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G3 Pipe Sizing: The Four-Step Method and Every Question Type

Pipe sizing questions look like the hardest thing on the G3 exam and are the most mechanical once you know the order. It is the same four steps every time.

The four steps

1. Find the LMR. The Longest Measured Run — meter to the farthest appliance. Always the longest single path.
2. Choose a column at or above the LMR, rounding up. Never down.
3. Add the section load. The total BTU/h of every appliance fed by that section of pipe.
4. Read the pipe size. In your column, the first row whose capacity meets or exceeds the load. Round up again.

Two rules that never change

• The column value is at or above the measured run — round up.
• Pipe capacity is at or above the load — size up.

Where a value falls between two, take the larger one. Every time.

Which table

Natural gas uses Annex A; propane uses Annex B. Inside each, pick the sub-table matching the material — steel pipe or copper tubing. Reading the right table is half the marks; the arithmetic is the easy part.

Identify the scenario first

Every pipe sizing question on the exam is one of ten shapes. Recognising which one you are looking at tells you what to do before you touch a table.

#	Scenario	How you know
01	Straight run	Only one appliance shown, no tees
02	Single pipe system	Several appliances, one material, tees shown
03	Combination system	Two materials, such as steel and copper
04	Manifold system	The word "manifold" appears in the question
05	Find the missing pipe size	One section marked with a "?"
06	Find the maximum BTU	Pipe size given, asks for capacity
07	Column selection only	"Which column?" or "Is this correct?"
08	Medium pressure	Supply pressure given in psig, above 14" w.c.
09	System extension	The words "adding" or "extending" an appliance
10	Propane system	"Propane" or "LP" mentioned — Annex B, not A

Where people lose marks

• Rounding the column value down because the run is "almost" that length.
• Adding only the appliances downstream of the tee, not everything the section feeds.
• Using Annex A for a propane system.
• Sizing to the load exactly rather than to the first capacity above it.

Common questions

What does LMR mean in gas pipe sizing?

LMR stands for Longest Measured Run — the distance from the meter to the farthest appliance on the system. It is always the longest single path, and it is the first thing to establish when sizing pipe.

How do you size gas pipe using the B149.1 tables?

Find the Longest Measured Run, choose a column value at or above that length rounding up, add the total BTU/h load carried by the section, then read down that column to the first pipe size whose capacity meets or exceeds the load — again rounding up.

Do you round pipe sizing up or down?

Always up. Round the column value up to at least the measured run, and take the first pipe size whose capacity meets or exceeds the load. Where a value falls between two, take the larger.

Which annex do you use for propane?

Natural gas systems use the Annex A tables and propane uses Annex B. Within each, use the sub-table matching the material — steel pipe or copper tubing.

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