2.2 Gas Pipe Sizing & Demand

2.2 Gas Pipe Sizing & Demand

Quick Answer: 248 CMR 5.06 states that Massachusetts made no modifications to NFPA 54-2012 Chapter 6. Size pipe by converting each appliance to CFH, adding downstream load on each segment, and entering the longest-length column of the correct gas / pressure-drop / specific-gravity table. Default Massachusetts pressure is 0.5 PSIG / 14 inch WC. Do not print or memorize a bootleg copy of Annex C; learn the method, then read the row and column on the closed-book exam materials you are expected to know.

Pipe sizing is a calculation-heavy topic on the Massachusetts journeyman plumber exam (written gas items and a practical gas-piping portion). Independent OpenExamPrep teaching here is the arithmetic and the Massachusetts overlays — not a substitute for the NFPA 54 capacity grids themselves.

What 248 CMR 5.06 actually changes

Open Cornell or the Mass.gov 5.00 PDF to 248 CMR 5.06: Modifications to Chapter 6, Pipe Sizing. The entire amendment is one sentence: “No modifications have been made to Chapter 6, Pipe Sizing.”

That does not mean Massachusetts has no sizing rules. It means you still use NFPA 54-2012 Chapter 6 (longest-length method and the Chapter 6 / Annex C capacity tables) together with Massachusetts rules that live in other 248 CMR sections:

• 248 CMR 4.03(1)(b) — default low pressure ≤ 0.5 PSIG (14 inch WC).
• 248 CMR 5.05(4) — elevated pressure is an engineer-designed system (with two small exceptions), labeled, and fully welded above 5 PSIG.
• 248 CMR 5.05(3) — Table 5.4.2.3 diversity for domestic ranges only in multi-family dwellings.
• 248 CMR 5.05(8)(l) — CSST sized and installed at a maximum 14 inch WC (0.5 PSIG).
• 248 CMR 5.05(1) — systems over 5,000,000 BTU/hr are PE-sized, not a napkin sketch.

248 CMR 8.07 likewise makes no modifications to NFPA 58 Chapter 15 LP pipe and tubing sizing tables. Propane interior piping still uses the LP tables, not the natural-gas tables.

Convert demand before you touch a table

Capacity tables are in cubic feet per hour (CFH), not BTU/h. The conventional exam conversion for natural gas is:

CFH ≈ (appliance input, BTU/h) ÷ 1,000

That 1,000 BTU per cubic foot is the usual approximate heating value used in NFPA 54 examples (specific gravity 0.60 natural-gas tables). If a problem states a different heating value, divide by that number. Do not invent a heating value when the item already gives one.

Propane (undiluted LP-gas) carries more energy per cubic foot. A common exam approximation is about 2,500 BTU/cu ft, so the same 100,000 BTU/h appliance is only about 40 CFH of propane vapor — but you still must enter a propane table built for the higher specific gravity (typically 1.50). Smaller CFH does not let you use a natural-gas steel table. Wrong table = wrong pipe.

Specific gravity is a table identity, not trivia

If the header on the table does not match the gas in the building, stop. Massachusetts dual-fuel buildings (248 CMR 5.05(2)) have two piping systems and two sizing problems.

Longest-length method (the procedure you actually run)

NFPA 54-2012 Chapter 6’s everyday method for low-pressure steel (and the method PSI-style items expect) is the longest-length method. Massachusetts did not replace it.

1. Confirm the gas and the pressure. For ordinary Massachusetts work, inlet pressure is not over 0.5 PSIG / 14 inch WC. Pick the low-pressure table whose pressure drop matches the design (commonly 0.3 or 0.5 inch WC on natural-gas steel tables — use the drop stated in the problem or in the table header; do not guess a drop the item did not give).
2. List every appliance input in BTU/h from the nameplate or the exam schedule.
3. Convert each input to CFH (natural gas: divide by ~1,000 unless a heating value is given).
4. Measure developed length from the point of delivery to the most remote appliance outlet. For natural gas that start point is the meter outlet (or the service regulator/shutoff outlet if there is no meter) per 248 CMR 4.03. Include the real run: offsets, drops, and the path the pipe actually takes — not a scale-ruler shortcut across a floor plan.
5. That single longest length is the length column for every segment, including short branches. This is the rule candidates miss.
6. For each segment, add only the connected load downstream of that segment. The trunk from the meter to the first tee carries everything. After a tee, each branch carries only the appliances beyond that tee.
7. Enter the table. Stay in the longest-length column (or the next longer published length if your measured length falls between rows). Read down until you find a capacity equal to or greater than that segment’s CFH. Read the pipe size on that row. Do not interpolate a diameter between rows unless the adopted table’s instructions say to; the exam-safe move is the next larger published size that meets or exceeds demand.
8. Repeat for every segment. A 20-foot branch to a water heater can still be sized from an 80-foot column if the remote furnace is 80 feet from the meter.

You will see the full numeric capacities in NFPA 54-2012 Chapter 6 / Annex C on the materials you are expected to know for the closed-book exam. This lesson will not reproduce those copyrighted grids. Once CFH and length are known, the skill is: correct table → correct length column → first capacity ≥ demand → pipe size on that row.

Worked Example 1 — Natural gas, longest length, add the branch loads

A Massachusetts low-pressure natural-gas house (0.5 PSIG / 14 inch WC system, approximate 1,000 BTU/cu ft):

Developed length meter outlet → furnace shutoff is 70 feet. The water-heater branch leaves a tee at 25 feet and then runs 12 feet to the heater (37 feet of actual branch pipe). Longest length for the table column is still 70 feet, not 37.

