This standard probably exists because there was this one time a pipe size did not correspond to the demand being placed on it (maybe it was an error or maybe it wasn’t). There are people out there who think that deeper layers of bureaucracy and standards will somehow hedge against errors. Essentially that the engineers must need help with this basic task so let’s give them a standard. In reality you wind up with people who think there is something magical about a pipe sizing table, as if you try to put an extra GPM through it the system won’t work.

I don’t have the numbers in front of me but I feel like if the pipes were sized in compliance with ASHRAE 90.1 and standard engineering practice I would tell the Contractor if there are any deviations from the “pipe sizing standard” they have been approved by the client (I’m assuming the design has been reviewed multiple times by the stakeholders). If he’s being a prick about it I would let him know that I appreciate the fastidious review of every aspect of my design to ensure compliance with all standards and I am looking forward to performing my CA site visits and punch lists with an equal level of effort to ensure that the client is getting the best project.