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Determine proper size

Proper sizing of the pipe is important so that each gas appliance receives enough gas to perform properly. Each appliance has a minimum input demand in BTUs per hour. The chart below gives some examples of typical BTU demands. Look at the chart on the back of this insert to assist you in determining the proper pipe size for your job. To convert from BTUs to cubic feet per hour divide BTU by 1100 (example: 50,000 BTU by 1100 = 45.45 cubic feet of gas per hour). See the example on the back of this insert to help further illustrate this. To get BTU from cubic feet, multiply cubic x feet 1100 (45.45 cubic feet x 1100 = 50,000 BTU.)

Approved gas piping fitting materials

Black iron and corrugated stainless steel (CSST) are commonly used approved materials. CSST requires certification from the manufacturer for anyone who is going to purchase and install the material.

Cutting pipe

If you are cutting iron pipe, you must ream the cut of your pipe so you maintain the full inside diameter of the pipe.

Special instructions

Do not use ground joint unions except directly at the meter or after the shutoff valve at the appliance. Each place where you will have a gas appliance must have a gas shutoff valve. Within the City of Seattle only, any person who installs gas piping on property not under their ownership must possess a Seattle Gas Piping Mechanic License. All gas piping installations require a permit and inspection.


Testing the system is your responsibility. The inspector does not perform the test or provide any of the equipment necessary for the test, including test gauges.

An air pressure test is required. The test pressure shall be at least 1 ½ times the working pressure, but no less than 3 pounds per square inch (psi). The test duration shall not be less than 10 minutes. The piping system shall withstand the test pressure specified without showing any evidence of leakage or other defects. Mechanical gauges used to measure test pressures shall have a range such that the highest end of the scale is not greater than five times the test pressure. For instance, a 3 psi test will require a maximum 15 pound gauge. A 10 psi test could be performed using a 50 pound gauge, but not a 100 pound gauge. The piping system shall be under test (pressurized) and the test gauge visible at the time of inspection. Where the gauge does not indicate the minimum pressure required for the test, or any reduction of test pressures as indicated by pressure gauges during inspection shall be deemed to indicate the presence of a leak.

If there is a drop in pressure, check for leaks. Check for leaks by filling a spray bottle with soapy water and spraying the solution on the pipe, where it meets the fittings. Bubbles show a leak and you should repair it appropriately. Continue this process until you have no leaks. Do not use an open flame to test for leaks.


At the time of inspection, be sure to leave all of the gas piping exposed so the inspector can look at the whole system.

Minimum demand of typical gas appliances in BTUs per hour

Appliance Demand in BTU/hour
Barbecue (residential) 40,000
Domestic clothes dryer 35,000
Domestic gas range 65,000
Domestic recessed oven section 25,000
Fireplace gas log 80,000
Gas refrigerator 3,000
Storage water heater, 30-40 gallon tank 35,000
Storage water heater, 50 gallon tank 50,000


Problem: Determine the required pipe size of each section and outlet of the piping system shown.

Gas piping system


  1. Maximum gas demand of outlet A - 35,000 BTU per hour/1100 BTU per cubic foot = 31.82 cubic feet per hour.
    Maximum gas demand of outlet B-3,000 BTU per hour/1100 BTU per cubic foot = 2.73 cubic feet per hour.
    Maximum gas demand of outlet C-65,000 BTU per hour/1100 BTU per cubic foot = 59.09 cubic feet per hour.
    Maximum gas demand of outlet D-150,000 BTU per hour/1100 BTU per cubic foot = 136.36 cubic feet per hour.

  2. The length of pipe from the gas meter to the most remote outlet (outlet A) is 60 feet.

  3. Using the column marked 60 feet on the size of gas pipe charge:

    Outlet A, supplying 31.82 cubic feet per hour, requires one-half inch pipe.
    • Section 1, supplying outlets A and B, or 34.55 cubic feet per hour requires one-half inch pipe.
    • Section 2, supplying outlet A, B and C, or 93.64 cubic feet per hour requires three-quarter inch pipe. Section 3, supplying outlets A, B, C, and D, or 230 cubic feet per hour, requires one-inch pipe.
  1. Using the column market 60 feet: Outlet B supplying 2.73 cubic feet per hour requires one-half inch pipe. Outlet C, supplying 59.09 cubic feet per hour, requires one-half inch pipe.

  2. Using the column marked 50 feet: Outlet D, supplying 136.36 cubic feet per hour, requires three-quarter inch pipe.

Size of gas piping

  • Gas type is natural gas.
  • The inlet pressure is 0.5 psi or less.
  • The allowable pressure drop is 0.5 inches water column.
  • The specific gravity of the gas being supplied is 0.60.
  Length in feet
Pipe size (inches) 10 20 30 40 50 60 70 80 90 100 125
1/2 172 118 95 81 72 65 60 56 52 50 44
3/4 360 247 199 170 151 137 126 117 110 104 92
1 678 466 374 320 284 257 237 220 207 195 173
1-1/4 1,390 957 768 657 583 528 486 452 424 400 355
1-1/2 2,090 1,430 1,150 985 873 791 728 677 635 600 532