not been asked for this information. But I typically show it anyway, at least on full-design/bid/build projects. The one-line diagram will show the fault current available at each panel, including the main service panel. The panel schedules will show the fault current rating of each panel. That provides a plan reviewer the information necessary to confirm that each panel will be rated to handle the fault current available at its location. For design/build projects, that responsibility is turned over to the D?B contractor.

But you can estimate the fault current available at the secondary of the service transformer by dividing the full load rated current by the percent impedance value. For example, a 500 KVA transformer at 480 volts has a rated current of 602 amps. Divide that by a typical impedance value of 5.75% (i.e., divide 602 by 0.0575) and you get 10,464 amps. You can "shop around" (i.e., check the Internet or your manufacturer's reps) for other impedance values, if you want to be more accurate.

Please note that this will give you the maximum fault current at the secondary, and assumes an "infinite bus" at the transformer primary. Since the utility will not really have an infinite supply of energy at the transformer location, the real fault current at the secondary will be lower than the formula's results. Also, there will be some energy lost in the service conductors. So you could accept a label that says 10.5KA at the service panel for my example, and not be concerned that it may be too low.

 

[cowboyjwc]

I have not been asked for this information. But I typically show it anyway, at least on full-design/bid/build projects. The one-line diagram will show the fault current available at each panel, including the main service panel. The panel schedules will show the fault current rating of each panel. That provides a plan reviewer the information necessary to confirm that each panel will be rated to handle the fault current available at its location. For design/build projects, that responsibility is turned over to the D?B contractor.

But you can estimate the fault current available at the secondary of the service transformer by dividing the full load rated current by the percent impedance value. For example, a 500 KVA transformer at 480 volts has a rated current of 602 amps. Divide that by a typical impedance value of 5.75% (i.e., divide 602 by 0.0575) and you get 10,464 amps. You can "shop around" (i.e., check the Internet or your manufacturer's reps) for other impedance values, if you want to be more accurate.

Please note that this will give you the maximum fault current at the secondary, and assumes an "infinite bus" at the transformer primary. Since the utility will not really have an infinite supply of energy at the transformer location, the real fault current at the secondary will be lower than the formula's results. Also, there will be some energy lost in the service conductors. So you could accept a label that says 10.5KA at the service panel for my example, and not be concerned that it may be too low.

Mostly this is just to fulfill the requirements of NFPA 70E, so yes you could err on the side of caution, but if it's a completely bogus number at the main, say 65K when if you did the calculation it would only be 42K, it changes everything downstream. Now of course we are talking about a fully rated system vs an series rated system.

The difference between "here, this is worse case" vs "actual" could mean the difference between a moon suit or face shield and gloves for the PPE.

Oh and FYI, all main transformers, here, are owned and supplied by the utility as well as the conduit and wiring to the main service panel. On the plans I get, it usually just says "utility transformer" and I don't have anymore information than that.

 

[electrofelon]

This is mostly for plan checkers and inspectors, but contractors should chime in.

110.24(A) Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current.

So I've been trying to get this at plan check on the plans or a letter from the utility. I have not been very successful, the utility seems to balk at giving this information even though they are the ones that know the specs on the transformer and the size conductors they will be using.

Of course when I don't get it on the plans then come final, when I have asked for it for the third time, I have to fight for it still.

On one project the utility simply gave me a letter with worse case scenario as the number, that ended up throwing everything down stream off and adding considerable cost to the job, when they were having to put in 42K subpanels.

So for plan checkers, are you getting it on the plans?
For inspectors, are you simply waiting and getting it at final?
For contractors or engineers, is anyone even asking you for this?

Thanks

I don't follow. Seems like one can't get very far on a project without knowing the AFC. How do you know what equipment to get?

Around here AFC coordinated with AIC is one of the primary things plan check looks far, so how can you not know it?

 

[petersonra]

I don't see how this is the responsibility of the plan checker or the inspector. it would seem to me to be the responsibility of whoever is doing the design work.

The plan checker should just red flag the plans if the information is not on the plans.

 

[bkludecke]

We have had similar problems with trying to nail down the serving utility for a real number. They agree that the information is important and required but what causes them to just throw out a bigger than life number is that there will be times in the future when they will be doing upgrades to their system and may replace transformers with larger and more efficient units which might throw some of the existing installations' labeling way off.

 

[texie]

This is mostly for plan checkers and inspectors, but contractors should chime in.

110.24(A) Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current.

So I've been trying to get this at plan check on the plans or a letter from the utility. I have not been very successful, the utility seems to balk at giving this information even though they are the ones that know the specs on the transformer and the size conductors they will be using.

Of course when I don't get it on the plans then come final, when I have asked for it for the third time, I have to fight for it still.

On one project the utility simply gave me a letter with worse case scenario as the number, that ended up throwing everything down stream off and adding considerable cost to the job, when they were having to put in 42K subpanels.

So for plan checkers, are you getting it on the plans?
For inspectors, are you simply waiting and getting it at final?
For contractors or engineers, is anyone even asking you for this?

