OK, here's a quick calculation.... heat is lost from the kiln essentially as a function of the surface area of the chamber, at a rate that is determined by the insulation (e.g. thickness of the brick as well as what kind of brick). Your kiln (assuming a rectangular volume 17.5"wide x 17.5" deep x 13.5" high) has a surface area of 1557.5 square inches. At 4800 watts heating power (20 amps at 240 volts), that works out to just over 3.08 watts per square inch. (if your kiln is round, 17.5 diameter by 13.5 high, then your surface area is 1222.6 square inches, which means you've got 3.93 watts of heating power per square inch).
Our Gare 2318 (round kiln 23" in diameter, 18 inches deep) has a surface area of 2130.5 square inches. It is rated at 7200 watts heating power (30 amps at 240 volts), so that works out to 3.38 watts per square inch. Our kiln is made of standard 2.5 inch brick. We understand ours is supposed to be able to achieve cone 6, but we only use it for bisque so don't know how high it can actually go (we got it 2nd or 3rd hand).
So if your brick is similar (2.5 inches), I would guess that the "should be capable of cone 6" advice is probably close....
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