Max Available fault short circuit current

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Jpflex

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
If finding and labeling with calculation date of available fault current for service equipment other than dwellings is required in 2020 - 2023 NEC Code, then why is this formula or process not available in current or former electrical calculation books.

I checked all over online, eBay, Barnes and nobles, uglies electrical calculation reference - doesn’t include fault current to conductor termination point)

Some online formulas vary from another and there doesn’t seem to be an agreed set formula. One says you need transformer specs such as impedance and other formulas don’t.

Where can you find a book on electrician formulas including max available fault current?
 
In general, you need to get the starting point from the utility. However if you know the impedance of the utility transformer, you can do an infinite bus calculation based on the KVA and impedance of the transformer.
One of the easiest ways to do the calculations, either starting from the fault current the utility gives you or from a transformer is to use Bussmann's FC³ app. It is free and works great.
 
don_resqcapt19 said:
In general, you need to get the starting point from the utility. However if you know the impedance of the utility transformer, you can do an infinite bus calculation based on the KVA and impedance of the transformer.
One of the easiest ways to do the calculations, either starting from the fault current the utility gives you or from a transformer is to use Bussmann's FC³ app. It is free and works great.
Click to expand...
NEC testing for journeyman’s licensing won’t allow phone apps or software to do your calculations. One of the reasons I could not calculate available fault current for the company I work for was because of too many variables, such as not having utility transformer impedance or label specs
 
With the app and other programs available the :"book form" is rarely used but here is a link to Bussmann's:
I have taken a ton of NEC/Inspector tests and rarely been asked to perform any fault current calculations..
Never on any of the journeyman's tests I've taken....
.. On those rare occassiosn when there was a calculation requiredd I was given the info needed to perform that calculation.

As far as your particular installation, the power company should be able to give you the available current at their transformer of service point,.
Occasionally you have to find the right POCO engineer.
 
Jpflex said:
NEC testing for journeyman’s licensing won’t allow phone apps or software to do your calculations. One of the reasons I could not calculate available fault current for the company I work for was because of too many variables, such as not having utility transformer impedance or label specs
Click to expand...
The calculation is very simple, for the highest (worst case) value as it is basically amps and resistance, or the percentage of resistance. The big problem is getting the correct starting information.
 
I have had this discussion with an inspector, its usually pretty short and ends with the start at the poco. I figured I might learn a little so I knew what I was looking at but its overhead.
 
Sberry said:
I have had this discussion with an inspector, its usually pretty short and ends with the start at the poco. I figured I might learn a little so I knew what I was looking at but its overhead.
Click to expand...

When you first learn electricity, you consider ideal voltage sources, such that if you would short-circuit them, the short circuit will draw infinite current. Of course, this doesn't really happen, as all voltage sources have an internal impedance in some form or another.

The available fault current is directly determined by what that impedance is, along with the voltage of the source. So every real-world conductor along the journey from the source of power to the point of a possible fault contributes to this impedance, and reduces the maximum possible short circuit current.

Transformers have a group of formulas that tells you how fault current propagates from primary to secondary, and I'll refer you to this discussion for more details. If you graph the result as a function of the primary fault current, you'll notice that it approaches a limiting value as the primary fault current gets large. This is what is called the infinite bus fault current, that assumes infinite fault current on the primary, and gives you a worst-case scenario secondary fault current, which equals the amps associated with the KVA value, divided by the impedance as a decimal.

Fault current calculations for transformers with Sub-600V on both sides

I'm familiar with calculating KAIC from the fault current calculation spreadsheet available on this website. However, I'm surprised to notice no intake for the voltage on the primary side of the transformer. I know this is geared toward applications where this would be medium voltage and...
forums.mikeholt.com
 
I used the method in firms finder about 35 years back. Uglys should have. Busman had in catalogs, today it’s all moved to an app
 
Jpflex said:
NEC testing for journeyman’s licensing won’t allow phone apps or software to do your calculations. One of the reasons I could not calculate available fault current for the company I work for was because of too many variables, such as not having utility transformer impedance or label specs
Click to expand...
I though you were asking about real world applications and not testing applications. There are differences as you pointed out.

If you can't get the available fault current or the transformer information from your utility, you cannot do fault current calculations for a service supplied system. You have to have some base to start with. If you don't have this information, you cannot even select your service equipment as you don't know what fault current it must be able to withstand.
 
