AIC Question

[Thirst4Knowledge]

Thank you in advance for any comments. I've been a long time reader of the posts but this is my first post. We built a small commercial service 120/208 3ph overhead. The Poco sent us a letter telling us the available fault current was 21,000. We installed 25,000 rated circuit breakers in a two meter modular stack. We then went to two MLO panels inside. The MLO panels branch circuit breakers are rated at 10k. We always do it this way, and we passed inspection without a problem. my question is why are we allowed 10k at the MLO's when we needed at least 21k outside?? You wouldn't have less AIC after the service disconnects. Again, Thanks

 

[iwire]

IMO Unless you can prove there is enough impedance in the conductors between outside and inside to reduce the fault current to below 10,000 you have a violation.

 

[Pierre C Belarge]

Welcome to the forum...as a poster


I also would have some concern. 21,000 to 10,000 AIC is a fairly big jump down in value.

How long are the conductors, what size are the conductors?
Did someone provide engineering to properly size the AIR/AIC rating?

 

[ivsenroute]

This is an engineering issue and will have to be addressed by one.

 

[iwire]

This is an engineering issue and will have to be addressed by one.

Code reference?

 

[ivsenroute]

240.86 & (a)

 

[ivsenroute]

We can refer to this for some info:

http://www.cooperbussmann.com/pdf/8744b1f2-9436-426d-a924-5c4e9d57d93c.pdf

 

[iwire]

240.86 & (a)

240.86 does not require an engineer if the EC is using equipment that is rated higher then the available fault current.

In the OPs example I suspect he is in violation but it is also possible that the feeder is long enough to drop the fault current under 10,000. If I could show those calculations to the inspector of wires he could very well except them.

 

[Thirst4Knowledge]

The wire size is #3 copper. We built this service to the plans approved by the town. When I asked the AHJ the same question I posted here he said you only needed to be concerned with this at the service. That didnt make a lot of sense to me.110.9 and 110.10 dont imply at service only.By the way we are about 60 ft away from the service with the mlo's. this equipment was purchased by a nationally known chain store and we were hired as a licensed company to install it. I'm not saying they know what they are supplying, thats why I posted here, its looks like a possible violation to me, but I got dirty looks when I said something to my foreman. I get in a lot of trouble for thinking to much I guess.

 

[don_resqcapt19]

It looks like you are good to go. If you have 60' of #3 copper in a non-metallic conduit and an available fault current of 21k at the line end of the conductors, you will have about 6.6k at the load end. (based on bussmann's calculator)

 

[Thirst4Knowledge]

thanks for the link and the insight.

 

[suemarkp]

Are the MLO panels from the same manufacturer as the main breaker panel? If so, do they have a series combination AIC rating for what you have installed? I think this is the other way around the 10K AIC breaker problem -- many normal branch/feeder breakers can be used at a higher AIC rating if using an approved series combination from the manufacturer.

You'll most likely never find an approved combination if you're mixing manufactures (e.g. Square D isn't going to test with CH branch breakers or vice-versa).

 

[tom baker]

Also some mfgs, such as Sq D, have series let thru charts that list the AIC for various combination's of circuit breakers.

 

[Thirst4Knowledge]

All the gear is Square D. I looked for a series rating on the equipment and didn't see anything. The breakers inside are QO style plug in.

 

[jim dungar]

All the gear is Square D. I looked for a series rating on the equipment and didn't see anything. The breakers inside are QO style plug in.

Look in the Square D Digest: page 1-2 foot note for QOxxxVH breakers, page 1-5 for main breakers, page 2-9 for meter centers, and 9-2 for panelboards.

 

[Thirst4Knowledge]

UL Listed for use ahead of QO 10K AIR circuit breakers to permit their application at 22ka fault level. thats interesting. thanks

 

[Thirst4Knowledge]

Im not seeing the 9-2 reference? what am I missing?

 

[jim dungar]

Im not seeing the 9-2 reference? what am I missing?

While this section is for NQ panelboards, the result is that, these are almost all of the QO series combinations that are available. It actually makes no difference if the breakers are in NQ panels or QO loadcenters.

 

[Thirst4Knowledge]

oh, i see what u mean. thanks

 

[cowboyjwc]

I was going to ask the same question as Mark.