Fault Current Calculations

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Mdfili

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Miami, FL
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Electrician
Some of our local jursidictions are requesting fault current calculations for service entrance installations and upgrades. Perhaps a 2020 code requirement? This is all new to us. We were recommended to Use the Eaton FC calculator App by a local field inspector.

Can anyone provide the actual formulas needed? When they talk about conductor length, is that between the transformer and service entrance, or the SE and the main breaker panel?
 
You need a "starting point" which is normally supplied by your power provider. They can give you the available current at a given point (transformer, end of service drop/lateral, etc).. with that number you enter the data into the FC app to determine the available current at the 1st means of disconnect.
Most new residential service panels are good for 22k (some may still be 10k).
What size service are you upgrading ??.
 
The formulas are a little involved, but there are step by step manuals available, such as these https://www.eaton.com/content/dam/e...nter/bus-ele-tech-lib-electrical-formulas.pdf.

I would suggest the App as the inspector suggested, which allows you to put in the simpler info. If you know the information of the service transformer, you can start with the top choice below. If the utility gave you their fault current available at the output of their transformer, then start with "add conductor" with the length from the given fault current from the utility. The "add bus", is where your panel is.

1712938060772.png
 
augie47 said:
You need a "starting point" which is normally supplied by your power provider. They can give you the available current at a given point (transformer, end of service drop/lateral, etc).. with that number you enter the data into the FC app to determine the available current at the 1st means of disconnect.
Most new residential service panels are good for 22k (some may still be 10k).
What size service are you upgrading ??.
Click to expand...
Its a simple 200A residential service entrance. We are a generator company and will be adding an SER/ATS. I have a fault current letter from the utility company with the transformer available fault current, the primary fuse amperage and the primary voltage. I was able to plug all this into the Eaton app and get the required main breaker interruption current. I suspected the "conductor length" was from the transformer to the first disconnect, but was unsure. Was also interested in the actual formulas used....couldn't find them on line.
 
ron said:
The formulas are a little involved, but there are step by step manuals available, such as these https://www.eaton.com/content/dam/e...nter/bus-ele-tech-lib-electrical-formulas.pdf.

I would suggest the App as the inspector suggested, which allows you to put in the simpler info. If you know the information of the service transformer, you can start with the top choice below. If the utility gave you their fault current available at the output of their transformer, then start with "add conductor" with the length from the given fault current from the utility. The "add bus", is where your panel is.

View attachment 2571102
Click to expand...
Thanks....That's exactly what I did. I have the data needed from the utility. Just wasn't sure what they meant about "conducter length". The field inspector who gave me the app is a super smart and very helpful guy...previously a big commercial elect contractor here in Miami. The reviewer asking for the calculation is from another jurisdiction and is a draconian pain in the..... I am afraid he will insist on manual calculations.
 
Here's another method from Mike's Free Stuff that might satisfy someone who doesn't care for the FC calculator.
(select "fault current calculator")

www.mikeholt.com

Mike Holt Enterprises - The leader in electrical training.

Mike Holt worked his way up through the electrical trade from apprentice electrician through electrical contractor, to become one of the most recognized experts in the world as it relates to electrical power installations. He was a Journeyman Electrician, Master Electrician, and Electrical...
www.mikeholt.com
 
Mdfili said:
Thanks....That's exactly what I did. I have the data needed from the utility. Just wasn't sure what they meant about "conducter length". The field inspector who gave me the app is a super smart and very helpful guy...previously a big commercial elect contractor here in Miami. The reviewer asking for the calculation is from another jurisdiction and is a draconian pain in the..... I am afraid he will insist on manual calculations.
Click to expand...
I do short circuit calculations for and through very large buildings, and have not (and would not) shared manual calculations with an inspector ever.
 
