Tri-fuel

The derating, fuel by fuel, and why it happens

The order is always gasoline, then propane, then natural gas, and the reason is the energy density of the intake charge. Natural gas is mostly methane arriving at near-atmospheric pressure, so it displaces a large fraction of the air the engine could otherwise breathe; propane displaces less; gasoline, injected or metered as a liquid that vaporises in the port, displaces least. Typical published numbers on a tri-fuel portable: 9,500 W on gasoline, about 8,500 W on propane, and about 7,000–7,600 W on natural gas. Manufacturers publish all three columns on tri-fuel units, and the natural-gas column is the one to use if the house line is the primary plan.

The surge rating derates by the same proportion, and surge is what starts motors. A tri-fuel unit that starts a one-horsepower well pump on gasoline may simply refuse to on natural gas. If a well pump is the reason the generator exists, size from the natural-gas surge figure, fit a soft starter, or accept that pump duty means switching to gasoline or propane.

What you buy with that lost 20–30% is the only fuel that does not run out. Buried gas distribution usually survives the storms that take down overhead power lines, so a tri-fuel portable connected to the house line behaves like a standby generator you can also wheel away. The failure cases are real but narrow: earthquakes trigger automatic and utility shutoffs, and a main break or an extreme cold snap can interrupt supply. Keeping the gasoline and propane paths is the entire point of buying tri-fuel rather than a natural-gas-only unit.

The gas connection: pressure, meter capacity, and pipe size

Residential gas leaves the meter regulator at about seven inches of water column — roughly a quarter of a pound per square inch, which is far lower than most people assume. Generator manuals typically want 7–11 in WC static and no less than about 5 in WC with the engine at full load. Pressure drop along an undersized run is what ruins these installations: the generator starts, runs well, and then falls on its face the moment the furnace also fires and the shared pipe cannot supply both.

Meter capacity is the next constraint and it is pure arithmetic. At about 1,030 BTU per cubic foot, a 22 kW air-cooled standby needs roughly 300 ft³/hr at full load and about 190 at half; a 7,500–9,500 W tri-fuel portable needs roughly 100–150 ft³/hr. Now add the existing house: a 100,000 BTU/hr furnace is 97 ft³/hr, a 40,000 BTU/hr water heater is 39, a range is around 65, a dryer around 22. A common 250 ft³/hr meter has no room left for a generator. Utilities will usually swap a meter for a larger one at little or no cost — but on their schedule, which is not during a storm.

Piping is sized from NFPA 54, the National Fuel Gas Code, using tables that combine run length, allowable pressure drop, and total demand. A sixty-foot run out to a generator pad commonly lands on 1 in or 1¼ in black iron where the ½ in branch that fed a barbecue would have been fine. Every installation needs an accessible manual shutoff valve and a sediment trap at the appliance, and the flexible connection must be a listed connector rather than a barbecue hose. This is gas-fitter work under its own permit, entirely separate from the electrical permit for the inlet and transfer equipment.

Quick-connects, hose length, and the 20-foot problem

The usual arrangement is a tee off the house line, a hard-piped run to an exterior wall or a pad, and a listed quick-disconnect fitting at the end. You wheel the generator out, snap the hose on, open the valve, and start. Connectors and hoses for portable outdoor gas appliances fall under ANSI Z21.54; length is limited, commonly to about twelve to fifteen feet, and joining two hoses to gain reach is specifically not permitted.

That creates a direct conflict with generator placement. CPSC wants a running generator at least twenty feet from the house with the exhaust pointed away, and a twelve- to fifteen-foot hose does not reach twenty feet from the wall the stub comes out of. The correct resolution is a longer hard-piped run terminating where the generator will actually stand, not a longer hose. Decide where the generator lives before the pipe is installed, because moving a stub afterwards means opening a trench.

Running cost is where natural gas justifies itself. At $1.20 per therm, a portable drawing 120 ft³/hr costs about $1.50 an hour to run. The same machine on gasoline at 0.9 gal/hr and $3.50 a gallon costs $3.15 an hour, and on exchange-cylinder propane closer to $5. Over a four-day outage running twenty hours a day, that is roughly $120 against $250 against $400 — and no queuing at a gas station that has no power to run its pumps.

Conversion kits and the legal position

Aftermarket kits that add natural gas to a gasoline or dual-fuel generator are widely sold and mostly work. They are also a modification of a certified engine’s fuel system, which the Clean Air Act treats as tampering unless the kit carries its own EPA certification for that engine family — and a CARB Executive Order if the machine is in California. Many are marketed with language about off-road or emergency use that does not create an exemption, and engine manufacturers routinely deny warranty claims on units carrying them. A factory tri-fuel generator is certified as built.

Whichever route you take, budget for two trades and two permits — a gas fitter for the tee, pipe, shutoff valve, sediment trap, and quick-connect, and an electrician for the inlet, cord, and transfer equipment. Both are inspected, and pricing them together avoids discovering the second job after the first is finished.