Available fault current past breaker, 6 handle rule

[Emhudson7]

I'm working on a new build in a strip mall. The main transformer is 3 phase 120/208 and the main service comes in behind the first unit. The transformer feeds into an 800A MGP36800 breaker, then into a triple meter pack, and then into a 200A breaker for each unit. There are two of these setups behind the first unit that feed all 6 units. So, from the transformer to an 800A breaker, to the meter, to a 200A breaker then inside to the unit. There is a label near the 800A breaker that lists the available fault current at 37,000A. We are working on unit three, which is about a 100' run away. We are installing an NQ454L2 - Three-phase main lug QO bolt-on panel.

My journeyman is insisting that regular 10KA-AIC breakers will be sufficient, "because that's what the other company used in unit 4 (about 50' further down from the transformer)". Is this a high enough rating, will the available fault current really drop over 27,000A in 100'? Does the 800A or 200A breaker rated at over 65KA-AIC do anything to the downstream available fault current?

Another question, in the past I have always installed main breaker panels, not main lug. Does this setup violate the 6-handle rule if we load up the panel, or because there is a main disconnect after the meter outside, is this not an issue?

 

[electrofelon]

There are basically two scenarios:
1. The (downstream) breakers are rated less than the available fault current, but are series rated.
2. The breakers are fully rated, which may mean taking into account the conductor impedance from the source.

There are also two scenarios of how to come to a conclusion in your situation:

1. Take a guess and/or make assumptions
2. Run the numbers using a fault current calculator and series ratings charts.

Sounds like you don't need a main breaker panel. The six handle rule is mostly just for service disconnects and a building or structure supplied by a feeder disconnect.

 

[augie47]

Your concern is warranted. Failure to address fault current is commonplace and can be catastrophic.
Hopefully since you have an 800 amp main and the SCA was displayed, you have a series rated system
When you have time you can look into that scenario here:

https://download.schneider-electric.com/files?p_Doc_Ref=2700DB9901&p_enDocType=Data+Bulletin&p_File_Name=2700DB9901R407.pdf

One important fact is that when series rating circuit breakers all breakers must be of the same manufacturer.

If there is a main breaker ahead of your panel then you do not need a main (see 408.36) unless the rem is a problem wit the branches series rating.\

The length of a and size conductor will reduce the available fault current at a panel. This works in your favor in this case.
Assuming a 100 ft run of 3/0 Cu, the 37000 SCA available reduces to almost 10,000 at your panel.

Bussmann has an app " FC²" that you can use if you want to see how a run of cable reduces the available fault current.

There are other factors involved such as motor contribution so a true study requires engineering but the app and reference above can give you a good layman's perspective.

 

[Fred B]

Sizing of equipment SCCR to match or exceed AFC is important and can lead to catastrophic failure of equipment if not done correctly. Video shows result of improper matching at higher levels but can be still catastrophic on equipment even at closer but still under rated matches. Seen one on a 120/240 that faulted on an open panel the breaker had a withstand great enough to prevent catastrophic failure but still shot flame and smoke from the arc venting of the breaker

 

[don_resqcapt19]

I'm working on a new build in a strip mall. The main transformer is 3 phase 120/208 and the main service comes in behind the first unit. The transformer feeds into an 800A MGP36800 breaker, then into a triple meter pack, and then into a 200A breaker for each unit. There are two of these setups behind the first unit that feed all 6 units. So, from the transformer to an 800A breaker, to the meter, to a 200A breaker then inside to the unit. There is a label near the 800A breaker that lists the available fault current at 37,000A. We are working on unit three, which is about a 100' run away. We are installing an NQ454L2 - Three-phase main lug QO bolt-on panel.

My journeyman is insisting that regular 10KA-AIC breakers will be sufficient, "because that's what the other company used in unit 4 (about 50' further down from the transformer)". Is this a high enough rating, will the available fault current really drop over 27,000A in 100'? Does the 800A or 200A breaker rated at over 65KA-AIC do anything to the downstream available fault current?