Thanks

In my view this has been a problem for years and I agree with you that this is something that needs fixing. There is a total disconnect between the POCO/NESC and the premises/NEC on this subject. There are an awful lot of installs out there with wildly over rated gear.
Most always argued that over rating equipment is no big deal except for cost. Well, that may be true for SCCR, but for incident energy calcs it gets more critical to be precise. And now with the new NEC requirement to show clearing time at the service this info becomes more critical.
Even if the POCO gives you accurate values most seem to have no concern about coordinating with owners when they change things such as the transformer at a later date.

 

[jap]

If the plans we receive do not show the required AIC rating, then, most times we receive gear quotes at standard 10k.

Most times there is a note on the plans that indicated "Contractor to verify existing AIC rating with the power company" prior to bid.
That's usually information that we cannot obtain prior to bid day.

We then have to note it on our proposal that the AIC rating was not known at time of bid and the gear is quoted at standard 10k, and, any increase in AIC ratings will increase the price of our quote.

There are jobs where the AIC is only available, as you said, After the coordination study is done, which is after bid day, but, prior to the start of the job.

Oddly enough, most breakers in Feeder Panels are already rated for 65k and indeed the downstream panels may series rate to those feeder breakers at 22k already, but, without the panels being worked up that way to begin with , the AIC labels on the panels will not indicate such.

And, your correct, the difference between 10k rated gear as compared to 42, 65 or even 100k if required, is a substantial amount of money.

I also agree there are probably many a service installed that are severely under the AIC rating they should be for this same reason.

JAP>

 

[electrofelon]

I have not had trouble getting a value from the utility, but I have had trouble getting an ACCURATE number from the utility. Almost always it is unreasonably and unrealistically high - like 2-3 times is routine. Occasionally this warranted, but usually not (for me, YMMV).

A job I have going on now I was quoted 110k at the transformer secondary. I called BS, knowing it would be significantly cheaper If I could get below 65 at the gear. Got a pump, pumped out the transformer vault, went in and took a peek. It was actually 77k at the secondary and mid 50's at the gear.

 

[kwired]

I don't follow. Seems like one can't get very far on a project without knowing the AFC. How do you know what equipment to get?

Around here AFC coordinated with AIC is one of the primary things plan check looks far, so how can you not know it?

I agree. Even before we were required to mark the fault current, we still needed gear that was rated no less than what was available or at least use series ratings where they can apply.

Now for most 200 amp and less applications and at least 25 or more feet of conductor to the source, the conductor has enough resistance to drop available current to 22k or less and a 22k main is usually series rated for 10k breakers so often wasn't any problems with gear selection even if you didn't know any better.

I don't see how this is the responsibility of the plan checker or the inspector. it would seem to me to be the responsibility of whoever is doing the design work.

The plan checker should just red flag the plans if the information is not on the plans.

Who assures the information is accurate? If you do this often enough you kind of get a feel of what to expect for some common situations, if someone has something that seems completely out of line it should at least be questioned.

If the plans we receive do not show the required AIC rating, then, most times we receive gear quotes at standard 10k.

Most times there is a note on the plans that indicated "Contractor to verify existing AIC rating with the power company" prior to bid.
That's usually information that we cannot obtain prior to bid day.

We then have to note it on our proposal that the AIC rating was not known at time of bid and the gear is quoted at standard 10k, and, any increase in AIC ratings will increase the price of our quote.

There are jobs where the AIC is only available, as you said, After the coordination study is done, which is after bid day, but, prior to the start of the job.

Oddly enough, most breakers in Feeder Panels are already rated for 65k and indeed the downstream panels may series rate to those feeder breakers at 22k already, but, without the panels being worked up that way to begin with , the AIC labels on the panels will not indicate such.

And, your correct, the difference between 10k rated gear as compared to 42, 65 or even 100k if required, is a substantial amount of money.

I also agree there are probably many a service installed that are severely under the AIC rating they should be for this same reason.

JAP>

If you have plans that show a 1200 amp main, a short conductor run from source to that main, but no AIC rating or mention of fault current - you are going to assume 10K equipment is fine and bid with that equipment?

First off the 1200 amp main is probably rated for more than 10k, maybe even more than 42k, but next question is series ratings of whatever is beyond that.

 

[cowboyjwc]

OK looks like we are all pretty much on the same page with this and have the same issues.

I just get so tired of the "you're the only ones asking for that".

Thanks for the input guys.

 

[mayanees]

110.16(B)

110.16(B)

How about the requirements of 110.16(B) for available fault current and upstream device opening time? John C, do you look for that? I can see 110.24(A) as using an infinite bus calculation on the primary of the upstream transformer. This assures that as long as the transformer size doesn't increase then max fault currents won't be exceeded. But 110.16 (B) Arc Flash Hazard Warning at Service Equipment requires available fault current.

We perform Arc Flash Studies and that information is critical and we have to have it to perform the analysis. One VA Utility gives the actual transformer primary contribution, but won't give transformer impedance values so we have to cover the whole range.

