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Generator Gas Pipe Sizing

meplumber Member Posts: 678
Ok great ones.  I was piping a generator today and also redoing the house and tank piping.  Once again, I was left slightly guesstimating my pipe sizing to the generator.

Over the years I have heard a hundred different methods.  I have been using the kW rating for the last few years.  i.e. 1 kW = 3,400 btu/hr, so 10 kW = 34,000 btu/hr.  I then up size one pipe size to compensate for the startup gas demand.  This works except for the larger "Summer Cottage" generators that have a huge startup demand.

So I ask the distinguished members of the board; How are you guys sizing generator gas piping?

I have asked Generac, Kohler and some of the other manufacturers and don't get any good answers for a multiplier.

Does a slide chart exist?  Is there a multiplier?

Thanks.  Keep on rockin'.


  • Henry
    Henry Member Posts: 995

    It is simple, each and every LP or NG generator must have a maximum gas consumption rating on its id tag. Before installing the piping, we check the specs for the generator either in the manufacturers website or shop drawings.

    Here is a typical Kohler spec sheet:


    Look for the fuel requirement and then on page 2 in this case the maximum capacity in BTU. You then use your gas code for proper sizing and not the manufacturer's minimal pipe sizing! ALso beware and don't assume that all generators will work between 5 and 7 inch H2O! Some will need 11 inch.

  • Tim McElwain
    Tim McElwain Member Posts: 4,555
    In many cases with

    natural gas the local utility requires a separate line direct to the generator and will give you higher pressure from a separate regulator/meter setup. As a rule of thumb on most generators a minimum pipe size of 1 1/2" is used.

    By separating the generator from the house piping system you can prevent surges when the generator kicks on and possible pilot outage.

    Check with your local suppliers as to their requirements for either Natural Gas or LP.

    For whole house generators I have in the past run about 24" of 2" right at the generator to compensate for surge.
  • meplumber
    meplumber Member Posts: 678

    Thanks for the input guys.  It appears that I have been on the same page as you.

    Everything up here in my area of Maine is LP.  The oil companies are our providers and most of the time they drop the tank and leave.

    I thought about this for a while after I posted the question and have come to the conclusion that most of the problems that I have seen in the past could be a result of vaporization rate of the tank or tanks.

    Since we are dealing with anything from twin pigs to u/g 1,000's, the scenario changes from job to job.  I am going to pay attention this winter to see if the problems occur when we are down around or below zero.  When vaporazation is at its lowest and the heating system is calling for its highest loads.

    Thanks again.
  • Tim McElwain
    Tim McElwain Member Posts: 4,555
    On LP systems

    generators can be a problem when they first come on and draw such a surge of gas. Many times the LP can't vaporize fast enough especially if there is a short run to the generator.
  • bobboots
    bobboots Member Posts: 1
    CSST Piping size question

    I am having an 18KW Generac Standby Generator installed and have a pipe sizing question.

    The gas company upsized the meter to handle a 660,000 BTU load and the meter has 1.25" piping.   The generator is located 125 feet away with a 50 foot underground run of 1.25" Gasite  Flashshield CSST to be run inside 2" Electrical PVC.

    The generator will be fed from a Tee within 24" of the meter, so the other house loads of 240,000 BTU will not be on the same run.

    Since the meter provides 0.5psi and the generator spec is only 5-7 WC (0.25 psi),  can I use the 1.5WC pressure drop table that shows 311,000 BTU capacity and use the 1.25" Gastite for the entire 125' length ?

    Thanks for your guidance.

    (Gastite table 7-3 is on page 84 of attached)

  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    Pipe sizing is only 1/3 of you planning

    With Gen sets are on LPG, you also have to keep in mind clearance requirements between, well,  everything(gensets are a source of ignition).  I usually do a 2 stage system with one 2nd stage reg for the house and one for the genset exclusively.  This usually involves either locating the 2nd more than 10 feet away, or piping away the reg vent.

    The other thing you need to consider the tank size and vaporization rate.  In my mildish corner of the world a 10kw genset needs a pair of 80 gal tanks, and a 15kw gets a 500 gal tank.  See attached delightful reading material from my fav regulator supplier.
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • Henry
    Henry Member Posts: 995

    Is this a DIY job? If not, you should know the code requirements for piping sizes! If it is, nobody should supply information as one can be liable!
  • jonny88
    jonny88 Member Posts: 1,139
    gas sizing

    henry,i have been reading meplumbers responses for quite a while and think i can assure you he is no weekend warrior.i think he has actually helped me out before.when it comes to gas questions we are really asking for tims help which he is always obliging to give.thanks for the link you put up though it is very informative..
  • Tim McElwain
    Tim McElwain Member Posts: 4,555
    If my math is

    correct 18 Kilowatts comes out to around 61,434 BTU's so 1 1/4 seems over sized.

    What size is the meter?

    What other equipment?

    If I am reading it correctly there is straight run of 125 feet from the meter to the generator correct?

    Why just a 50 foot underground  and why is it 1 1/4?

    Why are you using 1.5" W.C. pressure drop?

    By my calculations at 125 feet using chart 7-3 w
  • bob_46
    bob_46 Member Posts: 813
    Natural gas consumption

    Tim, If you go to the gen. mfr. web sites and look at their specs it looks like

    these gens use somewhere between 16 and 20 cu. ft. per hour at 7"wc per KW.

    They are not very efficient.
  • Tim McElwain
    Tim McElwain Member Posts: 4,555
    Yes they

    would have an output of around 1600 to 2500 BTU's with an input of around 3000 to 3500 BTU's. There job is to produce electricity as a temporary process not necessarily to produce it very efficiently. There are larger generators which have a little better efficiency per BTU.
  • icesailor
    icesailor Member Posts: 7,265
    Generator efficiencies

    Yes, they are. 12% is considered fantastic. Right up there with Mod/Cons.
  • SWEI
    SWEI Member Posts: 7,356
    genset efficiency

    A properly operating reciprocating engine will convert 30-35% of the BTUs in the fuel to electricity.  A large lean burn can get close to 40% thermal efficiency.

    A big box special with some cheesy air cooled engine and a cheapo alternator might be as bad as 12% - I've never instrumented one to find out.
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