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.
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.
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
Jamie Arnold says
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.
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.
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?
Thanks.
Dale Yaeger dcyaeger67@gmail.com
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.
Charlie
Ray says
some gens have a floating neutral, if u use equipment with a fault
and 2 wire cord, and neutral and ground are connected at gen, that could be a very unsafe situation to be in. I saw where this happen.
guy using 7 kw gen outside never had an issue until gen was hooked into house, neutral and ground connections were then made .
the equipment he used had a fault but as long as neutral and ground were not connected. . .once connected at house neutral tied in with ground
Ernie Borsellino says
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.
kannan c says
Dear Mr. Charles
Bonding of Earth and neutral in machine subpanels will cause a problem, but I want to know how it would affect PLC & VFD, etc.
The machine has 2 panels one for heaters (uneven load in each phase) another has VFD (250 kW) for the motor. since the last 4 months continuously facing a problem with drive most of the time control board failed without any error replaced with new VFD but no improvement. Also found some liquid from boards maybe electrolyte.
While checking found both panel earth & neutral & bonded together. I removed the bonding but want know to make sure it was the root cause of all the time drive failure.
Also After removing bonding in the sub-panel between earth & neutral showing some 100kohms. The main panel (transformer room) & subpanel earth are not connected then how showing some ohms and it keeps varying. In a cable have 4 wire i.e 3 phase & 1 neutral.
Charles Buell says
You are asking questions way above my pay grade. Discuss this with a qualified electrical contractor. Sorry.
troy says
your test showing 100k ohms is your meter’s current flowing through neutral to source to grounding bond and returning through earth. the neutral line has little resistance, but earth has much resistance and conductance, both. it may not be possible to overload earth.
DON GREEAR says
I have a service meter and panel with a 200 amp breaker and a 40 amp breaker at a power line pole which use to service a mobile home. Both the neutral and ground are connected together. A new home was constructed in 1994, the mobile home was removed and the power company went from that service to a new 200 amp home panel with the neutral and ground connected together. There is a ground rod at both service panels. All 120 volt circuits have both the white wire neutral and ground connected on the same neutral/ground bus bar in the home panel. I have not experienced any issues with any appliances.
I now want to connect a 7200 watt generator to the 40 amp breaker at the power line pole when the grid is down. I have an interlock device so the 200 amp breaker must be off so the 40 amp breaker can be turned on.
Do I connect the neutral and ground for the generator together inside the panel? That is the only option I see is available.
Do I run a ground wire from the generator to the ground rod?
Charles Buell says
I am afraid you are going to need a good sparky for this one. I would recommend connecting the generator interface to the house panel—forget the 40 amp breaker at the pole. I assume there is a 3 wire feed between the pole and the house? Are you saying there is a 200 amp breaker at both the pole and the house?
Jim Robinson says
Because of limited space in the main outdoor panel, my generator transfer switch is located in a sub panel which has two other breakers for some outdoor circuits. The generator transfer plug is connected to a standard 50 amp breaker in the subpanel which serves as the transfer switch. The sub panel connects to the main panel with four #6 insulated wires. The procedure for using the generator is to first switch off the 200 amp breaker in the main panel prior to throwing the transfer switch and warnings are prominently posted in both boxes. This was originally wired by an electrician to accommodate a floating ground generator.
I now have a Westinghouse portable generator that has a bonded neutral. Given the wiring arrangement described above, should I float the generator neutral before using it to power the house or should some change be made to the neutral and/or ground connections in the transfer plug on the house such as disconnecting the neutral and/or ground path to the generator?
Floating the generator neutral doesn’t void the Westinghouse warranty but, of course wouldn’t provide GFCI protection when the generator is used in a stand alone setting.
Researching this on various websites hasn’t been of much value because everyone seems to have a different opinion. However advice on the buellinspections.com website is always concise and clear.
Bob says
Excellent article. I actually understood about 95% of it.
I understand why the neutral and ground need to be separated in a sub panel,
but doesn’t the metal panel itself need to be grounded all the way back to the service equipment panel?
Charles Buell says
It certainly does and the ground bar connected to the metal panel with the ground wire from the service equipment accomplishes that.
Ivan Ellsworth says
I enjoyed watching the video you made four years ago called “Parallel Paths”, which was suppose to show why it would be a problem to bond the neutral and grounds together on the same busbar in a sub-panel, but the video is more of an example of why one does not want to join the ground wire and white neutral wire together in a fixture. Too bad one can not copy and paste to this message. Why is that?