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

Grounding, Bonding and Bondage—it is important to know the difference!

In my blog I usually strive to keep things from getting too “technical”—-or too “risqué.”  I have wanted to do a post about the differences between “Service” Panels (the place where your main breaker/disconnect is) and “Remote” Distribution panels–more commonly known as “Sub-Panels.  The main difference between the two–without getting too technical is how the ground wires and neutral wires are terminated.

Candidate for "worst sub-panel"

Candidate for “worst sub-panel”

“Grounding” and “bonding” get thrown around casually sometimes as if there was no difference between the two.  In Residences “grounding” of the electrical system happens at the Service Panel and consists of the grounding conductor being connected to the Neutral/Ground bar.  The grounding conductor will be connected to one or more of several options:  Ground Rods, Metal Water Service Pipes, and/or Concrete Encased Electrodes (Sometimes called a UFER ground and consisting of the rebar in the foundation footings/foundation).

Other metallic systems in the home will be “bonded” (connected) to the grounding electrode system.  So if you have metal water pipes, metal heating system pipes, gas supply pipes, or cable/telephone systems in the home, these systems will all be connected mechanically (bonded) to the grounding conductor of the home.

The critical thing to keep in mind about bonding and grounding is that at the Service Panel, the Neutral Wires (white wires) and the Equipment Ground Wires (bare copper—sometimes green coated wires) join together on the Neutral/Ground bar.  So in the Service Panel you will find all of these white and bare copper wires connected to the same termination bar.

I really don’t want to get into the reasons why this is so in this post.  Just keep in mind that in a Remote Distribution Panel (sub-panel) the equipment ground wires and neutral wires cannot be joined together.  In fact even at switches and receptacles or anywhere else wires are brought together—-the equipment ground wires and neutral wires must never be connected together.

The reason for this is that there is a small amount of current that always flows on the neutral wire in the normal operation of appliances etc and you do not want that current running on the bare ground wires–these wires are connected to things you might touch and thus become a shock hazard.

So now let’s look at the electrical panel in the picture below.

This panel is a sub-panel.

On the left side of the picture we can see all the bare copper wires that are all the equipment ground wires of the individual circuits.  There is  a big copper wire connected to the ground bar that goes off to a gas pipe—out through the top of the panel.  Now aside from the fact that there are some issues with the way all of these bare ground wires are terminated in terms of the number of wires under each screw, the thing I want you to pay attention to is that they are all connected to their own little metal bar that is connected directly to the metal box itself.

Now look at the big wire with the white tape like a barber pole.  That is the Neutral wire and notice how it is connected to the vertical neutral bar on the left side (the bottom end is visible below the breakers on the left side) and that there is a Crossover Arm connected to it that goes over to the vertical bar on the right side.  This is what we look for when we talk about the ground wires and the neutral wires being isolated from each other in a sub-panel.  All of the electrical components related to the Neutral wires are separated from the metal box with pieces of plastic–so they don’t touch each other.


Improperly wired sub-panel

But wait a minute Captain–we have a problem.

Can anyone see the problem?

Take a look at that crossover arm that connects the left neutral bar to the right neutral bar.  Do you see that very pretty green screw?  That green screw is shipped with the panel, from the manufacturer, so that when the panel is going to be used as a Service Panel the Neutral bar can become a Neutral/Ground bar and the metal box is bonded with the green screw.

The green screw is meant to be discarded when the panel is being used as a sub-panel–like this one is.  So in this installation we run the risk of running some amount of current onto all of those bare copper wires and to everything they are connected to–including you if you touch them.  It is an immensely easy fix–the green screw merely needs to be removed–by the licensed electrical contractor.  But not to worry–if there is this defect, there will likely be others–to soften the electrician’s “minimum” service call.

Now for all of those that are wondering when I am going to get to the bondage part–ask Dr. Ruth–that is what Google is for.


Charles Buell, Real Estate Inspections in Seattle

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