size of GEC

[skiprima]

Why is such a large wire required for a water pipe grounding electrode conductor when other electrodes of the same system are allowed to be bonded with smaller guage wires? The concrete encased electrode is the best electrode by far and only requires no bigger than a number 4. Why could a water pipe ultimately require up to a 3/0 bond?

Thxs

 

[augie47]

an even bigger question, in my mind, is why such a conductor is needed when you are only bonding a water line this is not an electrode per 250.104


As far as the bonding of a water line being used as an electrode I can only assume the rules relate back to the day when metallic distribution water systems were common and the entire town became your electrode.

 

[tom baker]

yes Augie I agree - the water pipe was used as it was commonly available, continuous, and would have a very low resistance.
I belive a 6WAG is the largest for a ground rod as the resistance of the ground rod will be fairly high. All of this GEC sizing was figured out a very long time ago.

 

[haskindm]

If a wire shorts to the waterline it will carry fault current back to the service to clear the fault. The code making panels recognize that a ground rod or Ufer ground will not and cannot carry enough current to clear a fault so there is no reason for the conductors to be sized to carry fault current.

 

[kwired]

If a wire shorts to the waterline it will carry fault current back to the service to clear the fault. The code making panels recognize that a ground rod or Ufer ground will not and cannot carry enough current to clear a fault so there is no reason for the conductors to be sized to carry fault current.

It is only likely to do that where there are other services also bonded to same metallic water piping, otherwise the actual resistance of the water piping is not really known, what if there is only the minimum of 10 feet of metal piping that is required to call it an electrode? In that case a CEE just may have less resistance, two ground rods may even have less resistance.

Building steel that qualifies as a grounding electode is also required to have a GEC sized to table 260.66 and not just water piping.

250.66 (A)(B)and(C) are the exceptions to using the sizes in the table, rod, pipe and plate electrodes don't need a conductor larger than 6AWG copper, CEE don't need a copper conductor larger than 4 AWG and ground rings don't need a conductor larger than the conductor used for the ring. All other GEC are sized to the table.