Electricians; why can't I just put in a larger main breaker ratherthan a new panel?

Take a good look at the sticker on the inside of the cover. If it says the panel is rated 100 amps you must change the panel. If by chance it says it it rated for 150 amps you can go that large. I would be most surprised if it says it is rated 200 amps, but you just might get lucky. However, I can just about guarantee that the wires from the meter to the panel are not rated more than 100 to 125 amps. To go all the way to 200 amps you may have to change the meter can as well. (In my neck of the woods I have never seen the electric company change their wires when a service was upgraded. But then I may live in the only area in the country that doesn't do that.) You may also be required to install a main breaker outside next to the meter. This is so the Fire Department can kill the power before they enter the structure.

But to really answer your question, it has to do with the current carrying capacity of the "Buss Bars" (the metal strips the breakers attach to) and the Service Entrance wires.

[You can't (safely) put 3,000 lbs in a truck rated for 1,500 lbs.]

RE:

Electricians; why can't I just put in a larger main breaker ratherthan a new panel?

I have 100 amp panel and want to go to 150-200amp service. What is the reason I can't just replace my 100 amp main breaker with a 200 amp main breaker? I have plenty of slots in the panel for additional circuits. This panel was in the house when I bought it and I want to upgrade without...

Main Breaker Panel

The busbars are probably only rated for 100 amps. It's possible someone put in a 150 or 200 amp panel with a 100 amp main breaker, but that would be unusual. The service wires (running from the pole to the meter to your main panel) are also likely only rated for 100 amps, so they will probably need to be upgraded as well.

The buss bars in the panel are not approved for the larger current. Also the amount of power from the provider transformer is not rated for the 200 amps. The meter will not be safe at the 200 amps, the feed wires from the meter to the panel must also be rated for the 200 amps. The basic reason is it is an extreme fire hazard that will probably void your fire insurance if you do that. It is cheaper to install the electric upgrades to code than to rebuild the home after you have a fire with no insurance.

The panel is not made physically for the 200 amp breaker normally. Smaller size Buss.