Just my 2 cents again but, I have seen where the bus bar was ruined when there were 2 large appliances on the same bus. For example a range was on the left side of a panel at took up spaces 1 and 3. Then on the right side in spaces 2 and 4 there was either an electric dryer or say a large central air conditioner that draws 40-50 amps. You would have a large load on those first top two bus bars. I have seen where the bus was warped, overheated and was damaged. You should check with the manufacture of the panel to see if there are any load restrictions. Some panels might say "no more than 80 or 90 amps. on any one bus bar".
I have seen bus damage due to loose spring contacts on the breakers, but never due to the arrangement of the breakers. As inexpensive as CBs are we should never re-use old breakers. Someday, however, all our CBs will be AFCIs, and then, it may become more of a temptation to re-use an old breaker.
I go big breakers on top down to smaller ones. I don't think it makes much difference. It does make it look like the EC put some thouht into the panel instead of anywhere. It makes some breakers that are added after the fact stand out.
On some panels it makes sence to put the multi pole breakers on top becauce the mini breaker slots are only at the bottom. So latter adding a mini does not mean rearanging the panel and legand. I think that is the reason how it really got started putting the big breakers on top. I normaly use 40 space panels so that is not a factor.
I recently wired a small woodworking shop. Prices being what they are, it was cheapest to put in a 42 space panel for 10 or 15 circuits. I spread everything out in the panel by groups - 240 all together, main floor 120 outlets, loft 120 outlets, lights all have their own sections with a few spaces between them. Makes it real easy for the owner to find what he is lookng for. He wanted no hassles so I did a 100 amp service for a 16x20 shop. The inspector made a comment about how many outlets there were.
Big Jim, if you used a 42 circuit panel, obviously it had to be a 200 amp. panel. Did it have a main breaker? If so, were you not overloading the wire. If it was a main lug panel, did not the inspector question you on the amount of circuits you used for a 100 amp. service? Since the biggest 100 amp panel that I can get is 20 circuit, it seems that is all that is allowed for a 100 amp. service, to regulate how much of a load can be put on the panel. I am just asking just in case I am missing something here. Thanks for the reply. Steve
There is no limits on the number of CBs we can put on any one feeder. Twenty, fourty, eighty, it doesn't matter. What does matter is the computed load. If your computed load, using the rules found in the NEC, is less than 100 amps, then that 100 amp service or feeder is just fine with the 42 circuit, 200 ampere panel (with or without 200 amp main), if fed with a 100 amp feeder, and protected by a 100 amp main CB. I have installed up to four, 42 circuit, 200 amp 3 phase 480 volt panels on a single 200 amp feeder, using double lugs on the first three panels. The panels were used for switching lighting, (SWD breakers) and were spaced throughout the building (a high school) at convenient locations in groups of two. I was an apprentice at the time, and I questioned the combined loads of all those CBs. The journetman took the time to discuss the concept of loading, diversity of loads, and computed loads with me. It was a valuable lesson. He also indicated that it did not matter where any circuit breaker was on the bus, but he preferred to have the higher ampacity ones closer to the main. It just felt better.