AIC Calculations & It's Significance

[Samardas]

I am designing an 800A, 240V, 3 Phase service landing on the 800A Main Switchboard (MSB). I only have a 200A panel for now coming off of the MSB. The SCA calculation at the entry to the panel shows about 35,000A. If I ask for 35K or 40K AIC rated circuit breakers in the panel, it becomes cost prohibitive. The standard circuit breakers are 10K AIC. Any suggestion shall be highly appreciated, Thanks a LOT.

 

[topgone]

I am designing an 800A, 240V, 3 Phase service landing on the 800A Main Switchboard (MSB). I only have a 200A panel for now coming off of the MSB. The SCA calculation at the entry to the panel shows about 35,000A. If I ask for 35K or 40K AIC rated circuit breakers in the panel, it becomes cost prohibitive. The standard circuit breakers are 10K AIC. Any suggestion shall be highly appreciated, Thanks a LOT.

The conductors you use from the panel will surely drop the fault current level. Ex. you use 3X 3/0 Al for the conductor run and the length is 20 meters, your fault current will drop from 35kA down to 26kA.

 

[Jraef]

The conductors you use from the panel will surely drop the fault current level. Ex. you use 3X 3/0 Al for the conductor run and the length is 20 meters, your fault current will drop from 35kA down to 26kA.

You are assuming that was not already part of the calculations that put it st 35kA.

Something is only "cost prohibitive" if there is a viable alternative. It might be possible to use a series rated panel where the main is current limiting and the feeders below it are individually lower rated, talk to your suppliers for options. But other than that, it just is what it is. If your SCA is 35kA, you must use a system rated 35 kA.

 

[ron]

Can you run the 200A feeder a little longer and drop the short circuit current down to 10kA?

The 200A panel short circuit rating must be equal to or greater than the calculated available fault current at the end of that feeder terminating in the panel.

 

[drktmplr12]

not usually much you can do about available fault current except specify equipment that can withstand it.

one alternative is to place 200A Class RK current limiting fuses upstream of the panel. If you are anticipating 35k A of fault current, a 200 A fuse will let through about 14-15k A RMS sym according to the peak let thru chart. consider if 22k rated breakers make the fuses worth it-or find a more sensitive time-delay fuse that can bring available fault current down below 10k.

like jraef says, it is what it is. if the owner doesn't like the options they can take it up with the POCO and see how far they get. my guess is not very.

 

[ron]

one alternative is to place 200A Class RK current limiting fuses upstream of the panel. If you are anticipating 35k A of fault current, a 200 A fuse will let through about 14-15k A RMS sym according to the peak let thru chart. consider if 22k rated breakers make the fuses worth it-or find a more sensitive time-delay fuse that can bring available fault current down below 10k.

That only helps if you can find a series rated tested combination by the manufacturer.

The old fuse let-through charts are no good anymore now that code requires tested combinations, other than control panel SCCR.

 

[smoothops10]

Are you sure 800A, 240V, 3 Phase is what your designing? No utilility my side of the country would offer that. You could get 800A 120/208 3Ph. A few offer 120/240 hi leg open delta but only up to 200A. I'd verify that first.

 

[Ingenieur]

can you post the single line?
what is the util xfmr kva, pu Z?
3 x 112.5's

 

[mayanees]

not usually much you can do about available fault current except specify equipment that can withstand it.

one alternative is to place 200A Class RK current limiting fuses upstream of the panel. If you are anticipating 35k A of fault current, a 200 A fuse will let through about 14-15k A RMS sym according to the peak let thru chart. consider if 22k rated breakers make the fuses worth it-or find a more sensitive time-delay fuse that can bring available fault current down below 10k.

240.86 covers use of an underrated device.
The OP should find a fuse that's series-rated with the intended breaker. 240.86(B)
The 240.86(A) alternative is for existing installations and the difficulty is in satisfying the condition statement for the breaker - that it remain passive during the line side fuse opening.

 

[ron]

The 240.86(A) alternative is for existing installations and the difficulty is in satisfying the condition statement for the breaker - that it remain passive during the line side fuse opening.

And in most cases in my experience, getting a PE willing to sign/seal the document. Not me!

 

[Samardas]

AIC Calculations & It's Significance

Thank you ALL Very Much for your valuable advice. Here is the Single Line Diagram for the same. The MSB and the Panel are 65K rated. I usually do not come across 240V, 3 Phase open Delta configuration that often. This is from Department of Water & Power, Los Angeles, where they want to provide 240V, 3 Phase, 4 Wire service for this facility. Thanks & Regards.

 

[Ingenieur]

based on the i fault provided by the utility looks like you have 2 choices
use mitigation techniques listed in this thread
bite the bullet and buy the rated aic panel

I would buy the panel

 

[smoothops10]

Interesting, thanks for sharing. You sparked my curiosity and you're right - they do offer 120/240 3 Ph 4W up to 800A and even a 240V 3Ph 3W.

