Yes, in a main lug box the rating is a declaration that the components there in (the lugs, the bus bars, etc...) are capable of safely handling that amount of load.

You can put as big of a panel as you want in. The main breaker here is only protecting the panel as you are correct that your existing disconnect is providing the protection for the service and the feeder to current fusebox or your replacement.

The existing 100A *IS* your service disconnect. The equipment grounding conductor (ground) and the grounded conductor (neutral) must be connected there *AND NOT ANYWHERE ELSE* such as in your new panel. Depending on what manufacturer you are dealing with you may have to remove a jumper or screw, and in the case of Square D, install an additional bus bar.

You may find a panel with a main costs less than a lug panel.

Volume manufacture favors the panels with a main.

Thanks for the response. Another question:

Most load center manufacturers advertise their centers as something like 12 space / 24 circuit capacity. I assume this is because you can put two half-inch circuit breakers into one space. Occasionally I see a box where the number of circuits is not double the number of spaces (like 20 space / 30 circuits). Why is this? What prevents you from putting two half inch breakers in each slot?

What prevents you from putting two half inch breakers in each slot?

The geometry of the breaker and the bus bar

"Occasionally I see a box where the number of circuits is not double the number of spaces (like 20 space / 30 circuits). Why is this? What prevents you from putting two half inch breakers in each slot?"

Depending on the manufacturer, some tandem breakers can only be installed in bus bar slots that are notched to accept them. Where that's the case, a panel may have some slots that are notched and others than are not -- a design feature that serves to limit the total number of circuits.

If you look at the printed information inside the panel cover, you may see either a schematic or a table indicating that certain slots can be used to feed two circuits (i.e., will accept tandems) and others which cannot.

Also keep in mind that a normal two-pole 240V breaker takes up two slots but only counts as one circuit. Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.

One of those times I wished I could edit a post...

Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.

Strike that. :-)

What was I thinking? Obviously, I need to give myself a remedial arithmetic lesson.

5 two-pole 240V circuits plus 10 tandems is only 25 circuits. DOH!

Also keep in mind that a normal two-pole 240V breaker takes up two slots but only counts as one circuit. Thus a 20-space box allowing for 30 circuits is at full circuit capacity when there are 5 two-pole breakers plus 10 tandems.

In a 20/30 box you have 20 full size positions but the capacity for 30 OCD's (NOT CIRCUITS!). The code counts a two-pole (e.g., 240V) breaker as two ocd'S for the limit purpose.