Gas Supply Sizing and Connection Standard
Purpose
A generator that starts clean on a no-load test and starves the moment the AC compressor and the water heater join the load isn't a generator problem, it's a fuel supply that was sized off the generator's nameplate alone and never checked against everything else already pulling on the same meter or tank. This procedure sets the standard for verifying supply capacity and connecting the gas line, so the pressure a tech reads at commissioning reflects what the system actually delivers under real combined load, not just what a static test showed with nothing else running.
Scope
Covers capacity verification, line sizing, connection, and pressure/leak testing for a residential or light commercial standby generator's fuel supply, natural gas or propane, from the existing meter or tank to the generator's own gas train inlet. It does not cover propane vapor-versus-liquid draw physics or the line-sizing math itself, owned by the propane vapor vs liquid line sizing reference; fuel-type selection or conversion between natural gas and propane, owned by the natural gas and propane conversion reference; or the annual inspection of an already-connected supply, owned by the annual generator maintenance SOP. This procedure is the one-time standard for a new or resized connection.
Roles and responsibilities
| Role | Owns | The handoff |
|---|---|---|
| Licensed gas fitter or plumber | Line sizing, connection, pressure and leak test | Signs the test record before the electrician or PM schedules commissioning, not after |
| PM or office | Capacity verification with the utility or propane supplier | Confirms meter or regulator capacity in writing before pipe is ordered |
| Electrician | Bonding jumper coordination | Confirms bonding point and size against the gas fitter's line material before either trade closes out |
| Customer | Disclosure of other gas appliances and any planned additions | Names every existing gas appliance before the load calculation runs, not after a shortfall shows up |
Procedure
Compute total connected load before sizing anything. List every existing gas appliance's BTU/hr rating from its nameplate or the customer's utility records, add the generator's full-load BTU/hr demand from its own nameplate, and total it in writing. Acceptance: a written total with a source cited for each appliance figure, not an estimate. Wrong looks like taking the customer's guess ("we've just got a furnace and stove") at face value and missing a pool heater or a second furnace on a zoned system. Stop rule: no line sizing until every existing appliance is accounted for with its actual rating. Hazard: none at this step; it is a records survey, not field work.
Verify the existing meter, regulator, or tank capacity against the new total before ordering pipe. Pull the meter's capacity rating from its nameplate or the utility, or the tank's first-stage regulator capacity for propane, and compare it to the step 1 total with margin. Acceptance: a documented capacity figure that exceeds the total connected load, sourced from the meter tag or the utility/supplier directly, not assumed from "standard residential service." Wrong looks like assuming a standard meter covers any residential addition, the exact failure the whole-house install SOP's own top warning exists to prevent. Stop rule: if capacity is inadequate, escalate to the utility for a meter upgrade or to the propane supplier for a larger tank before pipe is cut. Hazard: none at this step; it is a records check, not field work.
Size the line using the actual effective length and the applicable code table, not a rule of thumb. Measure the run from source to generator, add fitting equivalent lengths, and look up pipe size against the code table for the system's pressure class (see the propane line sizing reference for the LP math, NFPA 54's gas table for natural gas). Acceptance: a written pipe size and material selected from an actual table lookup using the measured effective length. Wrong looks like matching the new run to whatever size feeds the furnace because "that's close enough," which ignores that the new run is longer and now carries more total load. Stop rule: no pipe ordered or cut until the sizing calculation is written down. Hazard: none at this step; it is a table lookup against measured length.
Run and support the line, and install the drip leg, accessible shutoff, and bonding jumper before any joint is made up permanently. Route and support the pipe per code, install a drip leg (a vertical drop ahead of the appliance connection that catches condensate and debris before the gas train), a shutoff valve within reach of the unit, and a bonding jumper sized and landed per NEC 250.104(B), coordinated with the electrician. Acceptance: drip leg, shutoff, and bonding jumper each physically present and verified, not just noted on the plan. Wrong looks like omitting the drip leg because the run looks short and dry, which lets condensate or pipe-dope debris reach the gas train and fault the generator on its first cold start. Stop rule: no pressure test until all three are physically confirmed in place. Hazard: cutting and threading pipe throws metal shavings and carries pinch risk at the vise and threader; eye protection and gloves during cutting and threading, clear of the die head while it turns.
Before pressurizing, confirm no ignition sources are present and the work area is ventilated, then pressure test to the code-required test pressure and duration. Clear open flame, running equipment, and unnecessary personnel from the test area, then pressurize per NFPA 54 or the locally adopted code's test pressure and duration for the pipe material and pressure class, watching the gauge for any drop. Acceptance: gauge holds with zero drop for the full stated duration, start and end readings and elapsed time recorded. Wrong looks like testing at operating pressure only, or eyeballing the gauge for a minute and calling it good. Stop rule: any drop is a hard stop; find and repair the leak and retest from zero, do not proceed to connection on a failed or incomplete test. Hazard: a pressurized fuel gas system under test; if a joint fails audibly or a strong gas odor develops, stop, shut the source valve, ventilate, and do not introduce any ignition source until the area is confirmed clear.
