Is there an international standard that specifies the size of the grounding conductor to bond a metallic part of the installation to a grounding bar?.
And to connect the grounding bar to the grounding rod?.
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If you are in the US, you'll likely follow National Electrical Code (NEC).
Lets start with some definitions.
and a picture to help make things a bit more clear.
The size of the Main Bonding Jumper, Grounding Electrode Conductor, and Grounded Electrode is defined by table 250.66
The size of Equipment Grounding Conductors (the ones that run from the Ground bus bar out with each circuit), is defined by table 250.122.
There are various international standards that are trending towards "homologation" or which are being reissued under other standard numbers with no technical changes. BUT what applies currently varies by country and as grounding bars and rods all lead to ground in the country where the installation is (with a few very very unusual exceptions)(CERN would be interesting :-)) you need to check the regulations in the country of interest.