Bob.. what about a (3) pole 30 amp.. which is what I need
Are there any wild numbers like with the 100 amp. you mentioned?
A 3 pole 30 for a Square D I line panel with a high AIC rating could be very expensive.
But there are other types of Squre D panels.
You really need to detemine the catalog number of the breaker you need and get a price on it.
As soon as you go above 250 volts you start opaying a lot more for breakers and if you need a high AIC rating the price jumps some more.
How do you know the needed AIC? Well if you do not have an engineer to do the calculations for you it can be tough.
I will probably get slammed for this but many times I just match or exceed the AIC ratings of the breakers already in the panel I am adding one too.
If you do not make sure the breaker has a high enough AIC rating it could pretty much turn into a grenade if the circuit shorts out.
In general, the closer you are to the transformer supplying the panel the higher the available fault current is. (This is a huge oversimplification)
For instance I had to get a I-Line breaker with a 65K AIC (A typical home breaker is 10K) rating because the panel was supplied by a huge UPS capable of supplying very high fault currents.
The other issue is ............ plan to shut the panel down, do not try to install a 3 pole bolt on breaker into a hot 480 panel, sure it can be done but if it goes wrong it will get very bad, very fast.
One more thing, all of the above applies to all makes of breakers, not just Square D.