As a disclaimer, I'm working closely with my inspector and I'm going to subcontract my gas plumbing work to a professional - I just like to do my homework first so I know what's going on, what to expect, and so I can hire the right contractor.

I'm building out my backyard complete with a new spa, an outdoor kitchen, a covered patio, and a firepit. I'm a "homeowner GC," per my inspector, with subs on the different projects. Almost everything is going to have a natural gas appliance so my natural gas supply line will need to be sized properly. I have consulted my inspector and done my research on the Longest Length Method of sizing a natural gas supply line. I think I've done my homework right, but I wanted to confirm that because I'm confused by the 2 different low pressure natural gas tables. My situation looks to work out fine with the largest MDPE pipe (2") with a 0.5" water column drop, but no size pipe exists if I use the 0.3" water column drop table - that table only supports 745k BTUs and I need 911k. When would someone use the 0.3" w.c. drop table and when would they use the 0.5" w.c. drop table?

Here's my Longest Length Method plan:

Longest Length Method for Gas Pipe Sizing

Sorry if I'm confusing things here, but I think this method was created for iron pipe that can be mixed and matched by diameters. MDPE can apparently only be used as a consistent diameter throughout - is that correct?

Basically I have a spa heater with 400k BTUs, a kitchen with a combined total of 159k BTUs, a patio ceiling heater with 56k BTUs, and a fire pit ring with a max BTU rating of 296k BTUs. I got the BTUs from the appliances and the sizing chart from Home Depot's line of Home-Flex MDPE [link, pages 25-26] and I used the 100' row by slightly rounding down.

Wrapped galvanized pipe is also code here in Southern California, but my inspector says it has a 20-30 year life span while the MDPE has a 75+ year life span. I'd rather not have to dig this up in 20 years, so I prefer to use the MDPE. Should I be safe to use this 2" MDPE based on the 0.5" w.c. drop table or do I have to use the 0.3" table and run 2 lines - one to the spa heater and a second to the rest of the appliances?

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