Another question, in the past I have always installed main breaker panels, not main lug. Does this setup violate the 6-handle rule if we load up the panel, or because there is a main disconnect after the meter outside, is this not an issue?

What size and material are the conductors? Are they in a raceway and if so is the raceway ferrous?

I ran 100' of 4/0 copper in steel conduit in the Bussmann FC² app and got a load end fault current of 12,158 amps. (this is a free app that you can put on your phone)

The only way that the upstream breakers would make a difference is if they are series rated with the breakers in the panel.

Assuming that the meter disconnect is on the same structure that the panel is located in, the panel can be MLO and there is no limit on the number of breakers in the panel.

 

[Jraef]

It still boils down to the two things as previously mentioned: you either calculate it, or look it up in the manufacturer’s data.

The MGP36800 is a Schneider breaker, as are the QO panels. So Schneider should be able to tell you if there is a Series Rating for those QO panels when fed from those main breakers. If there is, and that rating is 37kA or more in series, you are good to go.

If it is series rated but the rating is LESS than the 37kA, then you might still be OK, but you still have to calculate what the available fault current really is at the QO panel terminals. So for example if the series rating is 22kA and the AFC at the QO panel terminals is 21kA, you are good to go.

If they are not series rated, you have to calculate. If the available fault current at the QO panel terminals calculates out to be 10kA or less, you are good to go.

The only way adding more breakers in the mix might help is if there WAS NO series rating with what is there, and you find that there IS another Main breaker that would GET YOU the series rating that you need, then that would work. But just randomly adding breakers does nothing for you.

What you CAN’T do is to say “well, the previous person did it so we can do it.” Totally NOT a valid response. Two wrongs just make two wrongs…

 

[Emhudson7]

Thank you all for your answers, that was very helpful and helped clear things up. I will be contacting Schneider to ask about the series rating of the breaker, and I will definitely be using that app from now on.

 

[winnie]

An answer to one of your explicit questions: Does the 800A or 200A breaker rated at over 65KA-AIC do anything to the downstream available fault current?

The upstream breaker does change the downstream available fault current, but in a fashion that is not well defined and not directly considered when evaluating the available fault current at the downstream breaker location. When calculating the available fault current at the downstream location you assume that the upstream breaker or fuse does nothing.

The only time you get to consider the protective value of the upstream breaker or fuse is if there is an official series rating. You still don't say that the upstream device has reduced the fault current at the downstream device; rather the series rated device is permitted to be used at the higher available fault current.

-Jonathan

 

[electrofelon]

An answer to one of your explicit questions: Does the 800A or 200A breaker rated at over 65KA-AIC do anything to the downstream available fault current?

The upstream breaker does change the downstream available fault current, but in a fashion that is not well defined and not directly considered when evaluating the available fault current at the downstream breaker location. When calculating the available fault current at the downstream location you assume that the upstream breaker or fuse does nothing.

The only time you get to consider the protective value of the upstream breaker or fuse is if there is an official series rating. You still don't say that the upstream device has reduced the fault current at the downstream device; rather the series rated device is permitted to be used at the higher available fault current.

-Jonathan

IT can all be rather confusing, and the NEC doesnt help by providing this definition, which IMO they should remove:

Current-Limiting Overcurrent Protective Device. A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.

Also, the NEC does not provide a definition for "Available fault current"

But essentially and practically, I definitely agree with all you said.

 

[Fred B]

This may become a more frequent issue on existing installations as POCO's increase xfer sizes to accommodate more customers on the same feed and thus increasing the AFC to each. Seen one already that the addition of new transformer pushed the installation form a max 10k SCCR to a 22K SCCR Min panel when a simple calculation was done. Now the existing10K panel potentially no longer safe. Further analysis of actual AFC with length of wire and other factors looked at to see what would be a safe installation.

 

[Jraef]

Also, the NEC does not provide a definition for "Available fault current"

It does as of the 2020 Code, in article 100. They added “Fault Current” and “Fault Current (Available)” to the definitions. You were not the only one to notice that there was no definition before, yet there were other articles that referred to it.