Leak test every mechanical joint at operating pressure with an approved solution or detector, including the joint made at the generator's own gas train inlet after the line test. Soap-test or electronically test each joint, paying particular attention to any connection made after the step 5 pressure test, since that joint was never inside the tested section. Acceptance: no bubble formation or detector response at any joint over a full check at each one. Wrong looks like trusting the earlier pressure test to cover the final union at the generator, the one joint made after that test passed and therefore never verified by it. Stop rule: any joint that shows a leak is remade and retested before proceeding, never tightened by feel and moved past. Hazard: testing at operating pressure with gas live in the line; keep ignition sources clear during the check, and if a joint vents visibly rather than just bubbling, shut the source valve before remaking it.
Commission with the generator running at a real load step, and read pressure at the gas train under that load, not at idle. Start the generator, apply a load (a resistive load bank step or the actual transfer test load), and read a manometer at the generator's gas train inlet during that load. Acceptance: the loaded reading holds at or above the manufacturer's stated minimum operating pressure for that train, recorded with the load percentage applied. Wrong looks like signing off on the no-load reading alone, which is exactly the combined-load starvation scenario this whole procedure exists to catch. Stop rule: no gas connection sign-off without a recorded loaded pressure reading that meets the manufacturer minimum. Hazard: a running, loaded generator near the manometer connection point; keep hands clear of moving belts, fans, and hot exhaust while reading the gauge.
Document and tag the completed connection before the job closes. Record pipe size, material, effective length, test pressure/duration/result, capacity verification source, and loaded pressure reading in the job file, and tag the shutoff valve's location for future service. Acceptance: every field populated in the job file, shutoff location tagged and visible. Wrong looks like closing the job on a verbal "it's all good" with the record filled in later from memory, which is how a test pressure or a joint result gets misremembered. Stop rule: the job does not close until the written record matches what was actually measured on site. Hazard: none; a documentation step.
The record this produces
One gas supply record per install, filed with the job's install package. Fields: appliance load list and total, meter/regulator/tank capacity source and figure, pipe size and material with the sizing calculation, pressure test start/end readings and duration, leak test results per joint including the final connection, loaded pressure reading with load percentage, and the shutoff tag location. Future service techs read the pipe size and material before any tie-in for an added appliance; the office reads the capacity figure before quoting any future load addition on the same meter.
One pass, with a step that failed
A 20 kW natural gas unit added to a home already running a furnace, a tankless water heater, and a gas range. Step 1 totaled the existing appliances plus the generator's full-load demand from its nameplate, sourced from each appliance's own rating plate.
Step 2 is where the pass failed. The gas fitter pulled the meter's capacity tag and found it rated below the step 1 total once the generator's demand was added in. The stop rule caught it before any pipe was ordered: the office contacted the utility, which confirmed a meter upgrade at no charge for the documented load increase, and the upgrade was scheduled ahead of the pipe work.
With the larger meter installed, step 3 sized the line off the new effective length and the upgraded meter's pressure class, landing on a larger pipe size than the original plan. Step 4 ran the line, installed the drip leg, shutoff, and bonding jumper, confirmed by the electrician. Step 5 pressure tested with the area cleared of ignition sources; the gauge held with zero drop over the full duration. Step 6 leak tested every joint including the final union at the generator train, all clean. Step 7 commissioned under a load bank step: the loaded reading held comfortably above the manufacturer's minimum, confirming the upgraded meter and resized line actually solved the capacity problem step 2 found, not just the paperwork.
When it does not fit the usual case
The supply is propane from an existing tank rather than a utility meter: step 2's capacity check runs against the tank's vaporization rate and first-stage regulator instead of a meter tag, per the propane line sizing reference. An existing line is being reused rather than replaced: steps 3 and 4 still apply in full, since an existing line sized for the old appliance load is not verified for the new total until it is recalculated and re-tested, never assumed adequate because it already exists. The site has multiple meters or a commercial multi-tenant gas system: identify which meter actually serves the generator's intended tie-in point before step 1's total is even built, since totaling against the wrong meter passes a check that means nothing.
References
- Propane vapor vs liquid line sizing reference, for the LP sizing math and vaporization-rate tables this procedure applies rather than re-derives.
- Generator natural gas and propane conversion reference, for fuel-type selection and conversion between the two.
- Whole-house standby generator installation SOP, for the install sequence this connection sits inside, including its own meter-capacity warning.
- Annual generator maintenance SOP, for periodic inspection of an already-connected supply.
- NFPA 54 (National Fuel Gas Code) and NFPA 58 (LP Gas Code), current adopted edition, for test pressure, duration, and line sizing tables.
- NEC (NFPA 70) Article 250.104(B), for gas piping bonding requirements.