AIC Rating and allowed series rating

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southernviewenergy

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I am trying to figure out when a series rated AIC value is allowed for breakers in a panelboard. My situation is a 480V 1000KVA transformer that has been set for an electrical service. I have a 400A Fused Disconnect installed 15' from the transformer and a 400A Main Lug Panelboard installed 30' from the Disconnect. Inside the panelboard are (10) 30A 3 pole breakers. I know how to calculate the available fault current at a given location with the given impedance of the transformer. However, my question is, when are you allowed to series rate the breakers downstream from the Main Fused Disconnect? The Main fused disconnect has (3) 400 amp fuses that have an AIC rating of 200,000 amps. The panelboard which I ordered comes standard with 18 KAIC breakers. Do I need to specifically calculate the available fault current at each breaker or am I allowed to series rate the breakers down since I have a fuse of 200 KAIC upstream from them.

Thanks

Scott
 
You must refer to the manufacturer of the panels for series ratings.
What the manufacture must do to achieve series ratings is to actually test the items is series. If not specifically listed as current limiting all breakers are current limiting to some degree. It is this current limiting characteristic that provides a breaker with the ability to reduce the let through fault current to the down stream device. The series combination rating is achieved as a result of this testing only.
So check the manufacture's catalog to see what series combination rating they have tested for.
 
Is this entirely new installation?
Then manufacturer's series rating. If there is 28,000 available, everything can be series rated for 30,000 until the wire reduces it to where it's not needed.

If this is an existing installation, the Code has some allowances.

Don't forget motor contribution.

See 240.86
 
sandsnow said:
Is this entirely new installation?
Then manufacturer's series rating. If there is 28,000 available, everything can be series rated for 30,000 until the wire reduces it to where it's not needed.

If this is an existing installation, the Code has some allowances.

Don't forget motor contribution.

See 240.86
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Please site the code reference. Also, should a coordination study be done, yes it is possible to find that there may be a reduction in the available fault current. BUT, just anybody can't magically be qualilfied to do such a study as it must be done by a PE or those certified is doing said study which is very common to do when systems are updated.
 
C)
templdl said:
Please site the code reference. Also, should a coordination study be done, yes it is possible to find that there may be a reduction in the available fault current. BUT, just anybody can't magically be qualilfied to do such a study as it must be done by a PE or those certified is doing said study which is very common to do when systems are updated.
Click to expand...

Not sure which part for Code reference

240.86(A) for existing installations

240.86(C) for motor contribution

What else?
 
southernviewenergy said:
my question is, when are you allowed to series rate the breakers downstream from the Main Fused Disconnect?

Thanks

Scott
Click to expand...

It sounds like you're unsure of what a "Series Rating" is...,
For instance, consider a panel supplied with an available fault current level of 40,000 amps with a 65kAIC main breaker, the branch circuit breakers if selected correctly can be 10kAIC rated and "Series Rated" for protection of that 40,000 ampere fault current level.
Now a fuse on the other hand, as in your question typically will reduce the available fault current applied to it. For instance, a 400 ampere class RK5 fuse will reduce a 40,000 applied fault current to a let through current of about 20,000 amperes. Now your MLO panel needs to have circuit breakers each rated to interrupt at least 20,000 amperes. Common interrupting rating would be 22kAIC.
Simply, "Series Ratings" are applied most often to circuit breaker main and branch device combinations.
 
SparkyHC said:
It sounds like you're unsure of what a "Series Rating" is...,
For instance, consider a panel supplied with an available fault current level of 40,000 amps with a 65kAIC main breaker, the branch circuit breakers if selected correctly can be 10kAIC rated and "Series Rated" for protection of that 40,000 ampere fault current level.
Now a fuse on the other hand, as in your question typically will reduce the available fault current applied to it. For instance, a 400 ampere class RK5 fuse will reduce a 40,000 applied fault current to a let through current of about 20,000 amperes. Now your MLO panel needs to have circuit breakers each rated to interrupt at least 20,000 amperes. Common interrupting rating would be 22kAIC.
Simply, "Series Ratings" are applied most often to circuit breaker main and branch device combinations.
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Highlighed phrase above means the designer will have to refer to published series rating data from manufacturers, as mentioned earlier by other posters.
 
petersonra said:
I agree. You cannot use the let through values on the fuse charts to make up your own series ratings.
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Generally speaking you shouldn't have to. All the breaker manufacturers have a chart to series rate their breakers with an upstream fuse. We saw that when the existing swbd was manufacturer A with breakers protecting the feeders and they bought a manufacturer B panelboard. Cut in a fuse of correct type ahead of the panelboard and we're in compliance.

