gas lines, water lines & duct work Bill
~RJ~
Unfortunately I don't think your question is a simple this or that. Too many variables are made within Article 250 that affect the size, and these variables need to be addressed to get the correct size. (Like so many other code sections. Do this, but if you do this or that you must now do this other thing.)
First off, Are you bonding for GEC or EGC? Makes a difference.
Gas line, and most duct work would be sized as a EGC and subject to 250.122 and not as GEC. Waterline could be either GEC or EGC, and if used as grounding electrode or meeting language of 250.53, 250.64 and it would be sized by 250.66. Or if EGC only sized according to the requirements of 250.122 as an EGC. All of these methods or reasoning for bonding use the associated system or circuit amperages, or ungrounded conductor sizes.
Duct work would seem to me to be associated only to the HVAC and amperage related to it and sized accordingly.
Any of these are are also affected by the ungrounded sizing and any increase in size that could push a normally #8 to need to be a #6.