Running a Natural Gas Line to a Standby Generator

Every generator lists its natural gas consumption on the spec sheet, in cubic feet per hour or BTUs per hour, at half load and full load. Natural gas carries roughly 1,000 BTU per cubic foot, so the two numbers are close to interchangeable. Size from the full-load figure, because an outage in January with the heat, the well pump and the kitchen running is when it will be at full load.

For scale: a popular 22 kW air-cooled model is rated by its maker at a bit over 300 cubic feet an hour on natural gas at full load. A typical gas furnace is somewhere between 40,000 and 100,000 BTU, and a tank water heater around 40,000. The generator can outweigh all of them.

A gas line has to deliver its full load over its full length without the pressure dropping below what the appliances need. Generators are particular about this. The installation manual will give a required inlet pressure range, in inches of water column, measured at the generator while it runs under load. Too little and it will start, then stumble or shut down when the load comes on.

Sizing follows the tables in the fuel gas code, by pipe material, pressure and developed length. The total is the generator's full-load figure plus everything else that could run during an outage: furnace, water heater, range, dryer, gas logs. Often the right answer is a dedicated line from the meter rather than a tee off the nearest pipe, and sometimes it is a larger pipe back toward the meter for the whole house.

Where the generator sits matters as much as its size. A pad far from the meter means a long run, and a long run means a bigger pipe. Manufacturers also set minimum distances from windows, doors, vents and the house, because generator exhaust contains carbon monoxide. Choose a spot that works for both the exhaust rules and the gas run.

Your meter has a rated capacity. A common residential meter is rated around 250 cubic feet an hour, which a single large generator can exceed on its own. The utility, Piedmont Natural Gas for most of Spartanburg County, decides whether your meter and service line can carry the new total, and replaces the meter or changes the delivery pressure if they cannot.

That means telling the utility about the added load early, with the generator's full-load figure and a list of the other gas appliances. A meter change is their work, on their schedule, and it can be the longest lead item in the project. Planning it before the generator is delivered keeps a new unit from sitting on its pad with no gas.

A standby generator job splits cleanly between two trades.

• Electrical: the transfer switch, the wiring from the generator to the switch and panel, load management modules, and the electrical permit and inspection. This is a licensed electrician's work.
• Gas: sizing and running the gas pipe from the meter to the pad, a shutoff valve and sediment trap at the generator, the flexible connector the manufacturer specifies to isolate vibration, pressure testing, the gas permit and inspection, and checking inlet pressure with the generator running.
• Shared: the pad location, which has to satisfy the manufacturer's clearances, the electrical run and the gas run all at once.

In Spartanburg County, gas piping and electrical work are permitted separately, and each trade calls for its own inspection under its own permit number. Inside the City of Spartanburg the city's building inspections office issues the permits; in unincorporated areas it is Spartanburg County Building Codes.

The gas line will be pressure-tested before it is connected. Under the fuel gas code the minimum is one and a half times the working pressure, never below 3 psi, for at least 10 minutes, and many inspectors ask for more. Only after the test and inspection should the generator be connected, purged, started and checked under load.