12mm - 1/2"
I don't have a copy handy of D14.3, but will ask a few questions to help narrow down some others who may be able to answer with that code in hand.
What position will these welds be welded in?
You may want to run some sample fillet welds to check for overlap or undercut, etch and cut to make sure that you are indeed getting penetration at least to the root while trying to run a pass that large in one pass(providing it is acceptbale per D14.3). This is inexpensive to perform and it provides an excellent way for the welder and QC to have confidence what is actually happening at the root.