Is 110.16(B) being given attention by inspectors?

 

[cowboyjwc]

How about the requirements of 110.16(B) for available fault current and upstream device opening time? John C, do you look for that? I can see 110.24(A) as using an infinite bus calculation on the primary of the upstream transformer. This assures that as long as the transformer size doesn't increase then max fault currents won't be exceeded. But 110.16 (B) Arc Flash Hazard Warning at Service Equipment requires available fault current.

We perform Arc Flash Studies and that information is critical and we have to have it to perform the analysis. One VA Utility gives the actual transformer primary contribution, but won't give transformer impedance values so we have to cover the whole range.

Is 110.16(B) being given attention by inspectors?

I know what the max here is (never been more than 65K) so as long as the main gear is that and you want to work your downstream stuff off of that, that's fine. Most engineers are getting lazy and one note will say that it needs to be fully rated and then the next note will say that if it's series rated it must be rated at ______.

The actual calculation would actually help these guys, so I don't understand why it seems to be such a big deal to get it. Like I said, the utility has the information.

 

[jim dungar]

Like I said, the utility has the information.

I deal with several large utilities that do not have this information readily available. For the most part they do not know the size of the transformer they will use until it has been installed. Typically they just grab some unit out of the storage yard as long as it meets their calculated loading requirement. Some of them do not even write down what they actually installed.

The POCO supplies a worst case or 'design' fault current which would not be exceeded by their current practices. This design level is often a value based on an infinite bus and at least one size larger transformer than requested by the customer.

The NEC (110.24) wants to know that installed equipment is sufficient to handle the available fault current.
NEC 110.16(B) requires a warning about arc flash and must have the available fault current and protective device clearing time, but only at the service entrance if a complete arc flash hazard analysis has not been performed. However, isn't clear what the service entrance protective device should be especially for 6-disconnect installations (one tenet of arc flash studies is that a device cannot protect itself).
NEC 240.67 and 240.87 require that an arc flash calculation be performed, so that the energy reduction method can be set below the available arcing current.

 

[jap]

If you have plans that show a 1200 amp main, a short conductor run from source to that main, but no AIC rating or mention of fault current - you are going to assume 10K equipment is fine and bid with that equipment?

First off the 1200 amp main is probably rated for more than 10k, maybe even more than 42k, but next question is series ratings of whatever is beyond that.

No.

I'm not going to assume anything.

It's not my job to do the electrical engineers job on an engineered set of drawings.

They simply need what they're paid to do, and, not keep leaving it up to us to take up their slack.

JAP>

 

[kwired]

No.

I'm not going to assume anything.

It's not my job to do the electrical engineers job on an engineered set of drawings.

They simply need what they're paid to do, and, not keep leaving it up to us to take up their slack.

JAP>

OK, so if they don't specify AIC rating what do you bid or install?

If they do specify 10k and it ends up not being enough - at least you have something to say you followed their design. Even better for you if they specify catalog numbers, as long as you installed what was specified.

 

[jap]

OK, so if they don't specify AIC rating what do you bid or install?

If they do specify 10k and it ends up not being enough - at least you have something to say you followed their design. Even better for you if they specify catalog numbers, as long as you installed what was specified.

Like I said.
I note it on the bid proposal what AIC it was quoted at with a price subject to change.

The Gear will have to go through the submittal process for approval.
If they approve it at the quoted AIC then I'm good with it.

If not, and the supply house can't get to the required AIC without a price increase, then I'm going to pass that additional charge on.

If the architects and engineers can't figure out what the AIC needs to be prior to bid day, why should we be expected too?

Jap>

 

[kwired]

Like I said.
I note it on the bid proposal what AIC it was quoted at with a price subject to change.

The Gear will have to go through the submittal process for approval.
If they approve it at the quoted AIC then I'm good with it.

If not, and the supply house can't get to the required AIC without a price increase, then I'm going to pass that additional charge on.

If the architects and engineers can't figure out what the AIC needs to be prior to bid day, why should we be expected too?

Jap>

I agree with that. How are contractors supposed to bid on plans that basically are not complete? Yet some will give a bid anyway, so that means you need to also if you want a chance at the project.

But we all make a lot of money all the time and it is always easy, right?
:happyyes:

 

[RICK NAPIER]

Here I don't have much of a problem requiring the AIC in advance of the installation the exception to this is when the utility hasn't decided on the transformer they are going to install in these situations the engineers often decide to engineer for the worse case scenerio and provide me a letter assuring the actual numbers prior to finals. Though the labeling is not required for dwelling units they still have to be rated for available AIC and have had sites that they had to use service equipment rated greater than 10,000.

 

[kwired]

Though the labeling is not required for dwelling units they still have to be rated for available AIC and have had sites that they had to use service equipment rated greater than 10,000.

Aren't nearly all loadcenters out there with factory installed main breakers using 22k mains and their branch breakers are series rated with that main?