If you measure the large OD of the impeller yeah it would be a 3A but there is typically an angle change at A and above and this has an effect on the load. For example you can run a big impeller in a berkeley style bowl that is only bored to 9.125 so the shroud is still small so it fits in the bowl but it could have a shallow angle machined on the exit side that can make it work like a large impeller. When you get into the larger cuts you have to look at all this to figure the cut size.
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