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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.

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  • 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, 2016 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, 2016 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, 2016 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, 2016 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, 2016 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

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  • 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, 2016 at 21:08
  • There's always an exception.
    – Tester101
    Jan 29, 2016 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, 2016 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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  • 1
    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, 2016 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, 2016 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, 2016 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, 2016 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, 2016 at 4:06
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The issue arises when you allow the electricity more than one return path back to the other panel. When the neutral is tied to ground on both ends, the path between the two ground rods takes a portion of the neutral conductor's return current. You are basically adding a second return path by connecting the ground to neutral at both ends. This can cause corrosion on grounding rods, water pipes, propane lines, any conductor connected to earth ground becomes part of the return path.

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    There is answer there, pity about the rant. Could you perhaps improve it ? Jun 13, 2023 at 13:46
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In most cases, the dirt that is near a structure will be electrically connected pretty well to the water table beneath, and in turn to any other regions of dirt that are likewise connected. If multiple structures which are connected to a common metal grounding conductor each tie that grounding conductor to a grounding rod, and the dirt beneath those structures also happens to be electrically connected, the grounding rods might be perceived as redundant.

The fact that on some particular day the dirt between structures is sufficiently moist to provide a reasonable electrical connection does not imply, however, that the dirt will always provide such a good connection. If an outbuilding didn't have a grounding stake, and the ground near it didn't have a good electrical connection to the building service it but did have a good electrical connection to a water pipe that happened to come in contact with the "hot" wire, that could cause the dirt near the outbuilding to become electrically live relative to both the dirt closer to the main building, and to the metal grounding electrode connected to the main building. Someone outside the outbuilding who simultaneously touched the dirt near the building and a piece of equipment that was connected to the main building's ground could receive a shock.

Such risk would be mitigated by the addition of a grounding rod near the second building. While the water pipe might be able to leak some current into the ground near the outbuilding, the grounding electrode should provide a good enough connection to carry away most of that leakage.

Note that if something like a lightning strike occurs vaguely near one of the buildings, some of it may go through one grounding rod, through the grounding wire, and down through the other grounding rod, but most of it would instead generally go through dirt, especially since lightning strikes are often accompanied by rain. In some situations, the presence of a grounding rod in the second building might lead to equipment damage that would not have occurred in its absence, but the grounding rod will improve safety more often than it hurts it.

Note, btw, that connecting the neutral to the grounding wire in an outbuilding's subpanel could create a dangerous situation if the main neutral connection fails. Normally, electrical systems are designed not only to ensure that single-point failures can't create dangerous conditions, but also to ensure that undetectable single-point failures couldn't leave the system one single-point failure away from disaster. If ground and neutral were connected at a subpanel, and neutral were fail, everything would appear to work normally, but a subsequent failure of the grounding conductor would cause everything that should be grounded to instead become electrically live.

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Ok Guys, I love all off this Stuff but it's consider;

  1. The neutral and ground in the main panel are Bonded and therefore the same potential in the main panel. There need be no ground from the meter to the panel as the neutral is the same potential as ground at that location and bonded to ground in the panel.
  2. A grounded conductor (ground wire) must run to the remote building that has a sub-panel and be mounted to the ground lug in the sub-panel, which is bonded to neutral inside the main panel.
  3. The grounded conductor to the remote building must connect to a ground rod with specified maximum resistance (or two ground rods by continuous ground wire)in the sub-panel. This means that the ground should have the same potential as the main panel ground and the main panel neutral, unless there is induced charge from some source (a Neighbor's house or pool, or bad transformer, etc).
  4. The neutral in the sub-panel is NOT bonded the the ground in the sub-panel. Therefore it will carry any current back TO the MAIN panel, instead of attempting to dump it into the remote building's ground. (Examine this: you have 6 circuits, 3 on A leg and 3 on B leg in the remote building. There are 14.5 amps active on A and 36 on B leg. The Neutral is carrying 36-14.5= 21.5 amps back to the main panel and MUST do so....or you would be imbalanced in the main, and possibly causing the problem listed in 3 above to your neighbor!!!).

Now we can examine a lightening strike. The way to cause lightening to WANT to strike is to set up a grounding plane and connect it to a tall sharp metal object. Bury 300 or whatever ft of wire in a circle and bring the end(s) to the middle of the circle. Attach them to a pole with a sharp tip, slightly higher than anything else around it. If the POTENTIAL for lightening approaches, the ground plane prepares the availability of electrons. When the potential is great enough the electrons RISE from the EARTH to the area of positive charge. Lightening strikes UP, not down. In sand, the heat generate by this kind of strike (electron flow) can turn the sand to glass with interesting fractal structures.

Houses used to be struck very often, church steeples even more often. However today we have power poles above ground with neutrals attached to ground rods everywhere. It is very unlikely to have a building struck by lightening directly as they are mostly lower than trees and power poles etc. IT does not matter whether it is your house, the transformer, your air compressor or any other device, the surge WILL come to your house through those lines and the earth. Best protection is a surge protector designed to capacitate, blow the caps instantly and blow your house main breaker or branch breaker in the process. This device will NOT protect electronics (you can do that but it is expensive and should be done by charging batteries and inverting thereby maintaining clean power separated from the grid). This surge protector generally WILL protect motorized equipment (AC compressors, washers/dryers, refrigerator motors, etc).

IF you have a surge protector, you should have one at each disconnect panel or sub-panel or even receptacle (to any exterior device). Example" pool pumps, well pumps, detached garages, AC Compressors, Pool Heaters etc. They are on concrete pads or poles which are excellent conductors (especially when wet, even wood when wet) and can help create the ground plane. Surge protectors need to be at every possible point of entry to the house that can be in that ground plane.

The Illinois Railway museum probably just needed a very good lightening rod and ground to their roof...or lightening rod on a tower higher than the roof and with a good grounding plane, to control any strike location. My guess is that the sub-station reduced the contracted cost of electricity substantially, and therefore was dual purpose and effective.

IS the GROUND System to protect from lightening strikes? Definitely NOT.

House grounds are specifically designed for PERSONAL SAFETY. Originally, there were no grounds. Most devices had metal casings. IF the hot came loose, no harm no foul, the device stopped. If the neutral came loose heaven help the poor guy squeezing the handle of the drill or touching the metal blender, the person became the NEUTRAL/Ground, and many homes had great conductors for floor materials and even water around. By having a ground, if the neutral fails, power is shunted to the ground, reducing the potential and saving the HUMAN from severe harm. Many devices can cause the circuit to blow when the neutral is loose and power is returned through the ground. I hope this helps resolve confusion about the code and the why.

Does the grounding system protect against lightening strikes? It does provide a more direct path to ground and therefore MAY help, however I would suggest that it might be the other way around. Because the path is more direct, it might actually increase the likelihood in some respect, however, it would most probably severely reduce any damage to structure.

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    Re: The wording here is important but can be confusing 1. In some cases the service provides a ground in addition to hot and neutral. 2. In the US a "grounded conductor" refers to neutral; this is different from a "grounding conductor", which is a ground (e.g. "equipment grounding conductor or EGC").
    – Armand
    Jun 14, 2023 at 4:25

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