Short answer
The maximum permissible pressure loss is the design limit on pressure lost between the meter outlet and appliance inlets at full load. For low pressure natural gas, a figure of 1 mbar is widely used, giving roughly 20 mbar at appliances from a 21 mbar meter outlet. IGEM/UP/2 lets the designer choose a different figure, but the result must keep every appliance above its minimum inlet pressure under the worst realistic load.
Working back from the appliance
Start with the lowest minimum inlet pressure among the appliances, compare it with the guaranteed meter outlet pressure, and the difference is the maximum loss you can afford. Leave margin for network pressure variation on cold days.
Splitting the loss
On larger installations the allowable loss is split between the main run and branches, so that no single branch consumes it all. Sizing tables or software then select the smallest diameter that keeps each section within its share.
Medium pressure
Where gas is distributed at medium pressure and regulated at plant rooms, a larger loss can be accepted upstream of the regulators, often allowing much smaller pipe sizes on long runs.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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