How to Size Natural Gas Pipe to a Standby Generator

Why this matters

An undersized gas line is the most common reason a new standby starts fine on exercise and falls over in a real outage. Exercise runs at no load and asks for a fraction of the gas; a full house load on a cold night asks for all of it, at the same time as the furnace and water heater. The pressure at the generator sags, the engine leans out, voltage and frequency droop, and the unit shuts down or drops the load it was installed to carry. The fix after the fact is a trench and a re-pipe. The sizing takes twenty minutes at the quote stage.

Who sizes it, who builds it

Gas piping and regulators go to a licensed gas fitter or plumber wherever the locality requires, and the utility owns the meter and service regulator. The generator tech's job is the takeoff: every load, every length, the system pressure, and a proposed size the fitter confirms against the code their jurisdiction has adopted. Doing the takeoff yourself means the quote carries the right pipe the first time instead of a change order after the fitter walks the job.

The shop's fuel-supply rules, including what pressure range to design for, are owned elsewhere. See related: Gas Supply Sizing and Connection Standard. The heat content numbers used below are owned by a sibling card. See related: Natural Gas and Propane Heat Content and Generator Consumption.

Step 1: Add every load at full rate, not just the generator

Pipe is sized for the total connected load, meaning every appliance on the system firing at its full nameplate input at once. Diversity does not apply: the code tables assume everything runs, and in an outage that is close to true, because the furnace runs harder on a cold night with no power to anything else. Never size the generator's line from the generator alone if it shares any section with the house.

Read inputs off the rating plates, in BTU per hour, then convert to cubic feet per hour of gas. Natural gas carries about 1,000 BTU per cubic foot, so divide BTU per hour by 1,000. The local utility can tell you its actual heating value if you need it closer; it runs a little either side of 1,000.

The generator's number comes from its spec sheet, at full load, on natural gas, in cubic feet per hour. Use the full-load figure, not the half-load figure the brochure often leads with. See related: Reading a Generator Spec Sheet and Derating.

Step 2: Measure to the farthest outlet

The longest-length method sizes every section of the system using one length: the developed length from the point of delivery (the outlet of the meter or service regulator) to the most remote outlet. Developed length means along the pipe, every rise, drop and offset included, not the straight-line distance across the yard.

On a retrofit the generator is almost always the farthest outlet, because it sits outside the house at the setback the siting rules demand. That is why adding a generator usually changes the whole system: the new longest length applies to the house sections too.

Measure with a wheel or a tape, and add the vertical runs: down from the meter outlet, under the slab or along the foundation, up to the generator's inlet. Round up to the next row of the table, never down.

Step 3: Pick the right table

The adopted code's capacity tables are organized by three things, and you need all three before you read a number:

• Pipe material. Schedule 40 steel, copper tubing, polyethylene and corrugated stainless steel tubing each have their own tables. CSST is sized from its maker's tables, not the steel tables.
• System pressure. Most residential systems run at low pressure, under 2 psi, typically about 7 inches of water column at the meter outlet. Some newer homes run 2 psi with regulators at the appliances, which uses a different table entirely.
• Allowable pressure drop. The common residential table assumes a 0.5 inch water column drop from the meter to the farthest outlet. That only works if what is left at the generator still sits inside the generator maker's running pressure range.

Those tables live in NFPA 54 (the National Fuel Gas Code) and in the International Fuel Gas Code, chapter 4, whichever your jurisdiction has adopted. The IFGC sets out the longest-length method at 402.4 in substance; check the current edition.

Step 4: Size each section for the load it carries

Walk the system from the meter outward. For each section, add up every load downstream of it, go to the row for the longest length, and pick the smallest pipe whose capacity equals or exceeds that load. The length stays the same for every section; only the load changes.

The worked sizing

A two-story house on natural gas at about 7 inches water column, Schedule 40 steel throughout, sized on the 0.5-inch drop table. The candidate generator's spec sheet gives 310 cubic feet per hour at full load on natural gas (illustrative; read the sheet for the unit you are quoting).

