Calculating available fault current.

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steve66

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Location
Illinois
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Engineer
Are they worried about the meter cans, or about the rating of the breakers after the meter cans?

Obviouslly, if they have a paralleled network of xformer secondaries, they couldn't expect everyone to have less than 10KAIC at the meters.

I think they may just be wanting a series rated listing between your 400A breakers, and the breakers in the meter cans.

If thats true, your 400A breakers would just need an AIC large enough for the calcualted available current (I think I saw 22KAIC?), and they would have to be series listed with the breakers in the meter cans.

That should be very easy to do.
 

dan1973ct

Member
The local utility wants me to show that there will be no more than 10,000 AIC at their meter because that is the rating of the meter that they would be installing. I would think the way to go is to do an accurate calculation with all the variables and if I did not meet the required 10,000 or under then I should redo the calculation with current limiting fuses in the meter can tap box. And hopefully that would work. I'm not an engineer but that makes the most sense to me. I just don't have the software or knowledge. And both of the engineers I hired for this can not get me the answers I need. I appreciate all the help.
 

don_resqcapt19

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Location
Illinois
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retired electrician
... I would think the way to go is to do an accurate calculation with all the variables and if I did not meet the required 10,000 or under then I should redo the calculation with current limiting fuses in the meter can tap box. And hopefully that would work. I'm not an engineer but that makes the most sense to me. I just don't have the software or knowledge. And both of the engineers I hired for this can not get me the answers I need. I appreciate all the help.
You just can't use current limiting fuse or breakers to do that. You can only get the available current at the meters down by decreasing the source current or increasing the line side impedance.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
The local utility wants me to show that there will be no more than 10,000 AIC at their meter because that is the rating of the meter that they would be installing. I would think the way to go is to do an accurate calculation with all the variables and if I did not meet the required 10,000 or under then I should redo the calculation with current limiting fuses in the meter can tap box. And hopefully that would work. I'm not an engineer but that makes the most sense to me. I just don't have the software or knowledge. And both of the engineers I hired for this can not get me the answers I need. I appreciate all the help.
Meters do not have an AIC rating, they have either a withstand rating or a SCCR.

The utility is asking you to ensure that the available fault current does not exceed 10kA or for you to provide equipment with a 'series combination rating' that will allow the use of a standard revenue meter.

First, you have a limited selection of group mounted circuit breakers rated for 200kA @ 208V, so you may need to consider using fuses instead.
Second, each meter center needs to be rated for the available fault current at its location, and not the 200kA available at the transformer. Metering equipment with UL listed series combination SCCR up to 100kA is readily available.
 

iwire

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Location
Massachusetts
Jim I have said it before and I will say it again .......... your knowledge of the available equipment for our trade is outstanding.:cool:

Thanks for your contributions here at Mike's!:grin:
 

don_resqcapt19

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Location
Illinois
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retired electrician
...
The utility is asking you to ensure that the available fault current does not exceed 10kA or for you to provide equipment with a 'series combination rating' that will allow the use of a standard revenue meter. ...
Jim,
How would a series combination system protect the meter if the fault is between the meter and the downstream breaker?
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Jim I have said it before and I will say it again .......... your knowledge of the available equipment for our trade is outstanding.:cool:

Thanks for your contributions here at Mike's!:grin:

amen
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Jim,
How would a series combination system protect the meter if the fault is between the meter and the downstream breaker?

It looks like it does depend on what meter socket you use. Square D individual meter sockets list only a 10KA SCCR, but notes "higher with utility approval". I guess that gives the utility some say on what provides enough protection for the meter socket.

Square D EZ meter packs have SCCR ratings listed up to 100KA listed on page 2-9 of Catalog 174. It looks like the upstream breaker is enough to protect these meter sockets.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Jim,
How would a series combination system protect the meter if the fault is between the meter and the downstream breaker?
Aah, the beauty of tested 'series combinations'.

Ooh, the problem with 'series combinations'.


I believe it has to do with the possibility of the bolted fault occuring in the few inches of conductor between the meter socket and its all but 'integral' breaker.

Meter centers are a unique application of series ratings because they may involve a combination of three protective devices, a meter socket, integral conductors, and external wiring.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
The utility is asking me to ensure that the available fault current does not exceed 10kA at their meters.
You are out of luck, as it appears the utility does not understand how electrical equipment is designed, tested and applied.

There is only one way to limit the fault current - add impedance.
Method one, add more conductor length.
Method two, add current limiting reactors.
 

tkb

Senior Member
Location
MA
Mike H also has a fault current calculations book in his free stuff that I learned from to get the original excel file corrected to the present 7.1 version.
http://www.mikeholt.com/documents/freestuff/faultcurrent.zip

I checked out this falut current file and noticed that he has a power factor for the transformer.
I didn't think transformers had power factors.

I get the same result on my Excel calculations using the Bussman Point to Point calculation.
They have you use a 90% factor in the calculation but they describe it as,
††Transformer %Z is multiplied by .9 to establish a worst case condition. See Note 3.

