Bonding grounds and neutrals together in sub-panels

Parallel Paths, be careful

Parallel Paths

One of the most common defects I find related to remote distribution panels (sub-panels) is ground wires and neutral wires bonded together. 

This is especially true if the work has been done by homeowners or handy persons. 

In simple terms, the only place we want to bond the grounds and neutrals together is in the service equipment. Many people refer to it as the “main panel” or a variety of other terms.  

Regardless of what you may improperly call it, the point where you can disconnect all power to the building is the service equipment.  At this point, the ground and neutral are connected to the earth through a system of pipes, rebar, rods, and or wires.  The purpose of connecting the system to earth has little to do with the function of the electrical system.  This provides a layer of protection against lightning surges or static charges that would otherwise build up on the electrical system.

It is a bit like the spark you get from nose to nose when static charges build up on you and the person with the other nose.  This happens because you have no means of sending that excess energy to the earth.

The second important function of all those ground wires running in all the circuits throughout the home is to provide an emergency path back to where they are connected together in the service equipment.  In this way, if there is a short between the energized conductors and some metal component that is grounded, there will be a path back to the point of connection to trip the breaker associated with that circuit.

Circuit breakers trip on heat curves and amperage curves and a short circuit represents many times the amperage rating of the breaker tripping it instantly.  Likewise if there is a problem with the circuit that is resulting in over-amperage, the breaker will trip within the time curve of the breaker–not necessarily exactly the rating of the breaker.  A 20 amp breaker could actually not trip for a few amps above 20 amps for X amount of time without tripping.  Depending on the appliance, the appliance might finish its job before the breaker trips and we would never know it is misbehaving.

But lets get back to not connecting grounds and neutrals together in sub-panels.  Installing the green screw in this sub-panel has resulted in connecting the grounds and neutrals together.  It needs to be removed.

Improper bonding

Green screw bonds the neutral bar to the grounded metal box

When we do bond them together we create two paths back to the connection at the service equipment.  The amount of current that will flow on the two paths will be proportional to the resistance of those paths.  For example if metal conduit or a very large wire is used as the equipment grounding conductor from the sub-panel to the service equipment a large percentage of the neutral current could flow on the bare conduit or bare ground wire (or coated ground wire as the case may be) back to the service equipment.  In some cases the metal conduit might be a proportionally better path than the neutral wire feeding the sub-panel and the majority of the neutral current could then flow on the bare conduit.

I consider it best practice to always provide  a ground wire inside metal conduit but there are probably millions of installations that rely on the metal conduit as the path back to the service equipment.  As long as neutrals and grounds are not bonded together in the sub-panel this is rarely an issue. 

Now if grounds and neutrals are joined together in the sub-panel, the current of all the 120 volt circuits that are operating will travel on the metal conduit, and the neutral wire, as well as the ground wire if present.  This is multiple paths.

So in the following picture where there is no ground wire inside the conduit, but instead the only path back to the service equipment, is the metal conduit, its being disconnected is a serious problem for fire safety and ability of the breakers to trip if there is a fault to ground.  The receptacles of the circuits in this sub-panel tested as ungrounded,.  Fortunately, in this case, the neutrals and grounds were properly isolated, so there was little risk of neutral current running on the bare conduit.

Disconnected electrical conduit

Disconnected conduit feeding condo sub-panel

If they are bonded together in the sub-panel, who is going to be brave enough to grab the two ends of the pipe and stick them back together?

A competent electrician will know enough to test the metal components and/or make sure electrical circuits are turned off, but what about the handyman?  What about your Honey that works on your Honey Do list?  Most people would be unaware of the dangers present and working with the exposed metal components with bare hands could be deadly.

Here is a video demonstration done with students at Bellingham Technical College to show the effect on different size “paths” in a simulation of grounds and neutrals connected together at a sub-panel.  The “light” is the load symbolizing the sub-panel.

A big thanks to Gary Smith for his improvements to this video.

Here is a picture of the wiring diagram for the demonstration in the video:

Charles Buell Real Estate Inspections in Seattle


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  1. Thank you, Charles for providing this service to all of us. I learn a lot from you and that helps my clients so I am in your debt.

  2. frank ingle says

    thanks for youtube video on good justification for safety aspects of not joining ground and neutral in subpanel.
    however i have another ecampke to share.
    i live in an old house with questionable wiring.
    i suddenly experienced flickering and bright flashes in the lights. bright flashes first: i measured sustained periods and measured voltage at wall socket to be as high as 195 vac. the neutrals wete joined in sub but not connected to gnd. i eventually realized that of the 3 circuits from the sub panel, they alternated phases as is usual. the neutral from sub to main was intermittent which when open put 240 across the two circuits and their loads. i moved one breaker in sub so all phases from sub were the same and flashes ended. however flicker down to 90 vac persisted. i then jumpered neutral to gnd in sub and problems ended. i await the electrician to remove my patches and rewire L1, L2, N, and G from main panel to sub panel. Conclusion: there is still one failure mode possible when N not connected to G in sub. A possible solution is to provide redundant N and G between panels. would have prevented the problem in my old house. Frank Ingle (EE, PE). thanks for your nice explanation and demo.

  3. Dale Yaeger says

    Hi Charles
    Thank you for your YouTube videos.
    I have a question which you may be able to answer: I have an small
    inverter power generator that has an open between the neutral and
    ground. This certainly violates the National Electrical Code. However,
    the Product Safety Commission says this posses no safety threat to consumers. Additional, Consumer Reports say they have seen this in some of the smaller
    generators they rate but suggest I talk to a certified electrician. Also, my 2000 watt generator fails operate a 900 watt microwave and the micro manufacturer says the electrical outlet must be properly grounded to work properly. Is there some provision in the NEC that permits an open ground in the engineering of some portable generators?
    If you can’t answer my question, could you refer me to someone who can?
    Dale Yaeger

    • Charles Buell says

      Dale, thanks for your question but my understanding of generators beyond the disconnect and how it is wired is limited. You should consult an electrical contractor. It should power the micro regardless of grounding as they work all the time plugged into older ungrounded circuits in homes.

  4. the photo showing the grounding wire on the neutral bus bar that is not allowed also shows 2 neutral wires under the same screw on the neutral bus bar which is also not allowed.

    • Charles Buell says

      Not sure what picture you are talking about but there is only one panel picture in the post which has 4 neutral wires and each one is under its own lug and I see no ground wires on the neutral bar.

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