My 10-gallon kettle is a Polar Ware 361BP. The 361BP is made in America and has all welded fittings (the 361BP is not from the same line as the economy kettles that are sold in most home brew supply stores).

I personally disagree with going with a 15-gallon kettle for 5-gallon batches. The geometry is wrong for the batch size. My advice is to go with a kettle that has at least a 1:1 height to diameter ratio at roughly twice the size of your final boil volume. Using a larger than necessary kettle results in less efficient hop utilization (like spices, hops benefit from a taller liquid column), a more rapid boil-off rate, and a shorter final liquid column, which is significant if you plan to use an immersion chiller because it will limit the effective height of the chiller.

With that said, I personally believe that a 15-gallon kettle is a waste of money. It is too large for efficient 5-gallon brewing and too small for 10-gallon brewing. The proper kettle size for 10-gallon batches is 20 gallons. A 10-gallon batch is really at least an 11-gallon batch of wort if one wants to yield 10 gallons of beer. With an evaporation rate of approximately 15%, one has to start with 13 gallons of wort. Wort expands by 5% at boiling temperature, pushing the volume to 13.65 gallons, which does not leave a lot freeboard to the top of the kettle.

Conversely, the liquid column with 5.5-gallons in a 15-gallon kettle is slightly more than 1/3 the height of the kettle. While Blichmann kettles sport better than 1:1 ratios, the 15-gallon kettle is 18.9" tall by 15.7" wide. Five point five gallons of wort displaces 5.5 x 231 = 1270.5 cubic inches; therefore, the column height in a Blichmann 15-gallon kettle is 1270.5 / ( 7.85 x 7.85 x 3.14) = 6.56 inches. That's the tallest immersion chiller than one can use with a Blichmann 15-gallon kettle with 5.5-gallon batch without wasting cooling capacity.

Now, lets run the same math against a 10-gallon Blichmann kettle. The 10-gallon Blichmann kettle is 16.5" tall by 13.8" wide; therefore, the column height with 5.5-gallons of wort is 1270.5 / (6.9 x 6.9 x 3.14) = 8.88". The delta between the two column heights is 8.88 - 6.56 = 2.32", which translates to 2.32 / 0.375 = 6 additional coils on a 3/8" diameter copper tubing chiller. With a chiller diameter of 9", 6 additional coils means that we can add 6 x 3.14 x 9 / 12 = 14.13 additional feet of copper tubing to our chiller.