Hi thermotek,
In addition to what Tim mentions, on the other end of the spectrum, per the AISC, the maximum size of fillet welds that is permitted along edges of connected parts is as follows:
Material less than 1/4" thick, the weld size is not greater than the thickness of the material.
Material 1/4" or more in thickness, the weld size is not greater than the thickness of the material minus 1/16", unless the weld is especially designated on the drawings to be built out to obtain full throat thickness.
The minimum weld size is dependent on the thicker of the two parts joined, except that the weld size need not exceed the thickness of the thinner part. Weld sizes larger than the thinner part joined are permitted if required by calculated strength.