fault current

[bark]

How is one supposed to find the available fault current?

 

[templdl]

How is one supposed to find the available fault current?

Since you didn't elaborate on what the spplication was Start by asking the POCO.

 

[petersonra]

Since you didn't elaborate on what the spplication was Start by asking the POCO.

that will tell you what it is at the terminals of the POCO xfmr. Ohm's law will tell you what it is downstream.

 

[don_resqcapt19]

If you need the fault current for compliance with 110.24 the infinite bus fault current that the utility typically gives you will be fine and is the worst case for equipment selection.

However if you are doing arc flash studies, you want a more realistic available fault current...you don't want the infinite bus fault current.

 

[bark]

It's just a single phase 600 amp CT service with 3- 200 amp disconnects grouped together. Two of the 3 panels are about 330' away using 350MCM to feed them and the 3rd panel uses just 26' of 250 to feed it.

Thanks

 

[kwired]

How about we discuss a little basic information on what contributes to the available fault current.

First being the rated capacity of the source, a 10 VA transformer likely doesn't even come close to putting out the current that a 10 kVA transformer of same output voltage does, it just isn't designed to handle that kind of current.

Second is the internal impedance of the source. If you short circuit the load side terminals of the source your external circuit impedance is essentially zero, but the internal portion of the source still has some impedance that will limit how much current can flow.

Third comes the impedance of the conductors between the source and the point you want to know the maximum available fault current. Short runs of course will have less resistance then long runs of same size/type conductor allowing the current at your measure point to be higher.

The fourth thing that isn't always considered is the primary side abilities of a transformer or prime mover abilities on generation, or something of that nature. With utility distribution there is still the same concepts as mentioned above on the utility side of the transformer, but many times those are harder to get an accurate calculation on and can also change as upgrades or changes occur to the utility system, so we often assume the primary side of a utility transformer has infinite ability to get a worst case possible result. Also with a generator (which will have low fault current compared to utility as a general rule) you have limitations on the prime mover and small generators usually do not oversize the prime mover by all that much over what the generator rating is, load the generator you also load the prime mover and will typically will slow it down which will in turn reduce maximum current the generator will produce.

So for typical utility supplied application if you want to calculate available fault current you at least need to know the kVA rating of the transformer, the impedance of the transformer, the size, type and length of conductor between the transformer and the point you wish to calculate for at the very least. The calculation gets more complex and there are other things to consider but you need to understand some of these basics first or you will be lost.

There was (probably still is) a Excel spreadsheet on Mike Holt's site that you can download free that does the calculating for you, you just need to gather some of the information I mentioned above to put into it.

 

[don_resqcapt19]

Bussmann has a nice fault current app, but you still need info from the utility to start with. You need either the available fault current or the kVA and impedance of the transformer (infinite bus). Then you can build your network and see the available fault currents at each point on the network.

 

[templdl]

that will tell you what it is at the terminals of the POCO xfmr. Ohm's law will tell you what it is downstream.

That would require a fault current coordination study which commonly is done by a qualified person such as a PE.
What the OP asked was a loaded question which did not include any clue as to if it were residential, commercial, industrial facility, or?
What would be the simplest answer to a residential facility, would using ohms law be a used?

 

[templdl]

Bussmann has a nice fault current app, but you still need info from the utility to start with. You need either the available fault current or the kVA and impedance of the transformer (infinite bus). Then you can build your network and see the available fault currents at each point on the network.

With an industrial/commercial facility that is where I would start also based upon a transformer to also include a 100% motor contribution.

 

[don_resqcapt19]

With an industrial/commercial facility that is where I would start also based upon a transformer to also include a 100% motor contribution.

That is correct....the app has a provision to add motor contributions.

 

[petersonra]

That would require a fault current coordination study which commonly is done by a qualified person such as a PE.
What the OP asked was a loaded question which did not include any clue as to if it were residential, commercial, industrial facility, or?
What would be the simplest answer to a residential facility, would using ohms law be a used?

