[LazyPup]

You will note in the example i cited that the total combined DFU load for the structure was 20DFU which is the maximum allowable DFU load for a 3" horizontal line. If we were to also add a floor drain in the laundry area it would then put the total over the 20DFU and require the main drain be enlarged up to the next size (4").

Many local codes also establish a minimum diameter line size that may be run under a slab. It may be that the actual DFU load would call for a 3" line and main vent, but your local code requires a minimum of 4" underslab, in that case it would be legal to run the 4" underslab with a 3" main vent, but it would required a written approval of the local code inspection authority.