Don't forget, the Regs are not the be all and end all. They are guidance and need to be complied with as far as is reasonably practical. It is better to bond where you can, than not to bond at all. I wouldn't be concerned about the 500mm rule, it wont take fault current much longer to travel 20 feet than it does to travel 500mm. Cable has a hell of a lot of redundancy built in, so i can't see your 6mm having any issues unless your PFC is off the scale.
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