Grounding Electrode

[BLA]

Is a grounding electrode in water at the end of a boat dock acceptable as a supplemental ground for a subpanel?

 

[LarrywdH]

no

 

[mopowr steve]

Interesting question,
I read on this forum about grounding of deep water oil rigs and how grounding is done by hanging rods in the water. I wonder if this would be considered in this case also.

I wonder how much distance from land one need to be before it can be just installed in water.

Maybe it only applies after an isolation transformer is installed between land and water?

 

[kwired]

Give us more details on "supplemental ground for a subpanel"

Recent editions of NEC require equipment grounding conductor run with the feeder to "subpanels". Grounding electrodes are only required if the feeder ends at a separate structure.

 

[SceneryDriver]

Give us more details on "supplemental ground for a subpanel"

Recent editions of NEC require equipment grounding conductor run with the feeder to "subpanels". Grounding electrodes are only required if the feeder ends at a separate structure.

Didn't the forum just have this discussion? It was pretty much decided that the code definition of "structure" meant anything more than (and including) a post whacked into the ground as a mounting site for a panel. I think a boat dock meets the NEC definition of "structure."

I'm less and less convinced that ground rods actually do anything useful; a buddy just re-did a 200A residential service, and before he connected the wire to the ground rods (driven regulation 6' apart) he threw his ohm meter across them, and got over 250K ohm... in soaking wet soil.

Tossing a ground rod in a lake can't be any worse.


SeneryDriver

 

[kwired]

Didn't the forum just have this discussion? It was pretty much decided that the code definition of "structure" meant anything more than (and including) a post whacked into the ground as a mounting site for a panel. I think a boat dock meets the NEC definition of "structure."

I'm less and less convinced that ground rods actually do anything useful; a buddy just re-did a 200A residential service, and before he connected the wire to the ground rods (driven regulation 6' apart) he threw his ohm meter across them, and got over 250K ohm... in soaking wet soil.

Tossing a ground rod in a lake can't be any worse.


SeneryDriver

My question is trying to get more details from the OP on what he has period, pretty vague otherwise. Subpanel already in/on same structure that supplies it doesn't need it's own grounding electrode. If it is in/on a separate structure then it is supplied from it very likely needs a grounding electrode. Have no clue if the boat dock is a separate structure or if the panel is even associated with the dock.

Suspending rod in the water is likely a better grounding electrode then driving it into soil in many instances, though NEC does not mention anything about doing such a thing.

 

[mopowr steve]

Didn't the forum just have this discussion? It was pretty much decided that the code definition of "structure" meant anything more than (and including) a post whacked into the ground as a mounting site for a panel. I think a boat dock meets the NEC definition of "structure."

I'm less and less convinced that ground rods actually do anything useful; a buddy just re-did a 200A residential service, and before he connected the wire to the ground rods (driven regulation 6' apart) he threw his ohm meter across them, and got over 250K ohm... in soaking wet soil.


SeneryDriver

I question your results as 250 Kohms. What do you mean between rods? That's not how it's measured.
Ground resistivity of soil near my location sets near 25 - 30 ohms when wet with one 8' Cu clad rod. Measured with a clamp on ground resistivity meter.

 

[SceneryDriver]

I question your results as 250 Kohms. What do you mean between rods? That's not how it's measured.
Ground resistivity of soil near my location sets near 25 - 30 ohms when wet with one 8' Cu clad rod. Measured with a clamp on ground resistivity meter.

He measured 250K ohms between two 8' ground rods driven 6' apart in wet soil; one lead of the meter to each ground rod. Meter was set to auto ranging.

How does one measure resistance with only one rod? Where does the other lead of the meter go?


SceneryDriver

 

[mopowr steve]

You can use a 3point fall potential method (It is described how to do it in the handbook see exhibit 250.25) or use a clamp on ground resistivity meter.

 

[gar]

170727-2335 EDT

To measure resistance between two points you need two points. Or you need a closed path into which you induce a current and deduce the impedance of the loop by its reflected impedance.

If remote earth is one point, then you need a means to connect to remote earth, or deduce it. Various means have been invented to do this.

I drove an 8 ft ground rod in my backyard to use for experiments. Originally I could connect 120 V to this and draw less than 20 A, not enough to trip the breaker, but there was a large current, possibly over 10 A.

This rod was left unconnected to anything for a number of years. Recently I connected 120 V and current draw was virtually zero. Obviously an insulating layer has grown on this rod in its inactive state. And that layer is not breaking down at 180 V.

A rod or plate in salt water would be a very good ground rod. A rod or plate in pure Michigan fresh water would be a poor ground rod.

My son uses a 12 V battery, and solar panel to power his boat lift. A battery and solar is a good safe way to have limited remote power on a dock. I don't believe there is any AC power on the docks at Portage Lake.

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