I have a detached garage whose wiring is fed by just two conductors running back to the main panel. There's no ground wire in the feeder and no separate ground rod at the garage. The feeder has only one black and one white conductor. Can I add a ground rod at the garage and rewire to update the wiring to support a bare ground wire in the new wiring? The garage subpanel is modern, replacing old glass screw-in fuses with circuit breakers. That subpanel also feeds a separate shed. In both buildings a circuit tester shows a missing ground, I assume that is because there's no ground rod at the garage and no bare ground wire running back to the main panel. Basically, is it OK to have each building equipped with separate ground rods used just for the bare ground wires in those buildings?

The distance between the main panel and the garage is about 160 ft. It is underground, probably without a conduit, as none of the other wiring uses a conduit and the feeder wire just appears out of the concrete floor of the garage. Most of the construction was done in the 1960s by a non-professonal. No 240v is involved. There's a single 30 amp breaker on the feed to the garage. I don't need more than 15 amps on any of the feeds going from the garage subpanel. I realize that feeds off that garage subpanel could add up to higher than 30 and trip the breaker back at the main panel.

I have looked at the previous, similar questions but see conflicting answers. Some folks say that you cannot have too many grounds. A satellite tech recently put in a new antenna on the distant shed. He grounded that to some rebar about two feet long. After he got done that particular circuit no longer showed a missing ground with a tester. Another circuit in the same building shows open ground. I realize that two feet of rebar is not much but don't see how that could be a hazard when used just for the bare wire. I just wanted to expand on that solution.

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    This question comes up fairly regularly. Have you glanced at the RELATED section in the sidebar?
    – isherwood
    Jun 25 '18 at 15:19
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    Is the power in the garage 120V or 240V? If the latter, where is neutral coming from? How much power do you want there and how big are the wires currently extant? Jun 26 '18 at 4:38
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    Hi, @DaveK. You've created a second account with the same name to edit your post. You should look into getting those two accounts merged. Jun 26 '18 at 23:27
  • Is your main panel indoors or outdoors? Jul 6 '18 at 2:47

Nope, the grounding rod is not a substitute for a ground wire back to the main service. It simply will not do what you desperately need it to do.

In fact, any electrical fault when the grounding system should be helping you, instead the entire grounding system gets lit up at mains voltage! Now every light switch cover screw, every bit of conduit, the service panel itself is electrified.

The two grounds have different jobs. Remember, current travels in loops, and it wants to return to source.

  • The grounding rod returns natural electricity (lightning, ESD) to its source, which is the earth.
  • The ground wire returns human-made electricity back to its source, which is the neutral lug on the transformer on the pole. It's supposed to do that via the neutral wire, but the ground wire is there on the hopes that some faults can return via the safety ground to the main panel's bonding wire, instead of via a poor human and some convoluted path back to source.

They are not interchangeable because dirt is a poor conductor of electricity.

Now the best way to safe this thing is to put a GFCI breaker on the breaker in the main panel. GFCIs do a pretty good job of rendering human electricity safe.

Once the GFCI is on board, having an isolated ground via your ground rod scheme would actually help the GFCI work. A ground fault in any of your equipment will still light up your grounding system with lethal voltage, but the GFCI should intervene before enough current flows to actually hurt you.

Of course this isolated ground would do a fine job of returning natural electricity to source.


It's transformer time, baby!

One of the wonderful things about AC power is that we can transmit it as an oscillating magnetic field without creating a conductive connection between the two points. This is how transformers work, and it means that not only can they be used to step up or down the voltage, they can be used to create a new neutral bonding point (a separately derived system in Code-ese) in the middle of an electrical system, as the primary and secondary sides of the transformer are effectively insulated from each other.

Given this, and the language in NEC 250.30 that covers separately derived systems, we can do a bit of rearranging at the house to get the situation at the garage to work. First off, the house end of the existing garage feeder cable needs to be brought up on the outside wall of the house, if it isn't already that way, and a new 3-wire circuit needs to be run from the existing house panel to where the feeder comes out onto the house.

Then, we work some magic. A 5kVA transformer with a 240/480V primary and 120/240V secondary can be had for roughly $500 new, or quite a bit less used in many cases (check Craigslist and such). The existing 30A breaker at the house needs to be swapped for a 15A, 2-pole breaker, and the 3-wire circuit needs to be wired at the panel as a 240V-only circuit (hot/hot/ground). The transformer goes at the point where the 3-wire circuit from the panel meets the 2-wire feeder to the garage -- this point must be on the outside of the house for this to work.

At the transformer, wires H1 and H2 connect to one incoming hot from the panel, and wires H3 and H4 connect to the other incoming hot from the panel. The ground wire in the circuit from the panel connects to a ground screw or wire on the transformer enclosure, completing the wiring of the primary circuit. On the secondary side, the X1 and X2 wires connect to the black wire going into the garage feeder, and the X3 and X4 wires connect not only to the white wire going into the garage feeder, but two two more wires:

  • A 6AWG jumper to a ground screw on the transformer case or transformer ground wire (this is a 250.30(A)(1) requirement for a system bonding jumper at the transformer if grounding electrodes are brought to it as per 250.30(C), and is permitted as a duplicate bonding jumper by NEC 250.30(A)(1) Exception 2)
  • Another 6AWG copper wire to a pair of ground rods spaced 8' apart (to protect the secondary side of this transformer from Mother Nature-induced insulation breakdown, again see NEC 250.30(C) for the gory details)

(P.S. This steps the voltage down from 240V from the panel to 120V on the feeder as an artifact of this arrangement, hence the breaker in the house getting downsized.)

Once this is done, then we move onto the garage. Another pair of ground rods are driven 8' apart and connected to the garage panel via more 6AWG copper wire, and the bonding screw or strap in the garage panel is left in -- effectively, the transformer makes it a main panel all over again. That way, faults to ground caused by man-made power get returned to the transformer secondary through the bond at the garage panel, while the ground rods deal with naturally generated electricity. (The duplicate bond at the transformer secondary will flow no current unless the transformer itself shorts from primary to secondary, which should not happen.)

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