Originally Posted by Monster Mash
With low pressure the more volume the better, 1/2" BIP will carry more gas. I don't think you will have to worry about any restrictions using 3/8" reducers at the connections.
Sure you have to worry about those restrictions.
If you had a clean 1/2" pipe with all 1/2" connectors then there is no restriction - but the moment you add a 3/8" restriction (from a Marshall 2 stage low pressure regulator, lets say) you'll never "undo" that restriction. You may as well use 3/8" inside diameter from then on out - you will never increase the flow after that step down. You may say that the restriction is only at the "head" of the manifold since it's at the regulator, but that's all it takes.
I could understand someone plumbing their natural gas manifold to a larger size if their connection to their gas main was larger.. Then it makes sense. But for folks using LP, it makes no sense at all (other than the economy of finding 1/2" BIP) You're already restricted (generally most are 3/8") after the regulator.