Is a 250 Gas Meter Big Enough for a 24 kW Generac Generator

Not for a 20 to 24 kW Generac Guardian on natural gas. Honeywell's AC-250 is rated 250 cubic feet per hour at a 0.5 in. w.c. pressure drop, and every air-cooled 20, 22 and 24 kW Guardian on Generac's spec sheet burns 287 to 327 cfh at full load before the furnace or water heater lights.

The meter belongs to the gas company, so the decision is theirs, but the arithmetic is yours to bring. This guide reads the meter's rating table, pulls the generator's fuel figures from its spec sheet, adds up a real house from a forum thread in cfh, and sizes the pipe from the meter. The pipe sizes come from Table 402.4(2) of the 2021 IFGC, published by the International Code Council (ICC), the same table the free gas pipe sizing calculator reads.

How much gas does a 250 meter actually pass?

250 cfh at a 0.5 in. w.c. drop across the meter, with 0.60 specific gravity gas. Honeywell lists that as the AC-250's maximum flow capacity. The older American Meter bulletin also prints 565 cfh at a 2 in. drop, the kind of figure behind claims that the meter can pass more. It's the gas company's meter to size, and the published maximum is 250.

The 250 in the model name matches the rating at a 0.5 in. drop, and every rating is a flow at a stated pressure loss. Here are the ratings from the older bulletin and Honeywell's current pages, which list 250 and 425 cfh as "Maximum Flow Capacity." Honeywell's bulletin for the current AC-250NX says it "meets and exceeds ANSI B109.1 requirements," the American standard for diaphragm meters, at the same 250 cfh rating.

0.25 psig is about 6.9 in. w.c., since 1 psi is about 27.7 in. of water. One city gas utility we checked, Enumclaw, Washington, sets its standard residential delivery at "7 inches water column (nominal)" and doesn't offer 2 psig service to single-family homes.

The 2 in. rows are flows at four times the pressure loss, and the current AC-250NX bulletin doesn't print one at 0.25 psig at all. Whatever the meter drops comes off the pressure at its outlet, and that's what the house piping and the appliances share. Plan on 250 cfh for an AC-250 and let the gas company say otherwise.

How much gas does a standby generator burn?

Generac's spec sheet lists 306 cfh at full load and 203 cfh at half load for the 24 kW Guardian on natural gas, and 327 cfh at full load for one 22 kW model. It also says the fuel pipe "must be sized for full load" and that the generator needs 3.5 to 7 in. w.c. at its fuel inlet at every load.

In the house worked below, the generator outburns both furnaces together, 306 cfh against 180. These are natural gas figures by model number from Generac's specification sheet for its 20, 22 and 24 kW Guardian units, Rev. E, March 2022. Read your own model's column, since the two 20 kW models on the same sheet differ by 14 cfh at full load.

Generac converts with 1,000 BTU per cubic foot for natural gas, so 306 cfh is 306,000 BTU/h. That's the "300k BTU" in the thread below. The sheet also says maximum power drops about 3.5% for each 1,000 ft above sea level, and IFGC 402.2 requires the gas flow to be adjusted for altitude above 2,000 ft.

Generac 20, 22 and 24 kW Guardian specification sheet § Fuel consumption, note

"Fuel pipe must be sized for full load." - Generac Power Systems, generac.com

Is a 250 meter enough for the house in the thread?

Not with everything counted. With our assumed inputs, the house connects 242 cfh before the generator, about 97% of an AC-250's 250 cfh rating. Add a 24 kW Generac at 306 cfh and the connected load is 548 cfh, 2.2 times the AC-250 rating and 1.3 times an AL-425. Even a summer outage with only the generator, water heater and dryer running is 368 cfh.

The question comes from a Houston homeowner on r/Plumbing. They're getting a 24 kW Generac and have two furnaces on 3-ton and 4-ton systems, a gas dryer and a 40 gallon water heater on "a 250 meter." The installer says no upgrade is needed. In the post's photo, the meter's badge reads American Meter AC-250, rated 250 cfh at a 1/2 in. differential. The post doesn't list appliance inputs, so we assumed common nameplate values. Swap in your own from the rating plates.

1. The furnace on the 4-ton system is 100,000 BTU/h input, or 100 cfh.
2. The furnace on the 3-ton system is 80,000 BTU/h, or 80 cfh.
3. The gas dryer is 22,000 BTU/h, or 22 cfh.
4. The 40 gallon water heater is 40,000 BTU/h, or 40 cfh.
5. The Generac 24 kW at full load is 306 cfh.

Each cfh figure is the input divided by 1,000 BTU per cubic foot. Your gas company publishes the local heating value if you want it exact. Generac already gives its figure in cfh, so leave the generator at 306.

No row with the generator fits under 250 cfh. The closest is the last one, 6% over, and it assumes the generator never passes half load. The poster expects that, since the air conditioners rarely push the unit past 50% except on startup. That's a reasonable read of their own house, but it's a diversity argument. IFGC 402.2 allows diversity for pipe sizing only "where a diversity of load can be established," and the meter question goes to the gas company.

Who decides whether the meter gets upgraded?

The gas company. The IFGC defines the point of delivery as the outlet of the service meter assembly, and the meter, regulator and service piping ahead of it are installed by the gas supplier. Code piping starts at the meter outlet. Bring the supplier the existing load in cfh by appliance, the new load and the generator's required inlet pressure in inches of water column.

Liberty Utilities, on its Central region site, spells out the order of steps on its page about adding natural gas load, and it names backup generators.

