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 a 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. 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. I have the new 225 amp load center inside (not hooked up yet) and the POCO already upgraded the transformer and the wire from the pole to the meter. I am upgrading the wire from the meter pole to my house with 4/0 aluminum. The main breaker disconnect at the pole, under the meter, is rated for up to a 225 amp breaker according to Cutler Hammer's website and the label inside the disconnect enclosure. It currently has the 35 kAIC breaker in it right now. I have a 10 kAIC, 225 amp breaker on hand that I would like to put in there, however I did not want to have it put in the main disconnect, if it needs to be a 35 kAIC like is currently in there. Thanks for the help.

Not knowing how the 35KAIC rated main breaker came to be, you need to check with the utility for the KAIC rating of their system.

I would have to assume that the 35 KAIC was required at the time of installation.

There is a chance that only 10K is needed, and the installer had the 35K on hand.

Most residential services only need to be rated 10KAIC, but, again, this is dictated by the utility transformer.

Check with the utility, and do as they require.

If your breaker is intended for residential application, 1o kaic is enough, higher kaic applicable only for industrial use.

It is as TK states, it is predicated by the available inrush at the point of connection. The utility may provide the KAIC available at the disconnect, but sometimes they will only provide the KAIC on the secondary side of the transformer. If this is the case, you need to calculate what can reach the fist protection point. Faults dissipate quickly overlong wire runs.

Residential installations could easily require 22kaic if they are the first home after the transformer feeding them.

As stated the Utility "must" provide to you the available short circuit at the secondary of the transformer...
Then, if you can tell us what that is... and then tell us if there is a single transformer and the voltage (for residential it's normally 240V), the size and length of the cable to the first disconnect and whether it is in free air or conduit, the size and length of the cable to the next disconnect or panel and whether it's in free air or conduit...
Knowing the above, the short circuit at the various locations can be calculated with reasonable accuracy.

There is no rule of thumb here!... utilities have been using energy efficient transformers more an more often that can provide much higher short circuit levels so you must contact the utility before you can go any further.