Condensing boilers - 4 feet horizontal from meter... source?

[R Grab]

Regarding high efficiency gas condensing boilers, I'm finding a consistent regulation in manufacturer instructions, "a clearance of at least 4 feet horizontally must be maintained between the vent terminal and gas meters, electric meters,...". It's so consistent, there must be a regulation somewhere, but I can't find it in the 2012 IRC or 2012 NFPA 54. Any ideas where I can find the source for this?... tia

[Mike Lamb]

NFPA 12.9.4 talks about protecting gas meters from gas vents. I don't know why the 4' distance is shown in most manufacturer's instructions.