Let's talk about making up your own series ratings. From what I remember from the Code change analysis, that was the purpose of 240.86(A). I've never seen it used by an engineer so it's not really up to speed on it.

Any thoughts?
 
sandsnow said:
Let's talk about making up your own series ratings. From what I remember from the Code change analysis, that was the purpose of 240.86(A). I've never seen it used by an engineer so it's not really up to speed on it.

Any thoughts?
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It's allowed only if a PE signs his life away to do so. I can't imagine very many are willing to.

A few years ago such a provision might have made sense as there were not as many series tested combinations. These days, there seem to be plenty of them, and it just should not be required to make up your own thing anymore even if you can.
 
petersonra said:
It's allowed only if a PE signs his life away to do so. I can't imagine very many are willing to.

A few years ago such a provision might have made sense as there were not as many series tested combinations. These days, there seem to be plenty of them, and it just should not be required to make up your own thing anymore even if you can.
Click to expand...

I agree. I think it was for equipment that is really old.
 
sandsnow said:
Generally speaking you shouldn't have to. All the breaker manufacturers have a chart to series rate their breakers with an upstream fuse. We saw that when the existing swbd was manufacturer A with breakers protecting the feeders and they bought a manufacturer B panelboard. Cut in a fuse of correct type ahead of the panelboard and we're in compliance.

Let's talk about making up your own series ratings. From what I remember from the Code change analysis, that was the purpose of 240.86(A). I've never seen it used by an engineer so it's not really up to speed on it.

Any thoughts?
Click to expand...

AFAIK, the provision in the code regarding "series rated" breakers or fuses came about because there are problems with various existing distribution panels with increased loads and their existing interrupting current ratings become lower than the new expected fault current. Inserting a better rated breaker/fuse between the source and the old breaker; the new breaker being capable of withstanding the higher fault current and in isolating the downside elements safely was the quick fix. Provided the breaker below the new, uprated, circuit protection device "remains closed" during the duration of the fault. The requirement was for the manufacturers to conduct "series rating" tests (breakers tested in series) on the various models/types of breaker and certify that those sets of combination behave as expected and publish the results for the engineers' references. The positive consequence is that engineers are given the choice on breaker combinations that do not have the same interrupting capacities but are listed as "series-rated".
But I cannot remember people being allowed to make their own series ratings (according to my own understanding of the code).
 
topgone said:
AFAIK, the provision in the code regarding "series rated" breakers or fuses came about because there are problems with various existing distribution panels with increased loads and their existing interrupting current ratings become lower than the new expected fault current. Inserting a better rated breaker/fuse between the source and the old breaker; the new breaker being capable of withstanding the higher fault current and in isolating the downside elements safely was the quick fix. Provided the breaker below the new, uprated, circuit protection device "remains closed" during the duration of the fault. The requirement was for the manufacturers to conduct "series rating" tests (breakers tested in series) on the various models/types of breaker and certify that those sets of combination behave as expected and publish the results for the engineers' references. The positive consequence is that engineers are given the choice on breaker combinations that do not have the same interrupting capacities but are listed as "series-rated".
But I cannot remember people being allowed to make their own series ratings (according to my own understanding of the code).
Click to expand...

You mean for example, if the POCO puts in a bigger transformer?
 
topgone said:
AFAIK, the provision in the code regarding "series rated" breakers or fuses came about because there are problems with various existing distribution panels with increased loads and their existing interrupting current ratings become lower than the new expected fault current. Inserting a better rated breaker/fuse between the source and the old breaker; the new breaker being capable of withstanding the higher fault current and in isolating the downside elements safely was the quick fix. Provided the breaker below the new, uprated, circuit protection device "remains closed" during the duration of the fault. The requirement was for the manufacturers to conduct "series rating" tests (breakers tested in series) on the various models/types of breaker and certify that those sets of combination behave as expected and publish the results for the engineers' references. The positive consequence is that engineers are given the choice on breaker combinations that do not have the same interrupting capacities but are listed as "series-rated".
But I cannot remember people being allowed to make their own series ratings (according to my own understanding of the code).
Click to expand...

I just don't see where in 240.86(A), the manufacturer is required to do any additional testing. They might if it was some of their old models, but not another manufacturer. Since tested combos is in the next subsection I figured A was about non tested combos.
 
sandsnow said:
You mean for example, if the POCO puts in a bigger transformer?
Click to expand...

It follows. After you apply for a bigger load, the POCO will put the appropriate transforme size, else you'll experience supply voltage drops that's not desirable. Hence your available fault current goes beyond your old breaker kAIc ratings.:)
 
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