Note 3:

The marked impedance values on transformers may vary ?10% from the actual values determined by ANSI / IEEE test. See U.L. Standard 1561. Therefore, multiply transformer %Z by .9. Transformers constructed to ANSI standards have a ? 7.5% impedance tolerance (two-winding construction).
Is this a pf or what Bussman describes and should not be a variable.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
3-phase 3-winding transformers can have a 10% tolerance in the stated impedance when specifying. 3-phase, 2-winding have a 7.5% tolerance. All this means is that if you specify a 5.75% impedance, the actual value determined by test can be 5.75% +/- 7.5% when delivered. So your actual impedance could end up being 5.35%, which could increase the fault current.

This manufacturing tolerance should not be used when doing a calculation where the actual impedance is known, i.e. nameplate.

I have not heard of a transformer having a power factor and ANSI Standard C57.12.90 does not endorse any fixed or standard percentage power factor level
for transformers. According to ANSI C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers, a factory power-factor test will be of value for comparison with field power-factor
measurements to assess the probable condition of the insulation. It is not feasible to establish standard power-factor values for liquid-immersed transformers.

As I stated earlier, I believe your only real option is to use a current limiting reactor, which Jim also confirmed. Longer cable lengths would not be realistic.
 

iwire

Moderator
Staff member
Location
Massachusetts
Besoeker, You may be "misssing" the possibility that the transformers are part of a POCO grid as Jim notes. In our larger cities it is fairly common to have 200ka + where the POCO grid consists of several transformers paralleled

Very common in this area, but also a constantly changing target.

I was working with Connecticut light and power in Stamford CT and they were tying together more more transformers into a system at will to try to get the voltage back up.
 

Besoeker

Senior Member
Location
UK
Besoeker, You may be "misssing" the possibility that the transformers are part of a POCO grid as Jim notes. In our larger cities it is fairly common to have 200ka + where the POCO grid consists of several transformers paralleled
It might be but this from post #13 suggests just two:
They are using two 500 KVA transformers
The diagram gives the service as 2500A which is reasonably consistent with two 500 kVA transformers in parallel.
And the opening posts mentions 200kA at the load side.
If the system is configured like that, then a 50kA fault level would be in the ball park.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
It might be but this from post #13 suggests just two:

The diagram gives the service as 2500A which is reasonably consistent with two 500 kVA transformers in parallel.
And the opening posts mentions 200kA at the load side.
If the system is configured like that, then a 50kA fault level would be in the ball park.

They may be using two today, but they may be requiring the OP to designing for the future. most of the POCO's, in my area, require service equipment to be rated based on a possible worst case. For example, if they were to experience a transformer failure, they may want to install larger units.
 

dan1973ct

Member
UI did say that there was room in the vault for a third transformer. I have to figure for what they told me 200,000. If not they won't give me power. I realize if I don't reach the 10,000 AIC at the meters I will have to find a solution. And a current limiting reactor might be the way to go. At this point I am just trying to figure out what my available fault current is at the meters right now and then if I need to I can look into solutions.
 

Besoeker

Senior Member
Location
UK
They may be using two today, but they may be requiring the OP to designing for the future. most of the POCO's, in my area, require service equipment to be rated based on a possible worst case. For example, if they were to experience a transformer failure, they may want to install larger units.
True. But...
If, as dan1973 says, there has to be room for a third transformer, that would then be 75kA ignoring source impedance. To get 200kA would require three transformers each three times the rating of the 500kVA units. If it's designed to accommodate three 500kVA units, would it be reasonable to expect it to have enough space and cooling for three at 1500kVA?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
UI did say that there was room in the vault for a third transformer. I have to figure for what they told me 200,000. If not they won't give me power. I realize if I don't reach the 10,000 AIC at the meters I will have to find a solution. And a current limiting reactor might be the way to go. At this point I am just trying to figure out what my available fault current is at the meters right now and then if I need to I can look into solutions.

Based on 200KAIC, 10' of (7) 500KCMIL conductor/ph, and assuming that you would load out your service to 2500A and of that 20% is motor contribution, then you would need reactors rated 2500A, 208V, 0.017ohms/phase. That would limit you to approx. 9200A. Using 0.02ohms/phase you would see about 8200A at the gear. Without reactors your looking at about 176,100A at the gear.

If you give the particulars to an equipment supplier they will help you. Try TRENCH they are very reputable.

But, on a 2500A service, the meter is going to be fed from CT's, which means the fault current on the meter is essentially 5A (2500A/5A CTs) and therefore the bigger concern would be the gear itself.

Eaton does make a switchboard that when using a bolted pressure switch with current limiting fuses is rated for 200kAIC at 240V. (Pow-R-Line C switchboard)
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
But, on a 2500A service, the meter is going to be fed from CT's, which means the fault current on the meter is essentially 5A (2500A/5A CTs) and therefore the bigger concern would be the gear itself.

Because there are multiple meter centers on this project, not a single 2500A one, it may be better to provide several smaller reactors.
 
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