Mdfili said:
Thanks....That's exactly what I did. I have the data needed from the utility. Just wasn't sure what they meant about "conducter length". The field inspector who gave me the app is a super smart and very helpful guy...previously a big commercial elect contractor here in Miami. The reviewer asking for the calculation is from another jurisdiction and is a draconian pain in the..... I am afraid he will insist on manual calculations.
Click to expand...
The calculator mentioned in Mike Holt's free stuff page is an Excel spread sheet. The printable output kind of mostly shows what the steps in the process are all about and is probably easier to understand than some peoples hand written calculations would be.

As far as NEC requiring markings at service equipment that has been around at least 10 years or more, but has never been required for single family dwellings, unless there is a change I am not aware of in 2020 or 2023.

Outside of that technically you still must use equipment that is rated for what is available you just were not necessarily required to mark the equipment with the available fault current.

And for lesser capacity single phase services, if you had at least 25 feet of service conductor you seldom need to be concerned whether your equipment had high enough rating. Maybe a temporary service for construction connected to a rather large transformer would have been the exception, in those cases often is best to just make sure to have more length of conductor as it will drop the available fault current pretty quickly with every foot of conductor used.
 
kwired said:
The calculator mentioned in Mike Holt's free stuff page is an Excel spread sheet. The printable output kind of mostly shows what the steps in the process are all about and is probably easier to understand than some peoples hand written calculations would be.

As far as NEC requiring markings at service equipment that has been around at least 10 years or more, but has never been required for single family dwellings, unless there is a change I am not aware of in 2020 or 2023.

Outside of that technically you still must use equipment that is rated for what is available you just were not necessarily required to mark the equipment with the available fault current.

And for lesser capacity single phase services, if you had at least 25 feet of service conductor you seldom need to be concerned whether your equipment had high enough rating. Maybe a temporary service for construction connected to a rather large transformer would have been the exception, in those cases often is best to just make sure to have more length of conductor as it will drop the available fault current pretty quickly with every foot of conductor used.
Click to expand...
IMO, if the utility transformer is quite big and very near to the consumer's building, even a 25-foot service conductor cannot bring down the available fault current level to the rating of most common electrical equipment. Been there, I got my share of burning candles in the night, so to speak! There was a project where it was more economical to have its dedicated transformer than to connect to the common residential distribution lines and buy a DP with a high IC rating.
 
Mdfili said:
I have the data needed from the utility. Just wasn't sure what they meant about "conducter length"..
Click to expand...
Like Augie said there is a starting point, and that is usually the value the utility gives you which is typically at the secondary of the transformer. The length is just the length to whatever your first OCPD is, usually the services disconnect. From there you may be able to use series ratings and not have to worry about any more calcs, but if no you keep going downstream to the next device.
 
topgone said:
IMO, if the utility transformer is quite big and very near to the consumer's building, even a 25-foot service conductor cannot bring down the available fault current level to the rating of most common electrical equipment. Been there, I got my share of burning candles in the night, so to speak! There was a project where it was more economical to have its dedicated transformer than to connect to the common residential distribution lines and buy a DP with a high IC rating.
Click to expand...
In general yes, but in cases where you maybe have a larger transformer for the permanent service but are connecting a temp service of lesser capacity to it for construction the smaller conductor to the temp service cuts down the available fault current because of conductor impedance pretty quick compared to what you may have at the permanent service via the larger conductors.

I just used some made up figures for a 500 kVA 208/120 transformer @ 6% impedance and 25 feet of conductor length.

Did it one time with 3 parallel runs of 600 copper and came up with about 22kA at the load end. Probably not a big deal as this maybe would be 1200 amp gear and has a main rating at least 25 kA and very possibly even 42kA.

Did it with single set of 3AWG for a possible temporary service and came up with about 11.3kA at the load end. Increased length another 10 feet and got under 10kA, which would let you use 10kA equipment for that temp service.

Decided to run it with 6 AWG for even smaller temp service and at 25 feet length it only had 7759 amps available fault current at load end. Reduced length until I got to 17 feet and still was under 10kA 16 was just a little over 10kA.
 
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