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I am going to be adding a 3-wire fed subpanel to a dwelling. My understanding of the plan of attack is attached below (Please forgive my crappy, minimalistic MS-Paint schematics).

Neutral bar in sub panel is bonded to ground bar. Problems?

As you'll notice, the ground and neutral are bonded in both the subpanel and service panel. My concern with this is any power on the neutral bar of the sub panel will also be carried by the ground bar, and my fear is that could cause current to flow to anything that's connected to ground, such as the electric stove chassis, any computer cases, light switches, etc.

My plan, attached below (Again, MS-Paint), isolates the ground and neutral bars, and adds a ground electrode at the sub panel. My thought is that this will keep power that is on the neutral off of the ground lines, and off of any metal objects attached to ground(stove chassis, computer cases, etc).

Neutral bar isolated from ground in subpanel.

Do my concerns with Plan A have any merit, and if so, would Plan B solve the issues I am worried about?

If Plan B doesn't address my concerns, what can be done to prevent anyone from getting shocked when they touch something that's connected to ground?

I should mention that I am aware that a 3-wire setup is not code. Adding a 4th conductor is not an option, as the person in charge doesn't have the money for that.He has been assured by a trusted, old-school electrician that a 3-wire setup will work, but will not be up to current codes. As such, code compliance isn't my goal, so much as base-line safety is.

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    Code compliance is base-line safety.
    – Tester101
    Aug 12, 2016 at 12:05

4 Answers 4

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Plan A is NO. An open neutral will put 120V on every grounded surface (i.e. light switch cover screws).

Plan B is NO. A ground fault will put 120V on every grounded surface.

Also, plan B will fail to trip breakers in the event of a high current ground fault. It is a terrible plan.

If you want to do it with 3 wires, you have two options, one bad.

Install 120V-only service.

Your 3 wires are L1 Hot, Neutral and Ground.

Install 240V-only service.

Your 3 wires are L1 Hot, L2 Hot and Ground. Use exclusively 240V loads (easier than it sounds) and install NEMA 6 outlets exclusively. It is not legal to do this in residential space or other domicile (hotel rooms etc.)

The problem with this strategy is keeping it pure. Some yutz will want to install a 120V load later, and he'll bootleg neutral which will render it unsafe the same as Plan A.

Transformer

This only requires 2 wires, but it requires a transformer. In this case you make the outbuilding its own main panel, with its own local grounding system. It is electrically isolated from the supply by the transformer, so neutral is locally derived and you don't (and shouldn't) connect it to the main building's neutral/ground in any way.

Costwise, there's a "tipping point" depending on distance, whether the cost of wire justifies the transformer. For very long runs, there's another tipping point where "step-up" transformers make sense. This is why Tesla won the war of the currents.

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  • While code may not provide for such things, I'm curious what hazards would exist with plan B if all current passed through a three-wire GFCI? If a difference in terrestrial ground potential exists between the two buildings, tying exposed metal surfaces to a grounding conductor which fed the first building would seem to create a potential difference between those surfaces and nearby water pipes, etc. Plan B would eliminate that difference, and I would think a three-wire GFCI should render it safe. A transformer may be a better approach, but would B+GFCI be dangerous?
    – supercat
    Aug 11, 2016 at 23:34
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    It's tempting for geeks and EE's not practiced in Code electrical, to second-guess the NFPA. That's a mistake. NFPA has been working these problems full time for years, with experts way smarter than us, and they have access to all the accident reports from all the inspectors kicking through the ashes. There's a reason, even if you haven't figured it out yet. It may be well beyond the fields you are thinking about, e.g. the behavior of other humans down the road.
    – Harper - Reinstate Monica
    Aug 12, 2016 at 1:30
  • I was not trying to suggest that someone should do plan B with a GFCI, but rather inquire whether you had any idea of what hazards might exist. Further, while plan B would not be suitable for new installations, the NFPA does acknowledge the use of GFCIs in some cases where existing wiring would not support grounding via approved means and I've often thought that safety might be improved with hardware designed for such situations. If one had an outbuilding that was already wired and had no approved grounding conductors, one could protect everything with GFCIs, leave the grounding...
    – supercat
    Aug 12, 2016 at 14:40
  • ...conductors of all receptacles open, and mark all receptacles "No equipment ground", but it would seem that a grounding lead which was limited to carrying 9mA and had its own current sensor and a device that would simultaneously connect it and all the mains wires in case of a ground fault, would improve safety by ensuring that short from hot to the grounding lead would kill power without anyone having to provide an alternate path to ground first.
    – supercat
    Aug 12, 2016 at 14:43
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Do not use plan B under any conditions! The earth is not a low-impedance path for electrical flow. If you had a hot-to-ground short in the sub-panel, the earth would not conduct enough current to trip the breaker. This would leave the sub-panel ground carrying a lethal voltage!

Plan A (bonded neutral-ground in the sub and common ground/neutral wire to it) I believe was legal under old code (I don't know when it changed). It is not legal under current code but is much safer than your plan B.

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  • Guess I should stick to computers. Lol. At any rate, Plan A is not going to create voltage on the grounds throughout the subpanel branches, in spite of them being bonded together with the neutral bus?
    – Gabe Evans
    Aug 11, 2016 at 20:30
  • @GabeEvans if you have an especially high load on the sub panel, so that you get a voltage drop on the neutral feed to the sub panel, you could end up with some voltage on the ground. However, if you ever lose the neutral wire to the sub, the ground will become energized from the hot (feeding through any connected loads). I think this is why the code was changed.
    – DoxyLover
    Aug 11, 2016 at 22:22
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The subpanel must have separate ground and neutral buses and feeds. You will need to add a fourth wire. The subpanel is likely to arrive from the manufacturer with a bonding screw which you must remove to isolate the neutral from the ground.

I think a separate ground rod is okay, but the grounds must be connected between panels through a #6 (or heavier) ground wire.

If Plan B doesn't address my concerns, what can be done to prevent anyone from getting shocked when they touch something that's connected to ground?

If someone is getting a shock from touching something grounded, there is a major problem. The purpose of grounding is to a) limit how far the voltage of "hot" wires is from ground, and b) to eliminate all potential (voltage) of grounded equipment from the earth.

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  • The person in charge is adamant about three wires as he lacks the funds to allocate to a fourth. He's consulted with an old-school electrician who assures him that the 4th wire, while proper, is not necessary, and that a 3-wire setup will work, although won't be code. As such, I'm not concerned about code compliance, so much as basic safety.
    – Gabe Evans
    Aug 11, 2016 at 19:37
  • @GabeEvans: Is this in North America?
    – wallyk
    Aug 11, 2016 at 19:51
  • Yes (char limit).
    – Gabe Evans
    Aug 11, 2016 at 20:01
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    If you can't talk him into a ground conductor, I presume you also can't talk him into metallic conduit, which used to double as safety grounds...
    – keshlam
    Aug 11, 2016 at 20:55
  • @GabeEvans: How far apart are the panels going to be?
    – wallyk
    Aug 12, 2016 at 0:34
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What about Plan A but add a Subpanel earth ground connected to the Subpanel ground bar (which is bound to the Subpanel neutral bar)?

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