I have a square d breaker box. Cat. No. QOC30CF Series SO1. Can you use mini breakers in this box? If mini breakers are possible, how many can you safely use in a box?

check fuses inside cat system

Use siemens for old ITE breaker box.
Square D will not work:
http://waterheatertimer.org/See-inside-main-breaker-box.html
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Look at label of marking on breaker box.
Square D would have D with a square.
Take one of the individual circuit breakers to local box store and see which brand breaker matches.
Or take circuit breaker to electric supply house and see who sells that main.
Not all mains are available.
And if main was getting hot, it might have damaged the busbar so whole panel needs to be replaced.
http://waterheatertimer.org/See-inside-main-breaker-box.html
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following links for general illustrations and information:
http://waterheatertimer.org/How-to-install-a-subpanel.html
http://waterheatertimer.org/Troubleshoot-household-electricity.html#stress
http://waterheatertimer.org/See-inside-main-breaker-box.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

check the instruction sheet included with the box,if none is available,then contact the manufacturer for ampacity/wire capacities of this particular box

Do you have a Square D "QO" panel - or a Square D "Homeline" panel (or a different brand panel completely)? If possible, please provide the catalog number of the circuit breaker.

Nearly all circuit breakers that are TRIPPED (handle is at the midpoint between ON and OFF) must be RESET by pushing the handle to the OFF position - then pushed the other way to the ON position. A circuit breaker that is in the OFF position, does not need to be RESET, and need only be pushed to the ON position.

It is advisable to shut off ALL the other circuit breakers in the panel, then turn on the 200 amp breaker. After the 200 amp breaker is on, turn on the largest value (in amps) double pole breakers and finally all the single pole breakers largest values down to 15 amps. This prevents switching one very large load all at once.

im going off memerory right now but i seem to recall that you may only be able to instal that breaker in the lower part of the panel maybe check to see if the buss bar is a little short to acomadate the 2 pole breakers i will check it out in the am sorry if im mistaken

Usually this is a defective breaker, possibly caused by water intrusion into the panel. This is NOT a do it yourselfer job -- refer this to a professional who knows what they're doing. The meter base needs to be pulled to cut the power to repair it and you are dealing with power that has virtually no fuse or breaker protecting it. If that sounds dangerous, there's a good reason -- it is. I get unnerved doing these. I hope I've helped you, though maybe not the answer you were hoping for.

Very simple: Unhook the wire from the breaker and then pry up at the center of the box... the breaker slips over a blade at the center of the box and tucks under a pair of tabs near where the wire connects.

That is, the breakers tip up from near the center of the box toward the outside.

You can use aa flat screwdriver to go right between the breakers at the center to nudge them up to start... DO NOT put the driver in more than 1/4 inch.

You won't find any "guts", like longer busbars, to upgrade. The panels pre-date most of the codes and you will not find replacement parts, except maybe a breaker or two. For panels that old you are looking at the possibility of having aluminum wiring and/or two conductor wiring (hot and neutral). In either case, your local code office or utility company will probably require you to update to current requirements. You cannot connect aluminum to copper without nickel connectors because of galvanic corrosion, and if you are disturbing any of the wiring you will probably need to upgrade to three conductor (hot, neutral, and ground). You should be able to buy "Square D" HOM series replacement panels with breakers for about $120 or less.

--Peter Nomikos, ICC Master Code Professional
Building and Zoning Director, City of Mauldin, SC