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Ontario HVAC technician that ventured out to "wild west" of Canadian hvac (BC), and had a disagreement with another tech about sizing gas lines.

Situation - 1/2 line feeds a WH and FP. The H/O wants to install a gas furnace to replace their existing hydronic furnace in the crawl space. The additional 40,000 btu furnace would make the existing 1/2 main too small.

Disagreement - I was taught in school that if you branch off within 3 feet of a regulator (I.e. tee a new line off the meter) that said load would now be sized with it's own individual LMR. The other technician argues that you still need to adhere to the longest measured run which would require the existing main to be upsized.

Thanks in advance!

I found this in the B149.1-15 Gas Code.  I believe Clause 6.6.2 agrees with you (but can only reduce line within 2' not 3' of gas meter) and Clause A.3.5 agrees with the "other technician".  I believe you could do it your way as long as you verified the maximum pressure drop was not exceeded to each appliance.

Quote6.4 Volume of gas to be used for sizing piping and tubing systems

6.4.1 - The total volume of gas required shall
(a) be determined as the total volume for all appliances supplied, except as permitted in Clause 6.4.3;
and
(b) include an allowance for known future extensions.

6.4.3 - When a diversity of load has been established to the satisfaction of the authority having jurisdiction, a percentage of the total volume may be used.

Quote6.6 Extensions

6.6.1 - When an existing piping or tubing system is of adequate capacity for an additional appliance, any required extension that also contains the existing system shall be sized to meet the requirements of Clause 6.3.

6.6.2 - When an existing piping or tubing system is of insufficient capacity for an additional appliance, the supply piping or tubing to any extension shall be sized to meet the requirements of Clause 6.3 to within 24 in (610 mm) of the point of the supplier termination or the line pressure regulator, at which point it may be reduced if necessary.

QuoteA.3 Use of capacity tables

A.3.1 - To determine the size of each section of gas piping or tubing in a system within the range of the capacity tables, follow the procedure outlined in Clauses A.3.2 to A.3.9.

A.3.2 - Determine the gas demand of each appliance to be attached to the piping or tubing system. Calculate the gas demand in terms of Btuh (kW) for each piping system outlet.

A.3.4 - Measure the length of piping or tubing from the point of delivery to the most remote outlet in the building.

A.3.5 - In the appropriate capacity table, select the row showing the measured length, or the next larger length if the table does not give the exact length. This is the only length used in determining the size of any section of gas piping or tubing.

A.3.6 - Use the horizontal row specified in Clause A.3.5 to locate all gas demand figures for this particular system of piping.

A.3.7 - Starting at the most remote outlet, in the horizontal row selected in accordance with Clause A.3.6, find the gas demand for that outlet. If the exact figure of demand is not shown, choose the next larger figure to the right in the row.

A.3.8 - In the top row above this demand figure, the correct size of gas piping will be found.

A.3.9 - Proceed in a similar manner for each outlet and each section of gas piping. For each section of piping, determine the total gas demand supplied by that section.

Here's the Enbridge Bulletin on gas piping sizing,

https://hvactechgroup.com/article.php/20140405101312855