 LOADS (rating plate BTU/h / 1,000 = CFH)
   furnace        100,000 BTU/h ->  100 CFH
   water heater    40,000 BTU/h ->   40 CFH
   range           65,000 BTU/h ->   65 CFH
   dryer           22,000 BTU/h ->   22 CFH
   house subtotal                    227 CFH
   generator, full load (spec)       310 CFH
   TOTAL CONNECTED                   537 CFH

 LAYOUT (developed lengths)
   METER --A 15 ft-- TEE --B 45 ft-- GENERATOR
                      |
                      C 20 ft -- house manifold
                                 (branches 10 ft or less)

 LONGEST LENGTH  meter to generator = 15 + 45 = 60 ft
 TABLE ROW USED  60 ft, Sch 40, under 2 psi, 0.5 in drop

 SECTION  LOAD    PIPE     ROW CAPACITY
 A        537     1-1/2    791 (1-1/4 = 528, short)
 B        310     1-1/4    528 (1 in  = 257, short)
 C        227     1        257
 furnace  100     3/4      137
 range     65     3/4      137 (1/2 = 65, no margin)
 WH        40     1/2       65
 dryer     22     1/2       65

The row capacities are from the commonly published Schedule 40, 0.6 specific gravity, 0.5-inch drop table at 60 feet. Read them again from the table in your adopted edition before the quote goes out; a transcribed table is the easiest place for a wrong number to hide.

Why section A jumps two sizes. The existing house had a 1-inch line from the meter carrying 227 cubic feet per hour, which the 60-foot row would just pass and the house's old shorter longest length passed easily. Adding 310 for the generator puts 537 through that first 15 feet, and 1-1/4 inch at 60 feet carries only 528. Nine cubic feet per hour short is still short. Section A goes to 1-1/2.

Why the range branch is 3/4. Half-inch at 60 feet carries exactly 65 cubic feet per hour, the range's full input. Equal technically passes, but it leaves no room for a fitting count the table did not anticipate or a range swapped for a bigger one later. The shop's practice is to step up where the capacity equals the load with no margin.

The cross-check. Re-add the loads: 100 + 40 + 65 + 22 = 227, and 227 + 310 = 537. Every downstream section carries less than A, and A's 791 covers 537 with room. Then sanity-check the generator line on its own: 310 cubic feet per hour is 310,000 BTU per hour, about three of the house's furnaces running at once, so a pipe one size up from the furnace's 3/4 would be the wrong instinct and 1-1/4 is the plausible answer.

What changes the answer

The meter is not yours to size. Many residential meters and service regulators are rated well below 537 cubic feet per hour. Before anyone trenches, the utility confirms its meter and regulator can deliver the new total at the delivery pressure, and if not, it schedules an upgrade. That lead time can set the install date.

A 2 psi system changes the table, not the method. On an elevated-pressure system, the section from the meter to the generator's own line regulator is sized from the 2 psi table, and the regulator's outlet must land inside the generator maker's range. Smaller pipe often works, which is why some installers ask the utility for elevated pressure on a long run.

A dedicated line from the meter means section A disappears: the generator gets its own outlet and its own longest length, and the house system stays as it was. The utility has to agree to the arrangement.

A long run to the generator. Under the longest-length method a long generator line drags every house section onto a longer row, so a 120-foot run can upsize piping inside the house that never changed. The branch-length method in the adopted code sizes each section from its own path and often keeps the house sections where they were. The fitter chooses the method; the code permits either.

How it fails, and how to catch it

The undersized line passes every test a tech runs at no load. Static pressure at the generator reads normal. The unit starts and exercises. It fails at full load, which nobody applies until the first outage.

So the finish test is a pressure reading under full load, at the generator's test port, compared against the maker's running range, with the furnace and water heater firing at the same time if the season allows. A drop well beyond what the design assumed points at a section, the meter or the regulator, and goes back to the fitter or the utility with the readings. See related: How to Measure Gas Pressure at the Generator, Static and Under Load.

References

• NFPA 54, National Fuel Gas Code, or the International Fuel Gas Code chapter 4 (402.4), in the edition your jurisdiction has adopted
• The generator spec sheet for full-load consumption on natural gas; the CSST maker's sizing tables where CSST is used
• The serving gas utility for meter and regulator capacity and delivery pressure
• See related: Gas Supply Sizing and Connection Standard; Natural Gas and Propane Heat Content and Generator Consumption; How to Measure Gas Pressure at the Generator, Static and Under Load