The drain pipe capacity can be found looking at plumbing sites for maximum drain flows by pipe diameter. The capacity is driven by diameter and gravity, which can be simplified to diameter since gravity is pretty constant where we reef.
The real question is, how many gallons per hour do you want to have going through each pipe when the system is running. I think a 3/4 inch pipe can handle ~400 gph, so two would be maxed at 800gph return pump flow.
The issue with larger pipes is that they require larger bulkhead fittings, making the box bigger to accommodate the bulkheads. I suspect that the mfgr is familiar with all of this, and their recommendation of 3 pipes is probably the simplest and safe configuration, but I also suspect it is overkill?
Look for the drain pipe capacities to get a better idea of what you need, and others will probably chime in. GL :cool:
