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Article 250.32(A) of the National Electrical Code, tells us that we need a grounding electrode at a detached garage. It also says that it has to be "installed in accordance with Part III of Article 250.".

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 250 Grounding and Bonding

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

(A) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

Then the first part of Part III of Article 250 says, "All grounding electrodes... ...that are present at each building or structure served shall be bonded together to form the grounding electrode system.".

III. Grounding Electrode System and Grounding Electrode Conductor

250.50 Grounding Electrode System. All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(8) shall be installed and used.

Does that mean that the electrode(s) at the garage, has to be connected back to the electrode(s) at the main structure?

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Since there is still some uncertainty here, I took a minute to create an image that might fix some of this.

Grounding electrodes

Lightning is a DC shot of electricity going to the ground, it's source. So it's easiest to consider it that way. Next, lightning is so powerful that it creates a surrounding zone of energy on any conductive material that is able, the yellow circles. Everything that is within a distance becomes energized to a point, but is again still from the lightning and is trying to go one way - to the ground. This is why everything metal is bonded or grounded: so that nothing needs to arc to find a path, since it has it's own. Regardless of how it gets to ground, it's going there.

Back to the electrodes though. Consider my image above as being of a main structure and a garage. For all purposes necessary with lightning, there is no need to tie them together. As mentioned above about 100,000A of power coming off of a lightning strike, more rods and such can help dissipate the power faster and catch a little more of it than is going through metal in your home, but regardless it's still 100,000A of power ... what happens, happens.

Whether it's your garage or your neighbors house that is splitting the distance of the strike zone, tying them together won't fully relieve the problem of getting struck by lightning.

For purposes of the code, which you fully understand, the connection that is made between the two of them is your EGC that is only for ground faults. The reason this is in the code is because some people think that running a ground rod will be the miracle solution to ground faults and that the wire isn't necessary. This isn't the case though.

For ground faults, the EGC is the answer because it sends the fault back to it's own source - the utility. For small voltage differences throughout a system, a ground rod (or multiple for higher systems) is driven. However, for lightning, you are again giving the ground rod as a path for the voltage difference caused by it, but tying the full system together doesn't help all that much more than one rod.

Hopefully this answer helps a little better.

  • This answer is very clear, and explains the effects of lightning on nearby buildings. However, I'm finding it hard to believe that the purpose of a grounding electrode is lightning protection. Especially seeing as there are very elaborate lightning protection systems, that could all be replaced by a simple #8 copper rod. – Tester101 Jan 29 '16 at 21:20
  • I think maybe I have a weak understanding of the purpose of a grounding electrode, so I'm having trouble grasping the concept. Also the wording of the code is not very clear. – Tester101 Jan 29 '16 at 21:23
  • @Tester101 No, it's not for lightning protection. It's more like lightning diversion. It is just there to shorten the distance for lightning overvoltage, but it will not protect the system. – TFK Jan 29 '16 at 21:25
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    Another function of the electrode system is to equalize the potential in the area. If all metallic parts are bonded together and taken to earth then the steel door at your work will be at the same potential as the mud puddle you are standing in when you touch it. We sometimes drive auxiliary ground rods for lamp posts in parking lots for the same reason. The ground and the post should be at the same potential at all times. – ArchonOSX Jan 29 '16 at 21:35
  • @ArchonOSX I always thought that too, but I'm not sure that's accurate. And that's kind of why I was thinking that interconnecting the two electrodes would be required. – Tester101 Jan 30 '16 at 0:55
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Short answer: No

Here's the quote you had:

that are present at each building or structure served shall be bonded together to form the grounding electrode system

Notice it says at each building not between buildings. It should finish that sentence with "for that building".

So, if you have a lightning protection system and the required grounding electrode for the communications at a building then the electrical service GE and the others all have to be bonded together to "form the grounding electrode system". This prevents potential differences between any of the systems.

You only need a grounding electrode at the second building if you have more than one circuit (and in this case a multi-wire branch circuit can be considered one circuit). But as soon as you have two circuits or if you have a sub-panel you must install a grounding electrode and grounding electrode conductor large enough for the panel as if it was considered a service even though it isn't.

Also, if you have a GE at the garage then it is indirectly connected back to the house grounding electrode conductor through the equipment grounding conductor that serves the circuit that feeds the garage. This is still considered an EGC not a GEC and needs to be sized according to Table 250.122 (for the size of the feeder) not 250.66 (for the size of service conductors).

In most ways the sub-panel is treated as if it was in the same building with the exception of the grounding electrode and the GEC, which is if it was a separately derived system.

Clear as mud eh?

Maybe this will help.

This is all you need to do for a separate building according to the NEC Handbook.

Remote Building Wiring NEC

  • 250.32 Exception: A grounding electrode shall not be required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non–current-carrying metal parts of equipment. Just in case you want to take power to a yard barn or a greenhouse. – ArchonOSX Jan 29 '16 at 21:08
  • There's always an exception. – Tester101 Jan 29 '16 at 21:09
  • Yes, the exception to the rule is there is always an exception to the rule even the exception rule. 😉 – ArchonOSX Jan 29 '16 at 21:13
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Yes, they should be connected so that the power has a set path back to the source. Rods in the ground are there for a direct shot of electricity (lightning.) Connecting back to the source is for actual grounding - to tie in with the neutral and cause a short if any sort of ground fault were to occur. Without this, running the ground rods and electrodes would not flip the breaker in most cases.

Electrode conductors connected
(source: nachi.org)

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    That diagram does not show the electrodes being connected. It simply shows that there's an equipment grounding conductor with the feeder. What I'm talking about is connecting one electrode to the other, using a #6 or #8 copper conductor. A straight electrode to electrode connection. – Tester101 Jan 29 '16 at 3:05
  • If there's a lightning strike near the garage, I don't want the surge to follow the feeders EGC. – Tester101 Jan 29 '16 at 3:09
  • It wouldn't be necessary, the conductor is connecting them together. The code is just to ensure that there is a feeder, conductor, etc. bonding the secondary ground back to the primary.. to form a complete system. – TFK Jan 29 '16 at 3:10
  • Lighting will not follow the EGC. Lightning, and the voltage surge induced on metal nearby, is trying to get to ground in the shortest path possible (as that is it's source.) Running the EGC is for ground faults, to get back to their own source. – TFK Jan 29 '16 at 3:36
  • If that was the truth, a near lighting strike would have no effect on a building (since it already reached ground). So a grounding electrode would only do anything in case of a direct strike, in which case the current will follow the EGC. – Tester101 Jan 29 '16 at 4:06

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