Fault Current at the Utility Side

[Electriman]

Greetings,

To specify the panel board short circuit current rating (AIC), one thing that plays a big role is fault current at the utility side before the meter. I have always had hard time to find that information. I was wondering if there is a good assumption to consider a realistic value for fault current at the utility side. Is reaching out to utility service consultant and ask for this information my only option? Is there any set rules or rule of thumb that say the short circuit current at 120,240, 208 and 480 Volts single phase or three phase based on the service size?

Thanks for your input in advance

 

[kwired]

Greetings,

To specify the panel board short circuit current rating (AIC), one thing that plays a big role is fault current at the utility side before the meter. I have always had hard time to find that information. I was wondering if there is a good assumption to consider a realistic value for fault current at the utility side. Is reaching out to utility service consultant and ask for this information my only option? Is there any set rules or rule of thumb that say the short circuit current at 120,240, 208 and 480 Volts single phase or three phase based on the service size?

Thanks for your input in advance

It all starts with the size and impedance of the source transformer. Many calculations assume infinite ability from the supply to that transformer, but conditions there can have some impact. Then you have conductors between the transformer and your service equipment, those will have resistance and will lower available fault current at your service, how much depends on size and length, and maybe properties of raceway or cable enclosing them.
Kind of makes it almost a situation of no two installations will ever be the same.

 

[ggunn]

Greetings,

To specify the panel board short circuit current rating (AIC), one thing that plays a big role is fault current at the utility side before the meter. I have always had hard time to find that information. I was wondering if there is a good assumption to consider a realistic value for fault current at the utility side. Is reaching out to utility service consultant and ask for this information my only option? Is there any set rules or rule of thumb that say the short circuit current at 120,240, 208 and 480 Volts single phase or three phase based on the service size?

Thanks for your input in advance

In the Austin Energy jurisdiction there is a department I can call and ask what the available fault current is for any commercial service.

 

[Electriman]

It all starts with the size and impedance of the source transformer. Many calculations assume infinite ability from the supply to that transformer, but conditions there can have some impact. Then you have conductors between the transformer and your service equipment, those will have resistance and will lower available fault current at your service, how much depends on size and length, and maybe properties of raceway or cable enclosing them.
Kind of makes it almost a situation of no two installations will ever be the same.

The problem is that I don't know what size of transformer the utility installs to provide me service. I don't know any information of the transformer size, and short circuit impedance.

 

[Hv&Lv]

The problem is that I don't know what size of transformer the utility installs to provide me service. I don't know any information of the transformer size, and short circuit impedance.

it should be as simple as asking. I usually have two or three calls a month asking for that data. With modeling I just click on their “house” and the fault current is shown (along with other data).

 

[electrofelon]

For residential services it is very unlikely that the fault current exceeds 22k, and probably 95% of the time will be under 10k. But there will be exceptions.

Commercial can vary a lot. For example I have a client that has a 1000 amp 120/208 service in each of two buildings. One has 150kva of transformers. The other has 500kva of transformers, so that is more than three times the fault current right there all other things being equal.

Generally overhead banks won't exceed 3x167 units so that at least puts a cap on the transformer size if you know it is overhead. I have never seen a transformer lower than 1.8%Z so you could use that figure if you have to guess.

Three phase pads obviously can be much bigger than overhead banks, but their impedance seems to be a lot higher in my experience, more like around 5%.

One final thing to consider is it is possible the serving transformer is serving other loads and if that is the case it could be quite a bit larger than you think.

So there are some rules of thumb, but definitely best to ask.

 

[Electriman]

For residential services it is very unlikely that the fault current exceeds 22k, and probably 95% of the time will be under 10k. But there will be exceptions.

Commercial can vary a lot. For example I have a client that has a 1000 amp 120/208 service in each of two buildings. One has 150kva of transformers. The other has 500kva of transformers, so that is more than three times the fault current right there all other things being equal.

Generally overhead banks won't exceed 3x167 units so that at least puts a cap on the transformer size if you know it is overhead. I have never seen a transformer lower than 1.8%Z so you could use that figure if you have to guess.

Three phase pads obviously can be much bigger than overhead banks, but their impedance seems to be a lot higher in my experience, more like around 5%.

One final thing to consider is it is possible the serving transformer is serving other loads and if that is the case it could be quite a bit larger than you think.

So there are some rules of thumb, but definitely best to ask.

