(breaker) Interrupting rating explained (simply)

[Hamberg]

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?

 

[ron]

There is a definition in Article 100, but essentially, if there is an accidental short circuit, the breaker will experience very high current for a short period of time and the interrupting rating is the tested conditions that the breaker has been put through to be sure it opens the circuit rather than catastrophically failing. The enclosure or panel it gets mounted into, has to have a similar or better short circuit rating (withstand rating) to be sure that during the process of fault interruption, it doesn't meet its untimely end either.

 

[romex jockey]

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?

basically how much of a blast it can take

~RJ~

 

[Hamberg]

@ron, @romex jockey - thanks! Think I get the "max blast" part (especially as it would relate to serious voltage/amperage) but how do the numbers play with the panel rating?

So, for example, say the panel is rated at 22kA, if a 10kA breaker is used what are the potential problems (or not)? Or the opposite, if the panel is rated at lower than and a 22kA breaker were used??

Also, for residential, is there an easy (he says jokingly) way (or at those voltages is it even worth doing?) to do an Available Fault calculation to determine the correct (max?) panel kA rating and or circuit rating?

 

[augie47]

You are moving above 5th grade
The panel (buss) & breakers should be rated as higher or higher than the available current but the ratings can be a bit tricky due to things like "series ratings"
It is highly unlikely for a residence to have an issue (other than a dwelling part of a multi-dwelling structure).
Our local inspectors do not even question individual services 400 amps or less

 

[jim dungar]

Think 'weakest link'. If you put a 10kAIC breaker into a panel, you effectively lower your panel rating to 10kA regardless what it started as. NEC 110.9 says the breaker must not be less than the Line side (i.e. the panel) short circuit current rating.

In reality you would only have a problem if your fault current exceeds 10kA, which is not unreasonable if the fault occurs at the load side lugs of the breaker.

Very few utilities provide more than 22kA to 200A single or duplex residencies and most load centers are available with 22/10kA series ratings. This means it is rarely cost efficient to determine actual available fault current for these locations.

 

[Hamberg]

@augie47 - exactly what I wanted to hear (from an "inspector").

So, follow-up question... Is there a best practice on mixing (say on a panel upgrade) 10kA and 22kA breakers?

 

[Hamberg]

Think 'weakest link'. If you put a 10kAIC breaker into a panel, you effectively lower your panel rating to 10kA regardless what it started as. NEC 110.9 says the breaker must not be less than the Line side (i.e. the panel) short circuit current rating.

(teachers/instructors always said) I'm thinking about this way too hard :O)!?

So, If the panel is rated at 65kA it could withstand 65k without (loose terminology) blowing up? If the main breaker was rated at 22kA, it would withstand up to 22k, again, without blowing up? If a breaker was rated at 10kA the same would apply to the breaker?

My thought process would be to never have a lower level device (panel<main<breaker) rated higher than it's predecessor? Is that logic sound?

 

[augie47]

IMO, generally decent logic. Definitely you would not want to go lower than the predecessor unless someone qualified looks at the system.
On large current (800+) there are many factors involved in the decision often to the point that engineering is required.

 

[romex jockey]

This means it is rarely cost efficient to determine actual available fault current for these locations.

b-b-b-b-ut.....aren't we suppose to being doing that Jim?

~S~

 

[jim dungar]

b-b-b-b-ut.....aren't we suppose to being doing that Jim?

~S~

Why?
Many POCO and AHJs require a minimum of a 22/10kA rated service, based on the utility never supplying more than 22kA even after system upgrades.

Remember I said 200A single or duplex residential.

 

[tom baker]

b-b-b-b-ut.....aren't we suppose to being doing that Jim?

~S~

Our poco states the available scc is 10 ka, when typically it’s 5ka, for dwelling unit services

 

[romex jockey]

Our poco states the available scc is 10 ka, when typically it’s 5ka, for dwelling unit services

My point is, we're suppose to be posting it on service equipment Tom

~S~

 

[jim dungar]

My point is, we're suppose to be posting it on service equipment Tom

~S~

The NEC requires a maximum not an actual fault current to be posted at the service. So if the POCO says their system will never deliver more than XXkA, the NEC is satisfied.

 

[romex jockey]

The NEC requires a maximum not an actual fault current to be posted at the service. So if the POCO says their system will never deliver more than XXkA, the NEC is satisfied.

110.24 applies, and any good EC will do the math & post it Jim

All one needs is the KVA & Impedance of the poco xformer

Fact is, last time i did a 600a service, it paid off, because a service rated RK fused disco had a higher AIC than and OCPD & won the bid

~RJ~

 

[kwired]

My point is, we're suppose to be posting it on service equipment Tom

~S~

exception for single family dwelling services though. Those that exclusively do that kind of work likely not so familiar with the process to determine the available fault current.

 

[Hv&Lv]

110.24 applies, and any good EC will do the math & post it Jim

All one needs is the KVA & Impedance of the poco xformer

Fact is, last time i did a 600a service, it paid off, because a service rated RK fused disco had a higher AIC than and OCPD & won the bid

~RJ~

Thats really an improper way to do it..
Problem is, when you do it that way, and the POCO changes the transformer out, your numbers are wrong based on that method.
plus that assuming sub is directly beside the XF. There’s wire to consider also..

That way your numbers are always higher than actual.

 

[kwired]

@ron, @romex jockey - thanks! Think I get the "max blast" part (especially as it would relate to serious voltage/amperage) but how do the numbers play with the panel rating?

So, for example, say the panel is rated at 22kA, if a 10kA breaker is used what are the potential problems (or not)? Or the opposite, if the panel is rated at lower than and a 22kA breaker were used??

Also, for residential, is there an easy (he says jokingly) way (or at those voltages is it even worth doing?) to do an Available Fault calculation to determine the correct (max?) panel kA rating and or circuit rating?

On mike holts main site there is a "free stuff" section and within that is a fault current calculator. Doing this calculation by hand is a little complicated for many.

Main things that impact this is first - size of the source, and in most cases we would be describing here is a utility transformer. A 100 kVA unit is naturally going to allow more current to flow than a 25 kVA unit.
Next thing is impedance of the source. This value means little during loading conditions but put a short circuit across it and it is about the only thing limiting how much current will flow.

Next thing that limits current is the conductors between the source and the short circuit location. Size type and length of those conductors all factor in and it doesn't take much length of small conductor to make a huge differece in amount of available current at source terminals vs amount available at that first disconnecting means. When installing say a temp service on the permanent source sometimes you may find (via the calculator) that fault current is too high real close to the source but just 15 or 20 feet of conductor might lower it enough that it is ok with your equipment ratings. (impedance of those conductors is what lowers the numbers)

Then you have the more complex series ratings where a 10kA branch breaker is stand alone rated 10kA, but is series tested and rated by manufacturer to specifically be used with say a (specific not just any breaker) 22kA main breaker ahead of it.

Also remember that when you have sub panel situation you have more conductor between the main panel and sub panel - which also lessens the available fault current at the sub panel. You could have 20k available at the main panel but because of feeder size, length, type of conductor may only have 4k available at the sub panel.

 

[jim dungar]

110.24 applies, and any good EC will do the math & post it Jim

I never said 110.24 did not apply. I clearly stated my comments were for a 200A residential service.

 

[romex jockey]

I never said 110.24 did not apply. I clearly stated my comments were for a 200A residential service.

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~