Manufacturers use the cabinet and cover for a variety of panels. Saves money by having fewer stamping dies to make/buy. The interior, as you have discovered, determines the maximum number of circuits.

By "piggy back" breaker, I assume you mean a tandem breaker - two breakers in a single slot. That type of breaker is still available from Square D in both the QO and Homeline series and Square D makes panels that accept them. Most panels do limit the number of tandem breakers they will accept, so read the specs before making a purchase.

I believe all the slots in this panel can hold tandems. SquareD only gets to the point of restricting them when you get up to the larger ones where they have to prevent you from exceeding the 42 device limit.

Mike, how can I tell if the existing breaker is a QO or Homeline series? The following is all the info labeled or stamped on the breaker:

1. for replacement use only, not ctl assemblies
2. series 2
3. interrupting rating 10,000 amps max rms sym 40 degree C
4. Two 1 pole units, 8-14 AL/CU 60/75 degree C wire
5. Type QOT

I did searches, I found prices ranging from $13-$50 for a sq d tandem breaker. I assume one is a homeline and the other is the QO type. This looks like the one I have:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100075651&langId=-1&catalogId=10053&ci_sku=100075651&ci_src=14110944&cm_mmc=shopping-_-googlebase-_-D27

I also assume line 5 above means I have the QO model.

In one's expert opinion, if your load center had to be replaced today, would you still try reuse these tandem breakers saving $320 or just get a load center that accepts the single breakers?

I also need help in selecting a new load center. The current one has all the cables feeding through one bushing approximately 3" in diameter. Not sure if the aforementioned will affect the selection of the new center- wires not being long enough. If I reuse the 8 existing tandem breakers, I will need at a minimum 16 slots and 24 slots if I use single ones.

I would VERY MUCH APPRECIATE if square d load center model numbers were provided for the 2 sizes.

I failed to mention this load center is protected by a 60 amp breaker in the entrance panel.

Homeline and QO would not be both in the same panel. It is one or the other.
Looking over all your recent questions, I sense that this project is well beyond your present abilities. It is time to hire a pro.

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.

I think it is better written that washing your house with a chlorine bleach mixture CAN BE DAMAGING. Chlorine bleach is a very valuable tool but is corrosive to metals. It is really only effective on nonporous (hard) surfaces. Ideally, you should look up what is required to kill what ever you are trying to kill. (For pathogens, the CDC and EPA are good sources.) As a rule of thumb, a 10% mixture of disinfectant bleach in water for 10 minutes should be sufficient.

The surface should be otherwise clean or you need more. The bottle of bleach should be 6% hypochlorite or higher. If it does not specify, don't buy it. If you don't remember the 6% part, look for the "disinfectant bleach" check the concentration on that, and buy the lease expensive stuff that says 6%.

Rinse stuff that is likely to be affected by bleach very well. This includes metals and some plastics.

Ionized, It's a common practice in the south to wash your house with a bleach mixture to control mildew. My city leads the nation in yearly rainfall. This along with high humidities causes a lot of problems. Those that live in arid regions really have no idea what we're up against in controlling mildew. The problem is my washer uses an industrial grade bleach but it does a good job. I think I will start putting plastic over the panel on wash days.

In my hot, humid, rainy city, it is algae. Plastic is a good idea, If you can apply it in a way that prevents the bleach from running behind the box and corroding the hub. If you are using much more than 0.6% hypochlorite in your final mix, you are probably wasting money. Longer exposure than about 10 minutes will often only do harm to the substrate and not help killing your target. TSP helps, the real stuff, but it is not available everywhere.

Are you pressure washing? If so, I hope that you are careful that you are not getting bleach and water into places it should not go. I just use a brush with an extendable handle.