(breaker) Interrupting rating explained (simply)

[romex jockey]

Problem is, when you do it that way, and the POCO changes the transformer out, your numbers are wrong based on that method.

True, that would be 110.24 (B) , which when the POCO updates may result in an entire neighborhood uncompliant

~RJ~

 

[EC Dan]

Problem is, when you do it that way, and the POCO changes the transformer out, your numbers are wrong based on that method.

Wouldn't that be the case whether or not the infinite bus method is used?

 

[kwired]

Wouldn't that be the case whether or not the infinite bus method is used?

Infinite bus just assumes the primary side can deliver whatever is asked for, you can still replace a transformer with one of higher capacity and/or lesser impedance and raise available fault current even if assuming infinite bus.

 

[kwired]

Yes you did Jim...

my point being many of those resi services, especially proximal to a padmount , would never and a day meet 110.24

do the ###'s , and what becomes apparent, is we are installing a potential bomb, 110.24's ex was made to protect the manufacturer, not the consumer.

~RJ~

Not saying it is impossible to have a potential bomb, but how many dwelling services are you running into that have a padmount right next to the house? Just 25 feet of secondary conductor is often enough to lower available current at service equipment to a level that the main likely can handle and the branch breakers are typically series rated to go with that main.

 

[Hv&Lv]

Wouldn't that be the case whether or not the infinite bus method is used?

Yes. Which is why I never give an exact number
But the infinite bus method gives you ridiculously high numbers sometimes
there’s a requirement to reevaluate every five years for us. At that time the numbers should be changed and reposted by someone.
I wonder how many people actually do that?

 

[augie47]

Hamberg,
25 posts later and we are working on the "simple explanation".
Be thankful you didn't ask for the detailed one

 

[Hv&Lv]

Hamberg,
25 posts later and we are working on the "simple explanation".
Be thankful you didn't ask for the detailed one

I had to go back to the OP to see what the question was to start with. Half the fun is seeing what’s down each rabbit hole

 

[kwired]

Yes. Which is why I never give an exact number
But the infinite bus method gives you ridiculously high numbers sometimes
Believe this is also the reason there’s a requirement to reevaluate every five years. I wonder how many people actually do that?

infinite bus gives you basically worst case, if that value is below what your gear can handle it is the simplest method of verification that your gear should be fine.

If you have more than 25 feet of "small conductors" (maybe 100 amp and less conductors) you often have enough impedance just in the conductors your gear rating is likely going to be fine. 200-600 amp conductors - may be something you need to watch more close, but if you have at least 50 feet of conductor to the source you likely are going to be fine in nearly all instances. This is all assuming you haven't sized conductors to any more than minimum ampacity or slightly over than what is necessary per code. If you have increased them is usually for voltage drop, and in those cases they are long enough there still is no problem with gear not going to be rated high enough.

But even with larger conductors and large gear often you are fine with SSCR/withstand ratings unless the source is extremely close

 

[tortuga]

It comes up on residential, especially for service changes, albeit rarely.
Years back we were upgrading a meterpack on a triplex, I noticed the transformer was huge.
I figured i'd call the utility and ask, they were like oh that's a 167 kva your AIC is 31000 amps or whatever it was.
Luckily we were able to order different mains for the meterpack, the job was not a money maker.

Another one is your replacing a main distribution panel in a building remodel but there are old sub panels in other areas that have old breakers that are not being replaced.
I think those old unmarked breakers are only 5 KAIC rated.

 

[Hv&Lv]

infinite bus gives you basically worst case, if that value is below what your gear can handle it is the simplest method of verification that your gear should be fine.

The issue I see sometimes is infinite bus giving you currents that put you just over a breaker rating and now you have to put in higher ka breakers because of wrong numbers.

 

[jap]

does anyone have a (5 grader) explanation?

Ok,,, you asked for it.

It's much like controlling the pressure of a sneeze when you have diarrhea....

It's a method used to divert a disaster.


JAP>

 

[romex jockey]

infinite bus gives you basically worst case, if that value is below what your gear can handle it is the simplest method of verification that your gear should be fine.

well that's the thing kwired, check this out>>>



~RJ~

 

[kwired]

The issue I see sometimes is infinite bus giving you currents that put you just over a breaker rating and now you have to put in higher ka breakers because of wrong numbers.

If infinite bus gives you numbers slightly too high you need to go through actual work of figuring the actual input situation or as you said put in higher kA equipment.

But there is no need to figure the input side info if infinite bus calculation gives you low enough results.

 

[jap]

Do they even make residential GFI and AFCI breakers above 22k?

I would think an AIC above 22k would make it difficult to satisfy the breaker fill for GFI and AFCI circuits.

Jap>

 

[romex jockey]

But there is no need to figure the input side info if infinite bus calculation gives you low enough results.

I'm reading 100A service gear tested to 3K kwired....

~RJ~

 

[kwired]

I'm reading 100A service gear tested to 3K kwired....

~RJ~

??

 

[kwired]

Do they even make residential GFI and AFCI breakers above 22k?

I would think an AIC above 22k would make it difficult to satisfy the breaker fill for GFI and AFCI circuits.

Jap>

I mostly familiar with Square D - typical loadcenter mains are 22k.

AFAIK all the typical 10k branch breakers are series rated to use with those 22k main breakers.

In a single family dwelling it is probably rare you ever need to consider this series rating anyway.

Just plugged some numbers into fault current calculator and kind of come to conclusion you need to have over a 50 kVA transformer to even have much chance of having over 10kA available at the transformer terminals. A lot of 320/400 amp meter sockets/services for dwellings out there but not many actually have the load for a POCO to justify over 50 kVA transformer, and quite a few that only have a 25 or 37.5. If is over that it is more likely to supply more than one dwelling and not be too close to any individual dwelling so conductor length lessens available current at the service equipment.

So only the multifamily and the single family with a true high demand are the ones you may need to be more careful about IMO.

 

[Strathead]

Pretty sure everyone understands the overcurrent rating on a breaker but does anyone have a (5 grader) explanation for what the Interrupting rating (240.83(c)) is and how it meshes-up with the panel it's being used in?

I explain to my second year students...

A breaker that gets too much current will either explode or melt before or while trying to trip. The interrupt rating is the amount of current it can take before either happens. The transformer upstream determines how much this current is (5th grade explanation because 2nd year apprentice gets the fact that it is transformer plus wire resistance).

 

[kwired]

I explain to my second year students...

A breaker that gets too much current will either explode or melt before or while trying to trip. The interrupt rating is the amount of current it can take before either happens. The transformer upstream determines how much this current is (5th grade explanation because 2nd year apprentice gets the fact that it is transformer plus wire resistance).

Maybe. Might need to do a demonstration before it will set in. Connect a 100 amp plus load to a piece of 12 AWG and measure voltage, current and even conductor temp after a certain amount of time and compare it to same setup with say a 4 or 6 AWG conductor of same length.

 

[Strathead]

Maybe. Might need to do a demonstration before it will set in. Connect a 100 amp plus load to a piece of 12 AWG and measure voltage, current and even conductor temp after a certain amount of time and compare it to same setup with say a 4 or 6 AWG conductor of same length.

Yeah, what I meant didn't come out as what I wrote. I can see I indicated the apprentices understand, what I meant was that I present it to them. They generally sit like lumps on a log and when I review it later, they can't seem to remember.