Available Fault Current

[jjefferson]

NEC 110.24 Service equipment shall be marked with the maxium available fault current. The power company will supply the fault current at the secondary of the transformer, how is the fault current calculated at the service equipment?

 

[texie]

NEC 110.24 Service equipment shall be marked with the maxium available fault current. The power company will supply the fault current at the secondary of the transformer, how is the fault current calculated at the service equipment?

You will have to perform an available fault current calculation based on the size, type, length, etc. Or you could just use the value supplied by the POCO and be covered as that will be a higher number than you come up with at the load end. If you have reason to think the result will be a lot lower at the service due to, say, long length and your gear AIC rating is in less than the POCO value than you will have to have a calculation of course to be sure you are in compliance.

 

[augie47]

The actual number can be very job dependent. As texie points out, the characteristic of the service conductors will play a major role in reading the figure at the transformer. On the other hand, there could be factors such as motor contribution that could increase the fault current at the gear.
There are programs available that will allow you to make a quick guesstimate as to how the service conductors will effect the number.
In a vast majority of the jobs I see the raw POCO number is used. If that number results in a higher gear costs, calculations are often performed. On larger jobs, engineering documentation that takes all factors into is often needed.

 

[texie]

The actual number can be very job dependent. As texie points out, the characteristic of the service conductors will play a major role in reading the figure at the transformer. On the other hand, there could be factors such as motor contribution that could increase the fault current at the gear.
There are programs available that will allow you to make a quick guesstimate as to how the service conductors will effect the number.
In a vast majority of the jobs I see the raw POCO number is used. If that number results in a higher gear costs, calculations are often performed. On larger jobs, engineering documentation that takes all factors into is often needed.

Yes, I should have noted that other factors such as motor contribution could be an important factor in some cases. In my view this is normally best left to an EE to do these calcs.

 

[cowboyjwc]

Here in our city we don't have very high AIC on the utility side, usually between 42-30K. After the transformer and with some run it's usually at the buiding at around 22-10K or less.

Now I've heard that places like Long Island that the AIC can be as high as 100K.

 

[kwired]

Here in our city we don't have very high AIC on the utility side, usually between 42-30K. After the transformer and with some run it's usually at the buiding at around 22-10K or less.

Now I've heard that places like Long Island that the AIC can be as high as 100K.

Available fault current on primary side of a transformer does have some impact on available fault current on secondary side, but that in no way means a particularr transformer will be limited to similar AIC as some smaller transformer. Transformer size and impedance are still biggest factors in available fault current, should the fault cause a drop in voltage on the primary side then you will see a reduction in available current on the secondary.

Most simpler voltage drop calculations assume infinite capability of the supply side of the transformer.

 

[jim dungar]

Here in our city we don't have very high AIC ...

Transformers, and other sources produce Short Circuit Amps SCA. You can do things to change the SCA (e.g. add impedance).
Protective devices have a tested Amps Interrupting Capacity. You cannot do anything to change an AIC.
Equipment has an overall Short Circuit Current Rating (SCCR). It may be possible to change an SCCR (e.g replace one AIC with another)


For some reason it seems many designers never determine the available SCA, specify an AIC based only on voltage and 'electrical distance' from the service entrance, and totally ignore SCCR.

 

[cowboyjwc]

Available fault current on primary side of a transformer does have some impact on available fault current on secondary side, but that in no way means a particularr transformer will be limited to similar AIC as some smaller transformer. Transformer size and impedance are still biggest factors in available fault current, should the fault cause a drop in voltage on the primary side then you will see a reduction in available current on the secondary.

Most simpler voltage drop calculations assume infinite capability of the supply side of the transformer.

You are correct, and the bad thing is that you could have designed your system on a 20 year old spec and then they come out and install a new low impedence transformer later on and your calcs are not longer good. I mean unless they are already down in the 2-3K range.

The utility here will give you the AIC at the building since most of the time, here all of that is utility owned.

 

[kwired]

You are correct, and the bad thing is that you could have designed your system on a 20 year old spec and then they come out and install a new low impedence transformer later on and your calcs are not longer good. I mean unless they are already down in the 2-3K range.

The utility here will give you the AIC at the building since most of the time, here all of that is utility owned.

True, but unless you have designed things to operate at the edge of the rating or if transformer is replaced with higher capacity, chances are a similar capacity replacement transformer will not have too significant of a change, unless maybe the service or feeder is very short length to start with.

I will add that if a change of service is made then new calculations are generally going to be needed, unless service conductors and transformer remain the same for some reason.