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I would appreciate your feed back to help me understand the logic behind Bonding the Neutral and Isolating Ground as shown in the attached pics. Subject sub panel is dedicated to one of four residential units / FourPlex. each unit is about 500 Sq Ft of living space with not significant load. Each unit has own sub panel, wired exactly in the same manner. The electrical work is newer, completed in late 2019 in southern California. I had no access to view the main disconnect. enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here

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    Yeah, someone got the neutral and ground bars crossed.
    – Hot Licks
    Commented May 10, 2020 at 12:11
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    SInce this is recent work, I'd explore the option of calling the installers back for a do-over and make-good.
    – Criggie
    Commented May 10, 2020 at 23:33
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    In B.C., where I live, if the inspector knows you and trusts your work, and is very busy, then especially on a very small job he may take it on faith that you did it correctly, have you sign to that effect, and not physically inspect the work himself. Or he may have seen the tidy wiring job and overlooked the issue also. But I would not "Explore the option" of calling the installer back. I'd insist on it, and if they pushed back I'd have the inspector look at it. This is a safety issue.
    – user689
    Commented May 11, 2020 at 16:43

3 Answers 3

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Whoever wired this panel simply was not paying attention

The installer who wired this panel did not pay attention to the schematic on the label, assuming that removing the bonding strap was enough here. However, the split ground design of these Murray panels means that the left-hand bar is factory bonded to the case, with the right-hand bar being the only neutral bar, when the bonding strap is detached from the neutral bar.

Sadly, the only fix for this is to move all the neutrals over to the right-hand bar and move all the grounds over to the left-hand bar, pigtailing extra wire onto them as needed, as the bonding screw on the left-hand bar on these panels is factory-fitted and not intended to be removed in the field.

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    Alternatively, could one remove the earth bar, fit an isolated neutral bar in its place, and ensure that the chassis is properly earthed to the bar on the right?
    – SomeoneSomewhereSupportsMonica
    Commented May 10, 2020 at 2:35
  • @SomeoneSomewhereSupportsMonica can't really do that without making a mess in a few spots, sadly
    – ThreePhaseEel
    Commented May 10, 2020 at 2:53
  • The next question is how did this pass inspection. This should have been permitted work and inspected by the city/county, correct? I have to assume that this is not up to code.
    – Will Hartung
    Commented May 11, 2020 at 18:23
  • @WillHartung it certainly is not up to code! And municipal inspectors, sadly, do not get to be nearly as thorough as they'd like to be in most places, so stuff like this slips by all too often.
    – ThreePhaseEel
    Commented May 11, 2020 at 18:43
  • @ThreePhaseEel Why don't they "get to be"? Are they overburdened?
    – Logarr
    Commented May 11, 2020 at 19:05
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understand the logic behind Bonding the Neutral and Isolating Ground

the green earth ground or bare copper always connects to the metal of the panel box, or any metal that you touch.

the [white] neutral only connects [bonds] to the green earth ground connection at the main panel.

in any sub panels wired from the main panel you isolate the neutral from the earth ground for the reason in the attached pic; you don't bond or connect them at the sub panel to prevent a ground loop.

enter image description here

in your pic if that is a subpanel it looks like they put the bare copper earth ground on the isolated bus bar which is wrong. The bus bar with the white neutral wires needs to be isolated from touching the panel box to be correct.

The fact they isolated neutral from earth ground prevents the ground loop in that sense is good. But it is bad because now earth ground, which is a safety mechanism, is now isolated from the sub panel box. That sub panel earth ground will reach out to all branch circuits and be ok but there is no safety earth ground on that metal sub panel box because it is isolated.

The neutral wire carries current, and should do so back to the utility and to no where else. When it does go somewhere else, that's called a ground fault and if the earth ground wiring is correct then that circuit to earth ground along the bare copper is very likely to always be less than through you to your feet/hands touching the ground.

Because your sub panel chassis is now part of the neutral circuit having the earth ground bus bar isolated, if you touch it and present a lower or near the same resistance to ground than what there is back to the main panel along the neutral wire you will get bit.

If you know what you are doing, take a volt meter and check for voltage between earth ground (bare copper) the sub panel box frame (scrape though its paint). There should be zero volts. Any voltage present is a hazard.

I don't even know if I would say your better off temporarily creating the ground loop by bonding neutral and earth at the sub panel to provide earth ground safety on the panel frame. You would just never make an exposed piece of metal part of the neutral wiring circuit, that is bad.

Definitely be careful touching the sub panel with power going to it, you are effectively touching neutral and there is a risk. Touching the painted gray part, probably insulated but touch any shiny screw fastener into the panel box and you are touching neutral.

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The steel chassis of a service panel is always grounded.

You can identify the neutral bar because it's designed with at least the ability to be isolated from the chassis. Hence the standoffs and fiber guard to prevent bare wires from hitting the chassis.

You can identify the ground bar because it is irrevocably bonded to chassis.

The screw that the home inspector is pointing out is an important screw. It physically attaches the ground bar to the chassis. It is not a bonding screw (as neutral bars are known to have) and it must not be removed, or the ground bar will flop around.

So as ThreePhaseEel says, the apprentice did this one, and the master missed it :)

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