2. SERVICE SUPPLY VOLTAGES [...]
c. 240/120 VOLTS, 3-PHASE, 4-WIRE
(1) This voltage supplies single-phase and three-phase loads through the same metered service and
is normally limited to a service capacity of 800 amperes. Consult the Department when the
proposed service exceeds 800 amperes.
(2) The service neutral is center-tapped and grounded at the Department's transformer and the
"Power Leg" measures 208 volts-to-ground.
(3) Where service is supplied from dedicated transformers located on the customers premises, the
Department may require the service to be supplied with a four-wire wye voltage. See items 5 and
6.
d. 240 VOLTS, 3-PHASE, 3-WIRE
This voltage is generally not available for new services. New services may be permitted where an
existing premises is already supplied at this voltage. However, the customer is required to consult the
Department when planning increases in load or service ampacity to an existing service supplied at this
voltage. Such changes may require conversion to a 4-wire service.
Note: Single-phase loads connected to a 3∅ 3-wire service shall be reasonably balanced. The
customer’s system power factor (measured at the meter) shall not be less than 0.5 lagging.

http://www.kallin.com/ParkDrive/DWP Code.pdf

They don't actually say it is open delta but I have to imagine it is. If they had 3Ph distribution there would be no need to use a delta and you'd think they would offer you a 120/208 3Ph or 277/4703Ph 4W Wye.

I realize your drawing is preliminary but one thing to heed warning on is it looks like you are using the B phase for line neutral loads in the panelboard schedule. There will be a high leg at 208V to ground not 120V to ground. Utility will probably want you to land this on the C phase in their meter but downstream of that NEC will require it be the B phase. You will have to skip B phase for all single phase loads in the panelboard schedule and probably put warning sticker over each space so they don't install 1P breakers in B phase in the future.

That is too bad you have such high fault current. You're correct about 10kAIC standard. MFR's will offer 'high interrupting' 240V breakers probably only rated for 22kAIC. You will likely have to spec a panelboard for 277/480V system with breakers that have a 35kAIC rating at 480V and 65kAIC at 240V and pay that premium.

 

[steve66]

A 35 or 40K rating doesn't seem like it should add that much cost to a project.

You can get series rated breakers, if they are all the same brand. For example, the 800 A breaker might be rated for 40KAIC, and the 200 A might be a series listed combination up to 40K even if the 200A breaker is rated lower. Your supplier should be able to let you know what is available.

Likewise, you might be able to series rate the smaller panel breakers with the 200A breaker.

 

[smoothops10]

It's a good point and probably a viable solution in this case. I've always been steered away from the series rated combinations because of 240.86(C). If his subpanel has 10kAIC breakers series rated to 40kAIC like in your example the panelboard is limited to supplying no more than 1% of the lowest AIC in the system or 100A. Is it likely that subpanel does not carry more than 100A motor load in this scenario? Yes probably but in many cases no. This is why I've stayed away from the combinations but then again I'm not paying for it.

 

[electrofelon]

I am designing an 800A, 240V, 3 Phase service landing on the 800A Main Switchboard (MSB). I only have a 200A panel for now coming off of the MSB. The SCA calculation at the entry to the panel shows about 35,000A. If I ask for 35K or 40K AIC rated circuit breakers in the panel, it becomes cost prohibitive. The standard circuit breakers are 10K AIC. Any suggestion shall be highly appreciated, Thanks a LOT.

The fault current value of 56,000 at the utility transformer is likely not realistic. Perhaps you can try to get the actual transformer impedance and request a change? Last time I did that, the utility engineer sent me a picture of the actual transformer date plate, and the fault current was a third of what he had supplied me initially, even assuming infinite primary.

I have a hard time believing that it is "prohibitive" to get a 200A panelboard with 35KAIC. Perhaps that person didnt understand series ratings. I would get a second price quote.

 

[Ingenieur]

800 x 240 x sqrt = 332 kva

assume they him 3 x 75 kva
Z will be ~1.5% xfmr
i fault 36 ka

3 x 100 kva
48 ka

Z could be lower or kva higher
surprising how low util xfmr Z cab be
the do not like losses and do not care about the mag of i fault on your end

this will give you an idea of how low they can go
and they use the worse case
https://www.fs.fed.us/database/acad/elec/greenbook/10_shortcalc.pdf

 

[electrofelon]

800 x 240 x sqrt = 332 kva

assume they him 3 x 75 kva
Z will be ~1.5% xfmr
i fault 36 ka

3 x 100 kva
48 ka

Z could be lower or kva higher
surprising how low util xfmr Z cab be
the do not like losses and do not care about the mag of i fault on your end

this will give you an idea of how low they can go
and they use the worse case
https://www.fs.fed.us/database/acad/elec/greenbook/10_shortcalc.pdf

Of course we are both conjecturing, there is no universal rule of thumb here. I never see transformers with very low impedance here. Not sure why. I see 75 KVA pole units being in the high 2's. This is why I often rant about theses charts that have "unrealistic" Z values. Perhaps they are better fitting in other utilities or geographic areas.

I still say the OP is not getting an accurate figure though. Your 35K figure is good if they are indeed using a low Z transformer, but note that plan states 56K at the transformer. No way.

 

[Ingenieur]

Of course we are both conjecturing, there is no universal rule of thumb here. I never see transformers with very low impedance here. Not sure why. I see 75 KVA pole units being in the high 2's. This is why I often rant about theses charts that have "unrealistic" Z values. Perhaps they are better fitting in other utilities or geographic areas.

I still say the OP is not getting an accurate figure though. Your 35K figure is good if they are indeed using a low Z transformer, but note that plan states 56K at the transformer. No way.

some are as low as 1%
if he has 3 x 112.5 kva at 1.5% i fault is 54 ka

they base this number on
lowest Z of a possible replacement inventory xfmr
the max xfmr kva they may use
system fault mva

he likely has a letter from them
I doubt the will lower based on a request

 

[electrofelon]

some are as low as 1%
if he has 3 x 112.5 kva at 1.5% i fault is 54 ka

they base this number on
lowest Z of a possible replacement inventory xfmr
the max xfmr kva they may use
system fault mva

he likely has a letter from them
I doubt the will lower based on a request

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.