Carultch said:
When you first learn electricity, you consider ideal voltage sources, such that if you would short-circuit them, the short circuit will draw infinite current. Of course, this doesn't really happen, as all voltage sources have an internal impedance in some form or another.

The available fault current is directly determined by what that impedance is, along with the voltage of the source. So every real-world conductor along the journey from the source of power to the point of a possible fault contributes to this impedance, and reduces the maximum possible short circuit current.

Transformers have a group of formulas that tells you how fault current propagates from primary to secondary, and I'll refer you to this discussion for more details. If you graph the result as a function of the primary fault current, you'll notice that it approaches a limiting value as the primary fault current gets large. This is what is called the infinite bus fault current, that assumes infinite fault current on the primary, and gives you a worst-case scenario secondary fault current, which equals the amps associated with the KVA value, divided by the impedance as a decimal.

Fault current calculations for transformers with Sub-600V on both sides

I'm familiar with calculating KAIC from the fault current calculation spreadsheet available on this website. However, I'm surprised to notice no intake for the voltage on the primary side of the transformer. I know this is geared toward applications where this would be medium voltage and...
forums.mikeholt.com
Click to expand...
I will read more but I get the idea that the bigger the wire and the shorter the path the higher the available current. Its a general concept I kind of get.
I think it can be on a way smaller scale,, Not a true fault but current rise. I told the helper to run 12 to my chop saw circuit,,, but more is better. Takes adding 25 ft of 16 and even then wants to trip up 20
 
This
don_resqcapt19 said:
I though you were asking about real world applications and not testing applications. There are differences as you pointed out.

If you can't get the available fault current or the transformer information from your utility, you cannot do fault current calculations for a service supplied system. You have to have some base to start with. If you don't have this information, you cannot even select your service equipment as you don't know what fault current it must be able to withstand.
Click to expand...
is what I would like to know more about. How do you choose service equipment based on max fault current or overcurrent devices based on max interrupting current.

My company has a pile of junk cabinets and switches but all I can do do is try to match voltage, phases fir projects
 
As
Jpflex said:
This

is what I would like to know more about. How do you choose service equipment based on max fault current or overcurrent devices based on max interrupting current.

My company has a pile of junk cabinets and switches but all I can do do is try to match voltage, phases fir projects
Click to expand...
As far as your company goes, I'm assuming it's a pad mount transformer. How many KVA? Delta or Wye? What service voltages? What size feeders? How far of a run from the transformer to the first OCPD?

And where in the facility are you concerned with fault current? Are there additional transformers (step down) involved in the circuit?

Lots of factors

Some places breaker selection based on available fault current is super critical such as the first OCPD, or for a large motor (motors can contribute to fault current) . Other places like a breaker for a receptacle branch circuit in the break room require as much thought as a homeowner buying a breaker at Home Depot.

As far as your boss having you pick through piles of used equipment, that's fine if you have the right equipment, but not so good if it's being utilized above it's ratings.

Why do I get the feeling this is scrap metal processing or mining?
 
As far as your real life application, as don states, there is little you can do until you get enough info to determine the available current from your POCO transformer.
In selecting the equipment from your scrap, this video from SquareD explains fully rated vs series rating:
 
Joethemechanic said:
As

As far as your company goes, I'm assuming it's a pad mount transformer. How many KVA? Delta or Wye? What service voltages? What size feeders? How far of a run from the transformer to the first OCPD?

And where in the facility are you concerned with fault current? Are there additional transformers (step down) involved in the circuit?

Lots of factors

Some places breaker selection based on available fault current is super critical such as the first OCPD, or for a large motor (motors can contribute to fault current) . Other places like a breaker for a receptacle branch circuit in the break room require as much thought as a homeowner buying a breaker at Home Depot.

As far as your boss having you pick through piles of used equipment, that's fine if you have the right equipment, but not so good if it's being utilized above it's ratings.

Why do I get the feeling this is scrap metal processing or mining?
Click to expand...
It’s not a scrap metal processing plant but it is a mine as you have properly guessed. The junk piled supplies can have rusted out contacts in starters and equipment from being outside and rained on or from being picked up or auctioned cheap somewhere else.

Additionally, the amperes and specs of these random supplies often do not match the project requirements so all of us electricians can do at this company is get as close to for example the proper size fuses for motor OCPD, and unknown preinstalled heaters for overload protection, with no cataloged to reference.

