Last wk we looked at a job that another contractor was in the middle of before the GC bounced him out. They asked us to verify his work up to this point and submit any recommendations or changes. There were several things that stood out such as the PVC venting that was run in 2inch that needed to be 3, return drops, etc. In 3 locations there were 4inch supply ducts(round) run..two to the underneath kitchen cabinets and the other to register in front of an outside sliding door...Question..I thought minimum supply ducts were to be 6", am I mistaken. If not where is the reference that points to this, IMC, SMANCA, etc..thank you for your time..Jim