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Natural Gas Pipe Line Sizing Chart
Example of a pipe system for a residential home
Make sure the gas line can handle your generator
To add a generator to a gas system you have to make sure you do not "starve" the other appliances. Often, the sizes can be modified for emergency use. In other words, a gas line that you would normally not connect a generator to you just may decide to you can because in the event of an emergency, you will not be using the gas grill or the gas dryer so the BTU loads of those appliances can be eliminated from your calculations.
Normally, correct pipe size is important so that each gas appliance has enough natural gas to function correctly. To verify that you have a correctly sized system you should first read each data plate on the equipment that is connected to the system. Each is rated in maximum BTU required.
If the plates are not available or you just want a good estimate use the appliance chart at the bottom of this page to estimate average appliance BTU load.
There is a more exact formula but you can round off the conversion of CF to BTU by using 1=1000 (for example: 174 CF on the chart would be equal to 174,000 BTU). This will give a more conservative total estimate. Also use 10 CFH per horsepower of engine at full load. So a 10 hp engine would be 100 CFH at full load. Even if used at 1/2 load on average the pipe must be sized to handle the full speed of the engine.
What size hook-up hose at the generator is needed?
If the hook-up hose is kept to 6' or less, the use of a 1/2" hook-up hose and quick disconnect coupler for up to 18hp engines is acceptable. The point is to have the correct size pipe up to the shut off valve for the generator. It is like would be necessary if using an electrical appliance on a very long extension cord. The extension cord may be quite large but the wire on the appliance may be quite a bit smaller. The same principles apply. Having a sufficient amount of gas at the shut off and QDC to feed the generator is the key.
What size Pipe can your generator connect to in the drawing above?
What has to be done is to reverse engineer the pipe system Write down the pipe sizes and the BTU load of each branch of the gas system.
For example let's use the pipe layout above:
BTU Load of the range is 65 CFH
BTU Load of the dryer is 30 CFH
BTU Load of the furnace is 125 CFH
BTU Load of the grill is 40 CFH
The length of pipe from the gas meter to the farthest appliance (which is the grill) is:
(A,B,C,E,H,J,K,L OR 8+10+8+9+20+14+14+2) 85 feet.
There are no 5' steps in the length chart so you have to go to the next higher column which is 90 feet. Once the longest run is determined, you must stay in that column for all other appliances.
Using the next higher column labeled 90 feet on the pipe chart, we will enlarge that section and eliminate all the others for clarity to show the CFH and pipe size needed:
Pipe A, B and C should be 1-1/4" (there is no 310 on the chart so you must go to the next highest number)
Pipe D to the range should be 3/4".
Pipe E trunk line should be 1-1/4"
Pipe F to the water heater and dryer should be 3/4"
Pipe G to the Dryer should be 1/2"
Pipe H trunk line should be 1"
Pipe I to the furnace should be 1"
Pipe J, K, and L to the grill should be 1/2"
So as you can see adding a 10hp 100 CFH load to the system is difficult. For instance the grill line can accept only 13 CFH more which equates to a 1.3hp engine. But if you did not use the grill while operating the generator, you can connect a 5.3hp engine to the pipe. The furnace line was on the border of the next size smaller so it can accept (208 - 125 = 83) another 83 CFH or an 8.3hp engine.
But remember the main truck line can handle up to 428 CFH and at this time only 310 is connect which leaves an additional 118 CFH or a 11.8hp engine can be connected to the system at A, B, C, or E unless the plumber installed 1-1/4" all the way which they will do sometimes to eliminate reducer fitting, etc. The use of line F would also be good if the dryer and water heater is not operated at the same time. This line can hand up to an 11.1hp engine.
Now you understand why we can not answer piping questions over the phone. If you want to trace out your system and mark all the loads and pipe sizes and request a review to see if your generator can be connected at a certain point we would be glad to do that for free. You can fax it to 304-872-3359 24 hours a day.
Note: Any nipple or other piece that is smaller in the system and then goes larger again is to be rated from the reduced piece onward. For instance, if a 1 inch pipe runs 30' then drops to 1/2" for 6" and then someone installed 3/4" pipe and ran an additional 40 feet, the 40 feet of 3/4" pipe would all be considered 1/2". You will never get more gas through the 3/4" pipe than will pass thorough the 1/2" nipple.
Pipe Size Chart
Length in Feet
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