What size gas line for a standby generator?
The gas line size for a standby generator is 1/2 inch Schedule 40 pipe on runs up to 10 feet, then 3/4 inch from 20 to 50 feet, then 1 inch from 60 to 150 feet, then 1-1/4 inch from 175 to 200 feet. A typical residential standby generator is rated 150,000 BTU per hour, which is 150 CFH of natural gas, and the required pipe grows with the length of the run because capacity falls as gas travels.
Required gas line size for a standby generator at every run length
A whole-house standby generator in the 12 to 22 kW range. Generators surge, so never undersize the run. Measure the longest run from the gas meter to the appliance, round that measurement up to the next row, and read the size. Rounding down is the single most common way a line ends up undersized. Demand here is fixed at 150 CFH; only the length changes.
| Longest run | Smallest pipe that works | Its capacity | Margin over 150 CFH |
|---|---|---|---|
| 10 ft | 1/2 inch | 172 CFH | 22 CFH spare |
| 20 ft | 3/4 inch | 247 CFH | 97 CFH spare |
| 30 ft | 3/4 inch | 199 CFH | 49 CFH spare |
| 40 ft | 3/4 inch | 170 CFH | 20 CFH spare |
| 50 ft | 3/4 inch | 151 CFH | 1 CFH spare |
| 60 ft | 1 inch | 257 CFH | 107 CFH spare |
| 70 ft | 1 inch | 237 CFH | 87 CFH spare |
| 80 ft | 1 inch | 220 CFH | 70 CFH spare |
| 90 ft | 1 inch | 207 CFH | 57 CFH spare |
| 100 ft | 1 inch | 195 CFH | 45 CFH spare |
| 125 ft | 1 inch | 173 CFH | 23 CFH spare |
| 150 ft | 1 inch | 157 CFH | 7 CFH spare |
| 175 ft | 1-1/4 inch | 296 CFH | 146 CFH spare |
| 200 ft | 1-1/4 inch | 275 CFH | 125 CFH spare |
Where the size steps up
The answer is not one size. It is 1/2 inch through 10 feet, then 3/4 inch through 50 feet, then 1 inch through 150 feet, then 1-1/4 inch through 200 feet. Each step happens at the first column where the smaller pipe can no longer carry 150 CFH within the code allowance of 0.5 inches water column. Anyone quoting a single size for a standby generator without asking how far the run is has skipped the half of the calculation that actually varies.
Worked examples at the four most common runs
Each of these shows the full NFPA 54 lookup for that distance, the nearby sizes and the margin:
- Gas line size for a standby generator at 20 feet — 3/4 inch
- Gas line size for a standby generator at 40 feet — 3/4 inch
- Gas line size for a standby generator at 60 feet — 1 inch
- Gas line size for a standby generator at 100 feet — 1 inch
If a second appliance shares the run
The code sizes the total connected load, so this is where undersized lines come from. At 60 feet a standby generator alone needs 1 inch. Add a 100,000 BTU furnace to the same pipe and the connected load becomes 250,000 BTU, or 250 CFH, which needs 1 inch at that same 60 feet — the same size, so the original run still carries both. Size the pipe for everything that will ever hang off it.
Propane instead of natural gas
Propane carries 2,516 BTU per cubic foot against natural gas at 1,000, so the same standby generator draws only 60 CFH on LP. PlumbCalc returns 1/2 inch at 20 feet and 3/4 inch at 100 feet. Those figures are deliberately conservative — they are read off the natural gas table, so they come back equal to or larger than a dedicated LP table would give. Confirm any LP work against an LP table and local code.
Before you cut any pipe
These figures come straight from NFPA 54 Table 402.4(2) for Schedule 40 metallic pipe, natural gas at 0.60 specific gravity, inlet pressure under 2 psi and an allowable pressure drop of 0.5 inches water column. That is the ordinary residential case, and it is the same table the PlumbCalc gas line sizing calculator uses.
Two things change the answer on a real job. First, the code sizes the total connected load on a run, not one appliance in isolation — if a second appliance shares the pipe, add its BTU rating before you size. Second, a 2 psi system, a CSST or copper material, or a longer allowable pressure drop all use different tables entirely. Size to the longest run, size to the full load, and confirm against your local code and the appliance data plate before installing.
The governing standard is the National Fuel Gas Code, published as NFPA 54 and adopted in most of the United States — see the National Fuel Gas Code for background on the code itself.
Related sizing references
Frequently asked questions
What size gas line does a standby generator need?
A typical 150,000 BTU standby generator draws 150 CFH of natural gas and needs 1/2 inch up to 10 feet, 3/4 inch from 20 to 50 feet, 1 inch from 60 to 150 feet, 1-1/4 inch from 175 to 200 feet, in Schedule 40 metallic pipe. The size depends on the longest run from the meter, so measure the run before you buy pipe.
Why does the run length change the gas line size for a standby generator?
Gas loses pressure to friction as it travels, and NFPA 54 Table 402.4(2) is built around a fixed allowable drop of 0.5 inches water column. A longer run spends that allowance sooner, so the same diameter delivers fewer CFH. That is why the table has a column for every length from 10 to 200 feet.
What if my standby generator is rated higher than 150,000 BTU?
Use the rating on the appliance data plate, never the typical figure. Divide the real BTU input by 1,000 for natural gas to get CFH demand, then read down the column for your run length until you reach a capacity that meets it. The PlumbCalc gas line sizing calculator does exactly that in one step.
Can a standby generator share a gas line with another appliance?
Yes, but you must size for the combined load. At 60 feet the standby generator alone needs 1 inch; add a 100,000 BTU furnace and the 250 CFH total needs 1 inch. Add every BTU rating on the run together before you look at the table.
Does this cover CSST, copper or a 2 psi system?
No. These sizes are Schedule 40 metallic pipe on an ordinary under-2-psi residential system with a 0.5 inch water column drop. CSST, copper tubing and polyethylene each have their own capacity tables, CSST varies by manufacturer, and a 2 psi system with appliance regulators uses a different table entirely and usually allows smaller pipe over long runs.