On 2010-07-17, HeyBub wrote:

But the pressure may not drop. The gas meter is, among other things,
a regulator - it tries to keep the pressure constant.

Right, the regulator at the gas meter tries to keep the pressure
constant at its output. So if no gas is flowing, the pressure
throughout the system is a constant 7" w.c.

But whenever gas flows, the pressure drops as it travels, due to the
friction with the side of the pipe. So turn on all your range burners
and draw 60,000 BTUs/hr (60 CFH), and while the pressure will stay at
7" w.c. at the regulator output, 10 feet away down a 1/2" iron pipe,
the pressure will have dropped by 0.071" w.c. to 6.929" w.c.

And the system is to be designed so that the pressure at each outlet
is at least 6.5" w.c. So you need to size your pipes large enough to
ensure at most 0.5" w.c. pressure drop between the regulator and the
appliance shut off valve.

Cheers, Wayne