Depends on the number of returns (or exits) and GPH. Right now I have ~600gph going through 2 3/4" locline returns. If it were half inch, it could handle it but it would be restrictive and have a lot more pressure/force at the exit.
I have 600 gph going through a single 3/4" PVC with 3/4" lockline on my 72. The Mag 18 on my 225 runs dual 1" returns and flows just fine. Or I should say was flowing fine when I picked it up
Still fine on the return but be aware that return and drain plumbing calculations are entirely different as return is working under pressure and the drains are operating off of gravity. 1" sch40 pvc is good for 600 gph of drain...I suggest 2 1" drains btw...just in case 1 clogs. Also consider the bulkhead size as you never want to reduce size at bulkhead. leaves way to much opportunity for something to clog.
Return line.....I cant imagine that you are using a pump with a larger output than 3/4 so with 3/4 should be perfect.
