I was looking at that kettle last night. I've only brewed 4 batches thus far in my 8 gal kettle, but am getting ready to go all-grain using BIAB. For some reason I don't feel like putting forth the energy to cut the top off and install valves into the keg my son brought home. I'm coming to the point that I need to get out of my rocking chair and brew something.
I have "agonized" over kettle size. I can't find the post where I asked a similar question about kettle size. It seemed that "everyone" was recommending the 15 gal kettle for 5 gal BIAB. It would allow for the full mash volume (no subsequent dunk sparge) as well as plenty of room to prevent boil-overs. Personally, I don't like the idea of looking down into a large pot and only using the bottom 1/3 or so. If you extrapolate the numbers I give down below, it's apparent that one cannot even do full volume mashes for 10 gal batches in a 15 gal kettle.
On this forum, someone suggested looking into the stainless steel kettles on this site. I'm looking at the 12 gallon:
There's a spreadsheet on the www.biabrewer.com
website into which you enter your desired brew length (final volume of finished beer you want), your original gravity, and the diameter of your kettle. The spreadsheet then calculates the total volume of water you need taking into consideration what will be retained by the grain, what will be boiled off during their recommended 90 minute boil, kettle trub, and what will remain in the fermenter at bottling time.
One of the extract kits I brewed had an O.G. of 1.062. If I plug this into the calculator (planning on all-grain BIAB) along with the 13.6" diameter of this 10 gal Bayou Classic kettle, I get the following volumes:
1. total water = 8.5 gal (14" high in this 16" tall kettle prior to adding grain)
2. total mash volume = 9.6 gal
3. start of boil volume (after removing bag assuming full volume) = 7.6 gal (12.6" deep in this kettle)
Just for further calculations, I entered 1.083 into the calculator, because I was interested in a beer that had that for the O.G., the volumes increased by appr. 1 gal.
So, if my reasoning is correct, if using the 10 gal kettle, you will need to begin with a gallon or so less water for your mash and plan to dunk sparge your bag into the remaining water. The kettle will probably hold the full start of boil volume if you're careful with the boil. If you want to be cautious, you could keep out a little of the runnings from your dunk sparge and add it to the kettle as your level goes down during the boil. According to the BIABrewer forum, this is called Maxi-BIAB - performing BIAB when your kettle won't hold the entire mash volume.
It is likely that you've already thought all this through. I confess that I'm spending too much time reading, studying, and trying to make a decisive move when I should just get brewing.