I like this. But how do I know what service transformer size the utility provide for this project? The disconnect size is 2500A. Can I assume utility will put a 1000 kVA transformer? It is an industrial project.

 

[electrofelon]

I like this. But how do I know what service transformer size the utility provide for this project? The disconnect size is 2500A. Can I assume utility will put a 1000 kVA transformer? It is an industrial project.

You really can't assume anything. In a perfect world, I find the POCO xformer capacity to be 40-50% of an NEC calc, however you could have a building changing use and ownership with reuse of what was there, a POCO sizing for motor LRA, etc.

 

[kwired]

I like this. But how do I know what service transformer size the utility provide for this project? The disconnect size is 2500A. Can I assume utility will put a 1000 kVA transformer? It is an industrial project.

Only if the real load is near that range. Nothing says you can't have 2500 amp of service but only about half loaded. POCO in that case will likely only have about a 500 kVA max if the load isn't there. If there are pretty definite plans for more load in the near future you may find larger. Customer probably paid for that up front in such cases as well.

 

[Bwas]

The problem is that I don't know what size of transformer the utility installs to provide me service. I don't know any information of the transformer size, and short circuit impedance.

For fault current calculations, assume infinite source on the primary side of the transformer and assume the transformer is sized big enough to carry the NEC calculated demand (or maybe one size smaller). I use 1.9%Z for transformers under 500kVA (lowest number I've ever seen on a transformer) and 5% for larger transformers. If you want better info than that, you have to get it from the utility or make some assumptions and model it yourself.

 

[powerpete69]

A good guess would be that the transformer will be sized at 50% of your connected load.

The long way (correct way) is too call your utility company and ask them for the three phase and single phase fault currents on the primary side of the transformer. (which you obviously can't do) They will also know all the transformer info if they are utility transformers. After you get short circuit info, you then enter them into a SKM or EasyPower software and make a model of your factory. Then short circuit that bus and look at the short circuit rating.

Short way, easy way, still correct, probably more conservative. Take the KVA of your transformer, 1000KVA for example. Get max secondary amps. Divide by impedance of transformer...this gives infinite bus short circuit which will work just dandy.
For example on 1000 KVA with 480V secondary with 5.75% impedance.
1000 E3/(480*1.73) = 1204 amps.
Divide by impendance for infinite short circuit bus calc on secondary of transformer. 1204/.0575 = 20,943 amps or 20.9kA.

 

[NewtonLaw]

The problem of obtaining the short circuit data from your local Utility can sometimes be daunting. It is however, the Utility's responsibility to provide you with the correct short circuit duty at the point of connection to your system. This is a legal requirement in every state I have worked in. When requesting this data from the Utility, make not of this requirement and request the actual AIC, not a cookbook answer that will be typically given from a local technician. In this day and age of Arc-Flash concerns, a ball park figure may actually give you the wrong answer for energy levels at the point of fault. This being said, yes, usually assuming an infinite primary side bus will give you a pretty good result, just no always. You need to satisfy yourself, since it is your reputation on the line, that you have provide a correct and reasonable answer to the question of AIC if that is your responsibility. Lawyers can be extremely good at pointing this type of stuff out when you thought you did eveerything in good faith.

As to getting a good ballpark figure, I worked as a Utility Engineer for 40 years and have performed the calculations required to give you this type of information. I can tell you that the Utilities today mostly use computer programs that follow the system design and can get this information fairly promptly. Getting past the IVR answering machine is the hardest part!!!!!!!!!!!!!!!!!!!!

Alternatives: Assume infinite primary bus. If it is an underground supplied system using a three phase pad mounted system, you may use the IEEE/ANSI standards for transformers made from 2010 on that lists typical transformer sizes and impedances. The list will also include pre 2010 transformers and typical impedance values. For example, a 1000 kVA three phase pad-mounted transformer will list a 5.75% typical nameplate impedance with a +/- 7.5% tolerance and an X/R ratio of 7.7. Pre 2010 the impedance is typically 5.3%. Measure the distance from the transformer to the service connection and add 10'. Determine the conductor size based on ampacity or aluminum secondary conductors (usually 750 MCM XLP Aluminum or 1000 MCM).

If an overhead transformer bank is used, get a set of binoculars and look at the nameplate attached to the mounting bracket of the transformer and obtain the %impedance and single phase transformer kVA. Measure the service wire length and determine the size conductor. Usually you will find it printed on the conductor insulation.

Hope this helps.