Liberty Utilities, Adding Natural Gas Load § Residential safety

"No customer should purchase and/or install additional equipment until Liberty determines that the service and/or meter can handle the increased natural gas load." - Liberty Utilities, libertyutilities.com

Liberty asks for "the total existing natural gas load (expressed in CFH or MMBtu/Hour), by appliance," plus the new load and its operating pressure in inches of water column. It also says customers will be held responsible for damage to its equipment from load added without notice. The table above is that list in the form a utility asks for it, 242 cfh existing and 306 cfh new. A meter swap also doesn't change the customer's pipe from the meter outlet, so the house piping still has to carry the new total on its own.

What pipe does the generator need from the meter?

On a 60 ft longest run, our version of the thread's house needs a 1-1/2 in. main from the meter with the generator, against 1 in. without it, and a 1-1/4 in. generator branch, per IFGC Table 402.4(2) at a 0.5 in. w.c. drop. If the gas company holds 7 in. w.c. at the meter outlet, 6.5 in. reaches Generac's 3.5 to 7 in. inlet range.

The longest length method in IFGC 402.4.1 sizes every section of a low-pressure system from the row for the longest run. Say the farthest outlet is 60 ft from the meter. On the 60 ft row of Table 402.4(2), 1/2 in. carries 65 cfh, 3/4 in. 137, 1 in. 257, 1-1/4 in. 528 and 1-1/2 in. 791.

The 548 cfh main misses 1-1/4 in. by 20 cfh, so it moves up a size. If the existing main is 1 in., which is what 242 cfh needs on this row, teeing the generator into it puts 548 cfh on pipe good for 257. IFGC 401.8 also requires pipe for an installation, extension or alteration to be "sized to supply the full number of outlets for the intended purpose." Stub a generator outlet now and it's in the sum. Add the generator next year and that extension gets sized for the new load on every section it passes through, the main included.

The table drop is 0.5 in. w.c. Enumclaw's generator guidance asks contractors for a design drop of "typically 0.3-0.5 in. w.c." and a pipe sizing calculation "based on NFPA 54 or approved equivalent" with total equivalent length. Generac's sheet says the unit's sediment trap "Meets IFGC and NFPA 54 installation requirements," and the pipe to it falls under whichever fuel gas code your jurisdiction adopted. Count the fittings too. Enumclaw's list asks for total equivalent length including fittings, so add the elbows and tees to the run before you pick the row.

The 7 in. at the meter outlet is an assumption until the gas company confirms it. An AC-250 pushed to 548 cfh would sit near its old 565 cfh row, rated at a 2 in. drop, which is one more reason the meter is the gas company's call. Enumclaw also asks for the generator's minimum inlet pressure at steady state and at start, so look for both in the installation manual and check them against what's left, along with every other appliance's minimum on its rating plate.

What mistakes show up on generator gas jobs?

Six mistakes, each with a number or a code line: piping for the half-load figure, reading the meter's 2 in. row without its drop, counting only the generator, buying before the gas company checks the service, assuming 2 psi service without asking the gas company, and skipping the altitude adjustment above 2,000 ft.

Here's the half-load mistake in numbers. Piping for 203 cfh on the 60 ft row gives a 1 in. branch, which carries 257 cfh. At full load the generator needs 306, so the branch comes up 49 cfh short just when the house is leaning on it hardest.

Running the numbers with BuildSolver

Enter each appliance input in BTU/h and the longest run in the gas pipe sizing calculator. It returns the main from the meter and each branch from IFGC Table 402.4(2). It doesn't check the meter or the regulator, so take the main's total in cfh to the gas company along with the generator's inlet pressure range.

The calculator is free without signup and runs in your browser. It sizes Schedule 40 steel at low pressure for natural gas or propane, and it holds up to six appliances, enough for the thread's house.

1. Read the input rating off every gas appliance's plate, and the full-load figure off the generator's spec sheet.
2. Measure the longest run from the meter to the farthest outlet, and add fitting allowance if the run has a lot of fittings.
3. Enter the inputs and the run in the gas pipe sizing calculator. For the thread's house at 60 ft it returns a 1-1/2 in. main and a 1-1/4 in. generator branch.
4. Send the gas company the load list in cfh, the new total, the generator's 3.5 to 7 in. w.c. inlet range from the spec sheet and any start minimum the installation manual lists, before anyone orders equipment.

One caution on the input box. The calculator divides BTU/h by the heating value you enter. If you set a local value instead of 1,000, enter the generator as 306 cfh times 1,000, or times your local value when it's above 1,000, so the pipe never carries less than the 306 cfh Generac lists. The result repeats that rule whenever the heating value isn't 1,000. Some jurisdictions also enforce NFPA 54 or the Uniform Plumbing Code for fuel gas instead of the IFGC, so check which table your inspector uses.

If the furnace input is still an open question, size it first with the furnace size calculator, then come back to the gas sum.

Sources

• Honeywell (American Meter), AC-250 Basic and AL-425 Basic product pages: honeywell.com and honeywell.com
• American Meter, AC-250 bulletin SB-3535.3, Rated Gas Capacity table: shopify.com
• Honeywell American Meter, AC-250NX diaphragm meter bulletin: shopify.com
• Generac Power Systems, 20, 22 and 24 kW Guardian specification sheet, Part No. A0000937814 Rev. E, March 15, 2022: generac.com
• International Code Council, 2021 International Fuel Gas Code, Sections 401.8, 402.1, 402.2, 402.4.1 and Table 402.4(2): codes.iccsafe.org
• International Code Council, 2021 International Fuel Gas Code, Chapter 2, Point of Delivery and Service Meter Assembly: codes.iccsafe.org
• Liberty Utilities, Adding Natural Gas Load: libertyutilities.com
• City of Enumclaw Gas Utility, Residential Gas Service Pressure Policy and Generator Guidance: cityofenumclaw.net

For estimation purposes only. The gas supplier sizes the meter and service, and a licensed gas fitter sizes, installs and tests the piping to the code your jurisdiction has adopted.