Ground rods on sub panel detached garage

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treyp3

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Fernandina Beach Fl
My inspector is telling me I'll need ground rods on a sub panel in a detached garage. I want to run a four wire feeder to the sub panel. 250.32(B)(1) states -

"an equipment grounding conductor shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrodes"

So, shouldn't the subpanel in the detached garage be tied in back at the GEC in one location only back at the point of service?

Code source please
 
tom baker

tom baker

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No. Any structure suppled by a feeder, 40 amp or 400 amps requires a grounding electrode system. This is for lightning protection
250.32 A
The EGC is for the return path to the main service where the MBJ is, for a line to case fault, the current goes back to its source.
So at a separate building, isolate the neutrals, install a ground bar for the grounds and GECs. Just like a subpanel in a building but no ground rods.
 
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treyp3

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Fernandina Beach Fl
tom baker said:
No. Any structure suppled by a feeder, 40 amp or 400 amps requires a grounding electrode system. This is for lightning protection
250.32 A
The EGC is for the return path to the main service where the MBJ is, for a line to case fault, the current goes back to its source.
So at a separate building, isolate the neutrals, install a ground bar for the grounds and GECs. Just like a subpanel in a building but no ground rods.
Click to expand...

First, I'm sorry to be dense. I keep all neutrals and grounds separated back to the first source of disconnect. The main source of power supplies the required grounding electrode system, correct?
 
Electromatic

Electromatic

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The feeder from "the main source of power" will supply the Equipment Grounding Conductor (EGC) for clearing faults. The detached structure is required to have its own Grounding Electrode System (GES) i.e. ground rods. The subpanel in the detached structure will have its grounds and neutrals separated. The EGC of the feeder and the Grounding Electrode Conductor from the [new] ground rods will connect to the ground bar of the subpanel.
 
tom baker

tom baker

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treyp3 said:
First, I'm sorry to be dense. I keep all neutrals and grounds separated back to the first source of disconnect. The main source of power supplies the required grounding electrode system, correct?
Click to expand...
No issue grounding and bonding is confusing. Since the Grounding electrode system (GES) is for lightning, we want a GES at the separate building as lightning wants to go to ground. If the GES was only at the main building, that path is long and has a very high impedance. The lightning will find another path to ground, causing damage and fires.
 
ptonsparky

ptonsparky

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NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
tom baker said:
No issue grounding and bonding is confusing. Since the Grounding electrode system (GES) is for lightning, we want a GES at the separate building as lightning wants to go to ground. If the GES was only at the main building, that path is long and has a very high impedance. The lightning will find another path to ground, causing damage and fires.
Click to expand...
It will anyway, but still required .
 
infinity

infinity

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treyp3 said:
I want to run a four wire feeder to the sub panel. 250.32(B)(1) states -

"an equipment grounding conductor shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrodes"
Click to expand...
What it's telling you is that you connect the grounding electrode conductor to the EGC at the detached structure and the enclosure that houses the disconnecting means. It is not required that you use ground rods you may use another type of electrode. If there rebar in the footing?
 
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petersonra

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Northern illinois
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engineer
tom baker said:
No issue grounding and bonding is confusing. Since the Grounding electrode system (GES) is for lightning, we want a GES at the separate building as lightning wants to go to ground. If the GES was only at the main building, that path is long and has a very high impedance. The lightning will find another path to ground, causing damage and fires.
Click to expand...
You have four wires feeding the garage. L1, L2, N, and G. Suppose lightning were to strike something out at the garage.

If it hits the structure itself, the GES at the garage does nothing.

If it hits L1, L2, or N at the garage end, there is no connection to the garage GES except to go to the house panel, and then travel back along the EGC to the garage GES. Does not seem like that would do all that much good either.

If it were to strike the EGC at the garage, there would be a solid connection to earth there, and if you believe in such things, perhaps it would be helpful.

Having the GES at the garage end does not even help stabilize the voltage to ground since there is no connection between that GES and the service except through the EGC.
 
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