• Meter → first tee (serves all four appliances): load 220 CFH, length column 70 ft.
• Tee → furnace: load 80 CFH, length column still 70 ft.
• Tee → water heater: load 40 CFH, length column still 70 ft.
• Size dryer and range the same way: each segment uses 70 ft and only its downstream CFH.

Trap: sizing the water-heater branch from a 40-foot column because “the branch is short” undersizes the pipe. Friction in the 70-foot remote run is the design length for the whole system under this method.

Now open the natural-gas, low-pressure, 0.60 specific gravity steel table that matches the stated pressure drop. In the 70-foot column (or the next longer listed length), find the first capacity ≥ 220 CFH for the trunk and ≥ 80 / 40 / 35 / 65 CFH for the respective branches. The diameters come from that table, not from a memorized “rule of thumb” like “¾ inch for a furnace.”

Worked Example 2 — Massachusetts range diversity (do not add 63,000 × apartments)

248 CMR 5.05(3) new 5.4.2.3 is the only Massachusetts Chapter 5 demand table. Diversity factor applies to domestic ranges only in multiple family dwellings, and only for ranges with connected load ratings of 63,000 BTU/h or more. It shall not apply to any other gas appliances.

Verified rows from Table 5.4.2.3 (demand in cubic feet per hour and percent of ratings):

Problem: Eight apartments, each with a domestic range rated 63,000 BTU/h or more. Natural gas, longest run from the meter to the remote range is 120 feet. Other appliances are sized separately at full nameplate CFH.

Wrong math: 8 × 63,000 = 504,000 BTU/h → 504 CFH of range load.

Table math: eight ranges → 212 CFH (42% of ratings). Use 212 CFH, not 504, when you enter the natural-gas table for the range portion of that multi-family stack. Then add water heaters, dryers, and boilers at full converted CFH. Diversity is not a license to haircut a boiler.

If a range is under 63,000 BTU/h, do not force Table 5.4.2.3. The table’s title limits it to 63,000 BTU/hr. or more per range. For any count not printed here, read the live table in 248 CMR 5.05 rather than interpolating a fake CFH.

Worked Example 3 — Same building, propane, and the mixed-table trap

Same four appliances as Example 1, but the fuel is undiluted propane from a first-stage regulator at the tank (Massachusetts LP point of delivery). Approximate heating value 2,500 BTU/cu ft:

• Furnace 80,000 → 32 CFH
• Water heater 40,000 → 16 CFH
• Dryer 35,000 → 14 CFH
• Range 65,000 → 26 CFH
• Total ≈ 88 CFH propane

88 CFH looks smaller than 220 CFH, so a rushed candidate grabs a natural-gas 0.60 SG table and picks a tiny pipe. That pipe is wrong. Propane is denser (specific gravity about 1.50). You must use the LP / propane capacity table (and Massachusetts still defaults the building piping to ≤ 0.5 PSIG / 14 inch WC unless 5.05(4) elevated-pressure design applies). Between the first- and second-stage LP regulators on the exterior, 248 CMR 5.05(4)(B)5.a does not require a PE solely because that short elevated run exists — but that exception does not let you size the interior low-pressure piping from a natural-gas table.

Second trap in the same item: using branch length for propane “because CFH is low.” Longest-length logic does not care that the number got smaller after you divided by 2,500. Remote length still governs the column.

CSST, elevated pressure, and giant loads

CSST. 248 CMR 5.05(8)(l) requires CSST systems to be sized and installed with a maximum 14 inch water column (0.5 PSIG). Use the CSST manufacturer’s capacity tables for that pressure — those published EHD capacities are part of the listed system, not interchangeable between brands (5.05(8)(a)). Do not “upsize CSST for 2 PSIG” as a Massachusetts shortcut; elevated CSST would still have to survive 5.05(4) and the 14-inch cap in 5.05(8)(l).

Elevated pressure steel. If the design is truly above 0.5 PSIG, 248 CMR 5.05(4)(B) requires PE stamped drawings, a serving-supplier letter, PE as-built certification, yellow labels with gas type and pressure every ten feet (light green for elevated dual-fuel propane), and all-welded pipe above 5 PSIG. The Chapter 6 higher-pressure tables exist in NFPA 54-2012; Massachusetts lets you use them only inside that engineer path (or the two exceptions: exterior LP first-to-second-stage piping, and meter-to-regulator piping not exceeding ten feet).

Over 5,000,000 BTU/hr. 248 CMR 5.05(1) takes sizing out of a journeyman’s sketchbook. The PE sizes it; you install the stamped diameters.

Exam traps to drill until they are automatic

• Using branch length instead of longest run for the table column.
• Adding nameplate BTU/h into a CFH table without converting.
• Entering a natural-gas 0.60 SG table for propane, or the reverse.
• Applying Table 5.4.2.3 diversity to water heaters, dryers, or boilers.
• Assuming 248 CMR 5.06 “rewrote” Chapter 6 — it did not.
• Sizing CSST as if Massachusetts allowed elevated CSST pressure above 14 inch WC.
• Inventing a pipe size from shop memory instead of capacity ≥ demand in the correct column.

When a numeric capacity or diameter is not printed in 248 CMR, say it is in the cited NFPA 54-2012 table and read that table. Do not fabricate a Schedule 40 capacity.