Grounding Electrode

[concur]

For a 3-phase 3-wire, 800-amp service with two sets of 600MCM conductors, where the distance from the main service disconnect to the water meter is 200 feet, what size grounding electrode can one use? The inspector requires 250MCM for voltage drop considerations.

 

[dereckbc]

Voltage drop on a GEC? No such thing. Table 250.66 dictates the size based on the phase conductors, not load or current. To my knowledge there is no voltage drop requirement anywhere in the code, only what has to be done if you upsize cables to compensate for voltage drop.

Have the inspector quote the article number for the requiment, I bet he backs off.

 

[Jljohnson]

Assuming your service conductors are copper (they must be to come up with 800 amps on 1200 MCM) your GEC should be 3/0 copper or 250 MCM AL. I agree, voltage drop on a GEC is a non-issue.

 

[concur]

Thanks you for your help. Would you agree that the GEC is not used to clear short circuits?

 

[iwire]

Thanks you for your help. Would you agree that the GEC is not used to clear short circuits?

Well I will agree that is not the purpose of the GEC but in some cases it does clear (or help) clear a ground fault.

That said I agree with the others on the original question.

 

[LarryFine]

The inspector requires 250MCM for voltage drop considerations.

Ask him what the current level is that he's proposing is creating the voltage drop.

 

[dereckbc]

Would you agree that the GEC is not used to clear short circuits?

No, it does in the very unlikely event if one of your feeder or branch circuits comes in contact, or if something like the primary distribuion blows or falls on to the secondary or your service drop.

 

[tryinghard]

Thanks you for your help. Would you agree that the GEC is not used to clear short circuits?

Past the service disconnect coming in, YES I agree because a ground fault seeks the source. If there is a short in the service entrance the ground electrode shouldn?t get any current but the serving XFMR will.

Lighting on the other hand flashes in all directions so the electrode is needed for lightning. If the serving grounded conductor fails the ground electrode will be used in stead. And if the serving XFMR fails in a way to cause high voltage on the service entrance the ground electrode can be used.

Usually there is enough resistance in any ground electrode to allow 20A current meaning a 30A circuit will not open with this short. Ohms law proves this I=E/R, say you have a 20A circuit 120V short that routes to a 7 ohm ground electrode this example allows 17A this circuit will remain closed! This example is actually very common and likely.

This is why I agree with your assertion, the clearing of ground faults needs to be done through an effective ground fault current path (250-4(A)(5) & (B)(4) and the electrode is described in 250-4(A)(1)