The owner is not an electrician, apparently doesn’t like to spend money and the company does not understand adding the entire calculated demand prior to selecting feeders, breakers, panels, generators because for starters they keep adding loads as the project progresses

We have a lab that will be fed from a pad mounted delta delta 4800 primary volt to 480 - 440 volt secondary with secondary system windings ungrounded and only cabinet and first point of disconnect grounded while bonded to building structural metal.

This will power a large shower room, then branch from a buss panel to underground pvc conduit planned at 3/0 (SOOW CORD - owners decision but not the best idea) to the lab 300 feet away for a three phase panel (disconnect ampere rating not known yet) to feed a pilot plant containing 3 phase motors at 440 volts and split to the lab to another 25kva transformer (engineer selected) for lab.

This 25kva concrete mounted transformer will feed a single phase 120/240 panel inside the lab fed from line 1 and line 2 of Delta 480 volts secondary of first pad mounted transformer mentioned. primary OCPD will have to be figured for this second transformer
 
Apparently your June 8th 5kva kva transformer has grown to a 25kva :)
With your 480v ungrounded source if you are going to meet Code you will need to be selective on your panel and breakers (no slash rated 480/277).
 
augie47 said:
Apparently your June 8th 5kva kva transformer has grown to a 25kva :)
With your 480v ungrounded source if you are going to meet Code you will need to be selective on your panel and breakers (no slash rated 480/277).
Click to expand...
Lol meet code
 
Jpflex said:
It’s not a scrap metal processing plant but it is a mine as you have properly guessed. The junk piled supplies can have rusted out contacts in starters and equipment from being outside and rained on or from being picked up or auctioned cheap somewhere else.

Additionally, the amperes and specs of these random supplies often do not match the project requirements so all of us electricians can do at this company is get as close to for example the proper size fuses for motor OCPD, and unknown preinstalled heaters for overload protection, with no cataloged to reference.

The owner is not an electrician, apparently doesn’t like to spend money and the company does not understand adding the entire calculated demand prior to selecting feeders, breakers, panels, generators because for starters they keep adding loads as the project progresses

We have a lab that will be fed from a pad mounted delta delta 4800 primary volt to 480 - 440 volt secondary with secondary system windings ungrounded and only cabinet and first point of disconnect grounded while bonded to building structural metal.

This will power a large shower room, then branch from a buss panel to underground pvc conduit planned at 3/0 (SOOW CORD - owners decision but not the best idea) to the lab 300 feet away for a three phase panel (disconnect ampere rating not known yet) to feed a pilot plant containing 3 phase motors at 440 volts and split to the lab to another 25kva transformer (engineer selected) for lab.

This 25kva concrete mounted transformer will feed a single phase 120/240 panel inside the lab fed from line 1 and line 2 of Delta 480 volts secondary of first pad mounted transformer mentioned. primary OCPD will have to be figured for this second transformer
Click to expand...
Remember, it's just temporary 🤣
 
Jpflex said:
Lol meet code
Click to expand...
There are other codes and standards you should follow. For example MHSA pretty much says you have to follow manufacturers instructions. There is no manufacturer that would approve the use of a 277V breaker on a 480V ungrounded system.

Do you really have a lot of 440V motors and equipment? That stuff went obsolete more than 60 years ago, as the standard was raised to 460V motors close to 70 years ago.
 
Jpflex said:
This

is what I would like to know more about. How do you choose service equipment based on max fault current or overcurrent devices based on max interrupting current.

My company has a pile of junk cabinets and switches but all I can do do is try to match voltage, phases fir projects
Click to expand...
Anything that interrupts the current flow must have an interrupting (AIC) rating equal to or greater than the available fault current at that location in the system. The bus bar and other equipment that does not interrupt current must have a short circuit current rating (SCCR) equal to or greater than the available fault current.
You still need data from somewhere about the available fault current on the system. If the system is directly supplied by a utility, you need the data from the utility.
If you are in a large facility, with customer owned transformers then you should be able to get the transformer data and do an infinite bus calculation, which is the worst case for the available fault current from the the supply. You also have to consider the motor contribution if you have larger motors. Many only include motors 50 hp and larger for this calculation and often use four times the motor full load current as the motor contribution.
 
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