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-   -   Main disconnect breaker (http://www.diychatroom.com/f18/main-disconnect-breaker-10302/)

coyote556 07-30-2007 08:11 PM

Main disconnect breaker
 
Can I use a 10 kAIC main breaker in a main disconnect or does it have to be a 35 kAIC main breaker? I have the 10 kAIC 225 amp breaker on hand, and I did not want to have to buy a new 225 amp 35 kAIC breaker if I did not have to. Thanks

SecretSquirrel 07-31-2007 10:23 AM

Excerpt from the SquareD Residential Catalog;

Quote:

The rating of the load center is equal to the lowest interrupting rating of any circuit breaker installed or combination series connected circuit breaker.
Additional or replacement circuit breakers MUST have an interrupting and voltage rating equal to or greater than that of the circuit breaker with the lowest interrupting and voltage rating presently installed.

I interpret that as meaning the breaker panel as a system will default to the lowest common interrupt rating. If you have any existing 10KAI breakers in the panel, then installing the 10KAI spare that you have should pose no issues. If all exisiting breakers are rated higher than 10KAI then you will lower the overall systems rating by using your spare breaker. Now whether that poses any problems or not is questionable. The higher the rating the more protection you have with respect to high inrush loads. My gut feeling is that you shouldn't have any issues. The SquareD 240V load centers come standard as 10KAI and it's up to the owner as to whether an upgrade is necessary. Not knowing the manufacturer of your breaker box it's hard to say if these comments apply to that as well. Others may have differing opinions.

coyote556 07-31-2007 01:14 PM

Well the one I would like to replace is in the main disconnect just below my meter, at the utility pole. My indoor load center has a 10kAIC, but the outdoor, main disconnect, the one at the pole is currently 35kAIC. This is the one I would like to replace. All panels and breakers are Cutler Hammer. THanks

coyote556 07-31-2007 01:19 PM

The more I think about this, after what you said, It seems to me you would want the lower kAIC breaker at the main disconnect, outside the residence. This way, if there was a failure, It would not be in your house. I am no expert, however that Is what I am thinking???????????????:confused1:

SecretSquirrel 07-31-2007 02:33 PM

The one with the lower inrush value is the one that would "hypothetically"
trip out first. The reason for the different ratings is to accomodate inrush current. If you had a large motor or a transformer... whenever it kicks in, it induces a momentary spike in current much greater than the full load running amps. You sometimes have to provide a breaker with a higher rating to accomodate that spike to avoid nuisance tripping.

coyote556 07-31-2007 03:23 PM

I know the kAIC value is different than the actual amp rating of the breaker, but what exactly is the inrush value? Is that the same as the kAIC rating?

SecretSquirrel 07-31-2007 04:01 PM

Quote:

Originally Posted by coyote556 (Post 55542)
I know the kAIC value is different than the actual amp rating of the breaker, but what exactly is the inrush value? Is that the same as the kAIC rating?

Sorry for changing up on you like that. I should have said interupt value. While discussing inrush the term stuck in my head. SquareD refers to the rating as 'AIR' which is Amperage Interupting Rating. Your guess as to what AIC stands for... Amps Interrupting Current??? I forgot to mention that it also the 'Maximum Short Circuit Current Rating'.

JohnJ0906 07-31-2007 06:57 PM

Quote:

Originally Posted by coyote556 (Post 55511)
The more I think about this, after what you said, It seems to me you would want the lower kAIC breaker at the main disconnect, outside the residence. This way, if there was a failure, It would not be in your house. I am no expert, however that Is what I am thinking???????????????:confused1:


NO! Usually the higher ratings are closer to the transformer, ie: the main.

The AIC rating is the current a breaker or fuse can interrupt internally... as in without exploding!

Available fault current is something that is different everywhere. It depends on the transformer, length of wire, and several other factors. If you have a short circuit, for a very short period of time, it will try to draw ALL available current. The breakers have to interrupt this fault current. Most residential breakers are rated 10000 amp interrupting current.

If someone installed a 35k AIC main breaker, I strongly suspect the utility required him to do so.

How close is the transformer? Any idea how many other houses are on with it?

coyote556 07-31-2007 11:21 PM

I am the only house on the transformer. The transformer is about 35 cable feet from the breaker, with the meter in between them.

JohnJ0906 08-01-2007 04:52 AM

You can check with the utility- as a matter of fact, you should, but be ready to get a 35kAIC breaker.


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