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I understand running the neutral wire back to the main service panel and not bonding the subpanel... but running the equipment ground back to the main panel seems to get conflicting answers.

If code requires it, then I'm assuming it's because the main panel should be adequately grounded but is it against code to also install a ground rod at the subpanel just to be safe? If the subpanel is mounted to a metal stake or metal building wouldn't it be grounded anyway?

Talking to different so-called experts some say on a long run to a subpanel just install a ground rod at that location and save the cost of a fourth wire.

  • Are you asking about adding an extra ground rod or using one to eliminate the grounding conductor? – isherwood Sep 27 '17 at 14:51
  • Those "experts" would be wrong. The three wire method is no longer legal by the National Electrical Code. You have to take the equipment grounding conductor to the remote building. – ArchonOSX Sep 27 '17 at 15:55
  • A metal sided or clad building is not grounded. In order to be considered grounded it would have to have a "Ufer" ground in the concrete or be a steel beam constructed building with buried steel columns. – ArchonOSX Sep 27 '17 at 15:57
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To make this easier, consider the grounding conductor (the ground wire) as a backup neutral. It's not used unless a ground fault occurs, in which case it's only purpose is to provide a path back to the source so that a protective device will trip. (breaker, fuse, etc.)

Your grounding electrode system on the other hand, the ground rod here, is only used for over-voltages on the line such as static or potentially lightning. Although we bond the grounding system, they serve two different purposes.

So for your first question: no, it is not against code to install a ground rod at the subpanel. It is actually required by code.

NEC 2014 - 250.32(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.

The only exception to that being that if you were only running a single circuit (or multiwire branch circuit) with a ground wire, then you would not need an electrode system.

Now on to your next question on if a metal stake/metal building works as an electrode: yes and no. There are a few conditions were these could apply, but they do have to have direct contact to the ground (the earth itself, the dirt or concrete below). Check out 250.52 (NEC 2014) for the list of electrodes that can be used.

Lastly, all new feeders to outbuildings must contain a ground wire. Previously they were not required, but this is no longer legal or up to code for new installations. However, modifications to existing 3-wire systems do not require the pulling of a new ground wire as long as it fits a few requirements for the exception; as seen in 250.32(B) (NEC 2014)

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    The exception is for "existing installations" which the inspector would interpret to mean an existing building with an existing 3 wire sub panel feeder built before this version of the code. The 3 wire method can no longer be used for a new sub-panel feeder. – ArchonOSX Sep 27 '17 at 15:53
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    @ArchonOSX Thanks for the clarity. I see it now. I had read it as such because I could not place why the NEC would make reference to an existing installation that would be grandfathered in - but I see now that it's simply to give the inspector reason towards maintaining the original wiring after any adjustments to the subpanel system. – TFK Sep 27 '17 at 15:57
  • There sure seems to be a lot of confusion about this even among electricians. I have had contradicting answers but to be clear any remote sub panel requires both a ground wire back to the main service panel and a ground rod at its location. Also would a subpanel in the same building only require a ground wire back to the main service panel but no ground rod at its location? – Roko100 Sep 27 '17 at 17:57
  • Sorry to be dense but I want to understand this! If a ground wire is required to a remote sub panel, to allow another path back to the main panel in-case you lose continuity in the neutral wire, how does installing a ground rod at the sub panel not effect the operation of the beaker back at the main panel? This would then be the shortest path to ground. – Roko100 Sep 27 '17 at 18:28
  • Yes, any sub panel outside of the main building requires it's own ground rod and a ground wire back to the main building. And yes, a sub panel in the same building as the main does not need a ground rod - only the ground wire. – TFK Sep 27 '17 at 19:07
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So you want to stick a rod in the ground, and use that as a ground instead of the ground wire. Let's see how that works.

Electric current travels in loops, and we are concerned with two separate loops. First, natural electricity (lightning and ESD) - it's sourced from the earth, and wants to return to the earth. Ground rods are great for this.

Second, human-made electricity (mains power). This is sourced from the generating plant, or in the local loop from the transformer. Since it's an artificial source, it wants to return to that source - not the earth. For that, you need a ground wire back to source (the service panel).

Ok, so without a return ground wire, what happens when a light or tool develops a hard short from hot to ground? It should draw about 300 amps and magnetic-trip the breaker. so let's follow the amps.

  • through the local grounding system
  • to the shed's ground rod
  • into dirt
  • ?????

Needless to say, if dirt could carry 300A, we wouldn't use wires! So the dirt will be unable to return the large fault current. As a result, the grounding system floats up to 120V. The next guy to touch a switchplate gets nailed.

The ground rod lets you do more

I really don't like the idea of an outdoor subpanel without a local ground rod. It's local for a reason, and that reason doesn't go away without a structure there.

For what it's worth, however, you can put a lot more power up that expensive wire. There would be no trouble provisioning a 240V/50A, 240V/100A or 240V/125A panel up there on those same wires... especially if you already have 480V, 575V or 600V in the building. Come on back and ask if you ever get to the point of needing that.

  • Sorry I mentioned the steel building and I'm not trying to be cheap. I'm installing an electric gate 400 ft from the service panel to a sub panel at the gate. I have run 3 #6, 1 hot, 1 neutral and 1 ground. The gate requires 20 amps, 110 volts. I already knew that a ground conductor was required but I asked around about installing a ground rod connected with a bare # 6 at the sub panel and I got conflicting answers and different theories for why it is or isn't needed. I just don't like having an inspector point out my mistakes if I can help it! – Roko100 Sep 27 '17 at 19:06
  • @Roko100 Since it's a subpanel, and outside not in the building, yes, you'll need both the ground wire and the grounding rod there. If it was inside the same building you'd only need the ground wire. Also since you only want 120V, the 3 wires will suffice. It will be a 120V subpanel. – Harper Sep 27 '17 at 19:47
  • @Roko100 If you are only feeding the gate from the panel, why use a panel? You could use a disconnect for cheaper and would be under the exception I mentioned in my post above about not needing an electrode system for a single circuit. But regardless of that, the ground rod at the subpanel (auxiliary ground rod) is optional by 250.54 if this is not feeding a structure. – TFK Sep 27 '17 at 20:01
  • I may want to install some led lighting at the gate. Just like to have options. The Home Depot small panels are cheap but I wish they came with a ground bar installed. – Roko100 Sep 27 '17 at 21:38
  • @Roko100 Oh, well don't buy there! Look up your local electrical supply houses and visit them and have a brief conversation about prices. Generally HD is only cheaper on certain loss-leader items, when you get into all the little parts, it's highway robbery, or they don't stock the items at all (because they're not popular) and send you off to the electrical supply anyway. An electrical supply will not only have the right stuff, they'll remind you that you need it. – Harper Sep 28 '17 at 5:18

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