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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.

isherwood
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Gabe Evans
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4 Answers4

7

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.

Harper - Reinstate Monica
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4

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.

DoxyLover
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2

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.

wallyk
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-1

What about Plan A but add a Subpanel earth ground connected to the Subpanel ground bar (which is bound to the Subpanel neutral bar)?

Town Piddler
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