The fault current downstream is going to be whatever it is at the POCO xfmr terminals and gets reduced as it goes downstream because of the increased impedance mostly due to the resistance of the conductors. This is not something that requires any kind of study to calculate for most simple cases.

Granted, it can become convoluted if there are motor contributions and downstream transformers to consider, but even so, in most cases, it is not all that hard to make a reasonable estimate of the available fault current at any point in the system.

What is much more difficult, and requires at least someone with some experience and training, is to calculate the incident energy that might be available at a point downstream. But they are two completely different and almost unrelated calculations.

 

[templdl]

The fault current downstream is going to be whatever it is at the POCO xfmr terminals and gets reduced as it goes downstream because of the increased impedance mostly due to the resistance of the conductors. This is not something that requires any kind of study to calculate for most simple cases.

Granted, it can become convoluted if there are motor contributions and downstream transformers to consider, but even so, in most cases, it is not all that hard to make a reasonable estimate of the available fault current at any point in the system.

What is much more difficult, and requires at least someone with some experience and training, is to calculate the incident energy that might be available at a point downstream. But they are two completely different and almost unrelated calculations.

When I go back to the OP and read the responses aren't we shooting at shadows yet again? With no aditional input from the OP regarding what the actual intent is when will the question be answered with any satisfaction?
I always have recommended that a worst case scinereo be considered based upon the KVA of the transformer, an unlimited fault current capability from the POCO, and a 100% motor contribution and then see how much dodo you are in with the down sream devices. If things don't look good get a coordination study done by a quilified person to see if you get lucky. If not so lucky the downstream devices must be replaced in order to meet the available fault current based upon the study. Either a fully raterd or posibly series rated devices may be considered.
But, is this the concern of the OP? Does anybody know?

 

[don_resqcapt19]

...
But, is this the concern of the OP? Does anybody know?

I don't really know, but my guess would be his concern is 110.24.

 

[bark]

It's just a 2400 sq foot office trailer. The largest load is the heat. I think it said the heat pump whih hangs on the outside wall has a locked Rotor amp of 75 amps

 

[bark]

The transformer on the pole is 37.5 kva. I just want this calc to beable to put the info on a plaque on the service. Just to satisfy the code.

 

[petersonra]

The transformer on the pole is 37.5 kva. I just want this calc to beable to put the info on a plaque on the service. Just to satisfy the code.

These days most POCOs have such information readily available, often on their web site. The numbers they give are the worst case at the output terminals of their xfmr, but it is good enough to use at your service point, unless it is so close to the rating of something that you need to look closer.

 

[kwired]

The transformer on the pole is 37.5 kva. I just want this calc to beable to put the info on a plaque on the service. Just to satisfy the code.

Now you need transformer impedance and then you will at least be able to tell what available fault current is at the transformer terminals. Find conductor sizes, type, length and type of raceway, and the Excel calculator I mentioned earlier will make this one fairly easy to come up with a number.

If you can't get transformer impedance one that size will likely be between 1.5 and 2.0 percent, lower value gives you worst case scenario in the results.

Unless your equipment is within 50 feet of transformer I doubt the number will be over 10kA that your breakers are likely rated for, but of course we now still need to come up with a label that states actual available current per the mentioned 110.24.

 

[bark]

The 3 mains are 25000kv and the main breakers for the remote panels are also 25000kv. Can't find the fault currents on their web site

 

[kwired]

The 3 mains are 25000kv and the main breakers for the remote panels are also 25000kv. Can't find the fault currents on their web site

25kA?

25000kV is 25,000,000 volts.

Is fairly common to have 22-25 kA main breakers in the average "loadcenter" and 10 kA branch breakers that are series rated with the main that is used.

Can't find fault current on what web site?

 

[templdl]

Wouldn't a simple call to the POCO provide the available fault current?