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Understanding the NFGC Gas Sizing Tables

cmaverick Member Posts: 4
Hi all. Could somebody please let me know if my understanding of these tables is accurate for sizing natural gas piping.

Is the right way to read this like this....
If my 4,000MBH appliance requires an inlet pressure of less than 2psi, and I only want a pressure drop of 0.5in WC, and I have 10 feet of piping length I can use, then I need to use 2" or larger piping. Right?
So if i use a 2" line, and if my supply gas pressure from the meter is 6"WC, then my inlet pressure, in theory, should be 0.5" less than that at 5.5" right?


    JUGHNE Member Posts: 10,909
    That is a very large appliance
    What are you hooking up?
  • cmaverick
    cmaverick Member Posts: 4
    I've got two 3,000MBH gas-fired condensing boilers. The question though is more of a general question to make sure I'm understanding the appropriate way to read the tables.

    As far as I'm understanding, it shouldn't matter whether I'm looking at the 0.5"WC pressure drop table, or the 3"WC pressure drop table, so long as I have sufficient volume and stay within the appliances minimum and maximum allowable range.
    EBEBRATT-Ed Member Posts: 14,543
    As far as I know the 0.5 wc pressure drop is allowed.

    Your sizing of the 2" pipe from 4000 cfh is right. I believe the code says your supposed to add for the fittings PD
  • ratio
    ratio Member Posts: 3,551
    For a developed length of 10 feet, that's right. All the fittings used add to the developed length, i.e. a 90° adds IIRC 5 feet. So, if you're 10 feet away and have only one 90° fitting in the run you're at 15 feet already.

    I'd look at 2½ minimum, but I get to deal with undersized gas piping all day long at work so I like to go strong, YMMV.

  • realliveplumber
    realliveplumber Member Posts: 239
    The appendix in the fuel gas code has instructions on how to use the charts. There are several methods listed.

    We use the longest run method. Each section of pipe is sized in the column that equals the longest run of pipe. If your longest run is 50', every section is sized using the 50' column, whether the section is 1' or 49'.
    EBEBRATT-Ed Member Posts: 14,543
    Or you can use the "branch method" same size main with smaller branches (usually)