Technically thats not true Robb......the primary should be able to handle the entire flow that the return puts into the tank, with the gate valve open all the way.....so if the emergency is the same diameter, then it should also be able to handle the flow if the primary drain is restricted more than the gate valve being closed to adjust the water line in the overflow......did that make sense? :noidea:

depends on the setup, an internal overflow for example could have two drains of equal size but the owner may decide to run the primary without a riser or a very short one which in a way is like a full siphon with the full height of the overflow adding to it's pressure. The back up will have a riser to the height of the overflow and won't be able to replicate the same pressure as the primary and fall short of matching the flow. However if the water can rise high enough over the emergency riser the effect will be very similar minus the friction and minor losses from created from the riser and it's entrance point but the amount you can allow the water to rise over the riser tube depends on how much clearance you have. The larger diameter to riser tube the closer you'll come to matching the capabilities of the primary.

On my set-up I don't run a riser on my primary and my emergency has a riser pretty close to 30" tall.

With the two pipes how far below the slots on the corner overflow does the main drain need to be ? An should the emergency drain be a 1/4", 1/2" , 3/4" , or 1" above the main drains height ? Can take pictures of my corner overflow if that helps too.

http://i797.photobucket.com/albums/y...2/IMG_0031.jpg
http://i797.photobucket.com/albums/y...2/IMG_0032.jpg
http://i797.photobucket.com/albums/y...2/IMG_0030.jpg