How do I apply AIC or Interrupting Rating.

[Pzlaket]

Hey, but I think the guy learns quick, he only "soft-deleted" the replies, in case he was mistaken. ;)

Marc/George,

I hope you aren't referring to me, I am new with forums and I would not have any idea how to delet what you wrote.

I really wish that all of you would easy up a little bit - all this hostility isn't healthy.

Regards,
Phil

[SolarPowered]

Phil,

OK, I see that you're apparently legal to do this work.

See NEC sections 110.9, 110.10, and 240.86 for information about this subject.

This document from Square D will probably be more helpful than the NEC text: http://ecatalog.squared.com/pubs/Ele...B9901R1102.pdf

The short summary is that all breakers (and fuses) have to be able to interrupt the maximum short-circuit current that can be supplied to them. There are basically two ways this is done:

1) What is called "fully-rated", where each breaker is independently capable of interrupting the maximum fault current, or

2) "Series-rated", where a bigger breaker upstream can help it out. The series-rating combinations are individually tested in order the get the UL listing, and must be specifically listed out in the label on the panel.

In theory, you could have more than two breakers in series to have a series rating, like a main breaker, followed by a breaker going to a subpanel, followed by a branch-circuit breaker in the subpanel. In practice, there are almost never any "three-in-a-row" series ratings that are tested and listed on the panel label. So, a 42KAIC main breaker can't protect a 10KAIC breaker in a subpanel when there is another breaker between them. I understand that there are situations where trying to do that actually could fail in a fault, so it's more than than just a legal technicality.

The most common configurations you're likely to end up with are:

1) A main breaker rated for the full fault current, no subpanels, and branch-circuit breakers that are listed to be series-rated with that main breaker, or

2) A main breaker as above, branch-circuit breakers in the main panel that series-rate with the main breakers, subpanel feeder breakers that are rated for the full fault current, and breakers in the subpanel that are listed to series-rate with the subpanel feeder breakers.

[Pzlaket]

SolarPowered,

Thank you for all the help!

I went to the NFPA website and found the three sections you gave, you're right, not much help.

I think I understand. If the "POCO" has an interrupting rating of let's say 42k amperes and my main breaker is rated for 10k amperes and I have a sub-panel that I back-fed with a rating of 22k ampere rating then I've constructed the curcuit incorrectlly - right?

I am waiting on the POCO to get me this information so I can install the correct main breaker.

If the transformer feeding the building has a rating of say 30k amperes why would the main breaker need to have a rating any higher then say 10k amepers?

Does the main breaker at the building need to have a rating at least as high as the POCO service equipment?

What does distance from the supply equipment have to do with choosing the correct ampere interrupt capacity?

Thanks again,
Phil

[SolarPowered]

Marc/George,

I hope you aren't referring to me, I am new with forums and I would not have any idea how to delet what you wrote.

I really wish that all of you would easy up a little bit - all this hostility isn't healthy.

Regards,
Phil

Phil, George was referring to himself--he's the moderator on the other forum who "learns quick." The context of those remarks is a recent rogue moderator on yet another forum who just deleted the entire forum, which has resulted it quite a bit of outrage in the user community.

As far as hositility, myself and apparently a lot of other people read your initial post and came away with the impression that you aren't licensed to do electrical work. I don't know the rules for your license class (I'm in CA, but I'm an engineer and so don't have reason to know the exact in's and out's of CA contractor licenses), so I'm taking you on your word that you're legal to do this work, even though you appear not to be specifically an electrician.

My basic objection was that I'm not willing to "aid and abet" someone in the commission of a crime. You have clarified that you are in fact legal, so I apologize for that incorrect assumption on my part.

A second issue is independent of that. While I'm not at all opposed to "DIY" work (when it is legal for the person to do that), I feel very strongly that the DIYer know what he's doing. In the case of electrical work, that includes having a copy of the NEC, a good knowledge of the basic provisions, and a working knowledge of where to find the more arcane things. Your first post violated that. (I'm not saying that you're a DIYer, just that you sounded like you were doing electrical work in a commercial building, and didn't even have a copy of the NEC.)

I hope that clarifies a few things. :)

--Solar

[SolarPowered]

SolarPowered,

Thank you for all the help!

I went to the NFPA website and found the three sections you gave, you're right, not much help.

I think I understand. If the "POCO" has an interrupting rating of let's say 42k amperes and my main breaker is rated for 10k amperes and I have a sub-panel that I back-fed with a rating of 22k ampere rating then I've constructed the curcuit incorrectlly - right?

Yes.

In your example, you'd need at least a 42kAIC main breaker, a 42kAIC breaker feeding the subpanel, and all the branch-circuit breakers would need to series-rate with those. (Which, in practice means that everything needs to come from the same company (no "Company A" main panel with "Company B" subpanel mixes), and further that the breakers must be listed to series-rate with each other.)

If the transformer feeding the building has a rating of say 30k amperes why would the main breaker need to have a rating any higher then say 10k amepers?

Because then a bolted fault might draw more than 10kA, which the breaker is not designed to interrupt safely. It might fail to open; it might explode; it might do other bad things.

Does the main breaker at the building need to have a rating at least as high as the POCO service equipment?

It might be less, because the current is reduced by the impedance of the wire between the POCO equipment and the breaker.

What does distance from the supply equipment have to do with choosing the correct ampere interrupt capacity?

See my previous comment.

[Pzlaket]

SolarPowered,

My wife just explained to me what a Moderator does. Sorry for the confusion.

And I'm sorry if I gave anyone the impression that I am a "Tim Tayler on steroids", I'm not. I am an exhalant electrician, I was installing new lights in my parents home at the age of twelve and have been working and learning ever since. I have worked in the building trades as long as I can remember and I am licenesed, bonded and insured.

I do own a copy of the 2005 NRC but I like the internet because the information is usually better.

Maybe I should know this subject cold but it just has never come up before.

Thank you again for your help. And rest asured that you haven't aided or abetted a criminal.

Regards,
Phil

[Pzlaket]

I got my answer from the POCO today. Good news, I configured the new service correctly with the correct AIC rating. See I am smarter then I thought!:)

Thank you to all the pros. that helped me on this with special thanks to SolarPowered.

I'm sure that I'll have more questions soon.

Regards,
Phil

[sinewave]

See NEC articles 110-9 and 110-10 for information concerning the requirement that your equipment be rated for the available short circuit current. You may also want to view the section for series rated systems. The website for Bussmann fuses also offers a free software to calculate the fault current through out your system. You can use this formula as a worse case calculation- Devide the secondary amperes of the transformer feeding the system by the percent impedance of said transformer. The resulting value is your available fault current. Note: Motors will also add to your available fault current values.