Bus_driver, I never said I had both type breakers. I was trying to figure out which type I had. Anyway, I talked to my cousin who is a ME and in turns out the load center will not have to be replaced. All the slots will accept a tandem breaker which will replace a couple of the singles. It's not like I'm adding additional loading to the box. I just had to add a dedicated circuit for the microwave and I want to add a dedicated 20 amp breaker for my home theater both of which are currently on a shared circuit. Also a minor decrease in loading was obtained by going with LED lighting throughout the kitchen. Additionally, the service (main) panel will have less loading due to changing to a gas cooktop. If the center required replacement, I had no intentions of doing this my self; and, I was only trying to find out what model box was needed and are the tandem breakers widely used. Believe it or not, the above was a blessing in disguise. My cousin who works for the local power company came by to look at the load center. He also wanted to look at the service panel; and, I'm glad he did because the insulation on one of the hot leads had been severely damaged to where it's almost was shorted to the box. He stated this needed to be addressed asap. It turns out that washing your house with a clorox mixture is very damaging to the service panel and its contents. The DW has been complaining about the lights flickering but I just kind of blew it off. I feel very fortunate that this was discovered before something bad happened. I already called an electrician. Anyway, thanks for the advice and believe me when I say that I know my limitations when it comes to electrical work but one can learn given time.

Older breakers are very brand-specific. FPE, Stab-Lok, etc., will not interchange, no way no how, with anything else. In terms of modern, currently-manufactured, non-bolt-on breakers, and speaking only from personal experience: Square-D QO will only fit Square-D QO. No interchanging. Square-D Homeline breakers will fit in every other panel currently manufactured, but not vice versa. No luck putting a GE, Siemens, C-H, etc. breaker into a homeline panel. Other panels are largely interchangeable NOW... this answer is ONLY in terms of what will physically fit. LEGALLY, you can only use breakers approved by the manufacturer for use with the panel in question. That said, I personally have a bunch of homeline and QO breakers, and use them everywhere.

Thanks for the excellent responses. Currently I can answer these questions. 1) Initially the service entrance voltage was 140 - 141V. Way too high. A transformer supplying power to our unit was changed and readings are now correct. Oddly, at 141V the tripping was just as frequent. I would have thought higher voltage would have lowered the Amps drawn, and there would have been less frequent tripping. Can this high voltage (for a couple of months during construction) have caused the breakers to become prone to tripping? Voltage is now correct at both ends of the run. No run is longer than 75 feet or so from the main panel. 2) I kept the nature of the equipment back as it might cloud the issue a bit. Yes there is an inrush as this is studio flash equipment. That being said, there is no problem using this gear anywhere else in the city (I have been). Even the 15A on the stove is fine, but even a 30A SqD trips repeatedly. The units plug in with standard 16GA cords. I have changed the cords to 15' 14GA heavy-duty high-visibility (orange and black) cords mostly for visibility. The equipment repair technician says a transformer charges capacitors and only basic switches and so-on are between the line-in and the transformer. No capacitors directly connected to the line-in. There is a 'slow' setting (or gentle) on the unit for situations where the units must work on non-dedicated circuits that already have significant loads on them. This 'slow' setting trips as often as the normal setting. Again, they work everywhere else (on normal setting), even the stove outlet. 3) Counterfeit: I compared a breaker to the photos in the article. #C is not identical. Most of my tab is exposed, not 1/2 as pictured otherwise they match. 4) Multi-wire branch: I think so, 1 Neutral for 2 circuits. A single unit can trip a breaker or several units at the same time can trip 1 or more breakers in no particular pattern. My previous set-up was 12GA 3-wire BX cables with 2-20A sharing 1 neutral with no issues. 5) The breakers are QO type HACR 20A. 6) An ammeter was put on the units. There is a very brief peak over 20A. It measures between 28 to just over 30A. There was some talk about this brief peak measurement being unreliable for an exact figure. A cheaper meter read anywhere from 20A-38A. A better quality Fluke was typically under 25A. The 30A SqD trips with the same frequency as 20A so simply swapping to 30A (even in the short term) isn't an option. Thanks for your knowledgeable responses and effort to help with this. Tom

Wayne: other than the fact that I will be out of room after adding the new circuits, I have not had any trouble with this panel per se. However, if you look how the 1/2 size breakers attach to the bus bar the connection does not seem nearly as secure as when using a full size breaker and the thicker, longer bus extension. We'll be living in this home for many more years, and while I don't want to spend money needlessly, the cost of a panel and breaker upgrade is a very small amount compared to the overall cost of our remodeling, and will give us good piece of mind that we have up-to-date equipment to go along with the rest of the upgrades. Mike: There aren't any circuits with MC cable that I'm aware of :-) There is an exterior circuit running to the A/C disconnect box though, but it runs in PVC. BTW, the reason 1/2 the drywall is missing is the house was recently retrofitted for Earthquake protecion and the drywall was removed to add anchors to the garage wall. The rest of it will be removed and replaced with plywood to create a shear wall. The plywood wasn't required per the approved plan, but will make it a lot stronger, and also give me the opportunity to insulate the wall. I ordered the Square D QO140M200C panel. Will pick up the breakers locally. It will be a few more months before it will be installed.