How to size natural gas or propane pipes?

Gas pipe sizing is governed by CSA B149.1 (2025 edition, B149.1:25), incorporated by reference into the Quebec Construction Code, Chapter II — Gas:

Basic principles:

• The maximum allowable pressure drop depends on the supply pressure: 0.5 in w.c. (0.125 kPa) for a natural gas system supplied at less than 7 in w.c. (1.75 kPa), and 1 in w.c. (0.25 kPa) for a system supplied at 7 to 14 in w.c. (1.75 to 3.5 kPa), which covers propane delivered at 11 in w.c. (cl. 6.3.2 and Table 6.1)
• For a flow of 20 m³/h or less, Énergir delivers a maximum pressure of 1.74 kPa (about 7 in WC) at the meter (Énergir, Guide d'intégration du gaz naturel à un bâtiment [Natural Gas Building Integration Guide], June 2024, consulted October 18, 2026)

Calculation method:

1. Calculate the total load in BTU/h of all appliances on the line from their rating plates (illustrative example: furnace 80,000 BTU/h + water heater 40,000 BTU/h + stove 65,000 BTU/h = 185,000 BTU/h)
2. Measure the actual length of the pipe: Tables A.1 to A.4 of Annex A already include a 20% allowance (factor F = 1.2) for a reasonable number of fittings (Annex A, cl. A.2.4); with Tables A.5 to A.7, add the equivalent length of fittings given in Table A.16
3. Use Tables A.1 and A.8 (0.5 in w.c. pressure drop) or A.2 and A.9 (1 in w.c.) for natural gas, Tables B.1 and B.6 for propane, or the manufacturer's tables for CSST (cl. 6.3.2), to find the required diameter

Illustrative example: for a load of 185,000 BTU/h over an actual length of 20 m (65.6 ft; read the 70 ft/21.3 m column):

• Schedule 40 steel (Table A.1 of CSA B149.1:25, 0.5 in w.c. pressure drop): ¾ in = 114,000 BTU/h, not enough; 1 in = 214,000 BTU/h, sufficient
• CSST (Gastite, Design and Installation Guide [French edition], Section 7, January 2020, Table 7-1, natural gas, 0.5 in w.c. pressure drop, consulted October 18, 2026): ¾ in = 75,000 BTU/h and 1 in = 163,000 BTU/h, not enough; 1¼ in = 239,000 BTU/h, sufficient

Important rule: Never reduce pipe diameter downstream of an appliance. If adding an appliance, verify that the existing pipe can handle the additional load.