Is Your Subpanel Wired Wrong? Let’s talk about Improper Subpanel Bonding
Whether you’re a homeowner, a real estate agent, or a fellow home inspector, you need to recognize this one specific method of improper subpanel bonding. It can be dangerous and it’s just something you don’t want to mess around with.
Here it is: Your ground and neutral wires definitely need to bond (or connect) together. But this is ONLY allowed in the main panel— never a subpanel, or anywhere else in the home.
This is a very common mistake we see in the electrical part of your inspection. Although some “professionals” have made this mistake by failing to remove the bonding screw/strap, it’s usually more a sign of amateur electrical work.
That’s why today, we’re going to go over how to properly layout your subpanel wiring and what can happen if it’s not fixed right away.
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What Are Subpanels?
Inspectors, you can probably sit this one out. But as a brief overview, your subpanel is one of the electrical boxes at your house (sometimes it’s in the garage). Not all homes have them, as some only have a main panel only. But if you have 2 or more panels, the one with the main “service disconnect” (i.e. shut-off) is the main panel, while any other panels are considered subpanels.
You may have also heard the main panel referred to as a service panel. This is the area where you have complete control over the power within your home.
One flick of a switch, and you can turn out all the lights.
A subpanel has its own breaker switches, but not the main. These typically control specific rooms or appliances in your home or other buildings on your property.
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Hot, Neutral and Ground Wiring— What’s The Difference?
In both of these panel boxes, there are four types of wires:
- Two hot wires (black or red casing)
- One neutral wire (white casing), and
- One ground wire (green casing or bare metal wire – usually copper or aluminum)
The hot wires carry the energy to a fixture (e.g. a wall plug or the stove in your house). For a 120V circuit, it will be just one hot wire. If it’s 240V (e.g. a dryer, water heater, or range/stove), then two (2) hot wires will go to those fixtures.
The neutral wires will then carry the energy (technically known as current/amperage) from those circuits back to the panel. This effectively completes the electric circuit, thus generating power.
Congratulations, you now understand how electricity works (and with just three wires)!
So what’s the point of the fourth one— the ground wire?
This one provides safety.
Hot and neutral wires are dangerous because they’re constantly running electricity through them. When you have something like frayed wiring cords or loose wire connections, it can cause those wires and their electrical currents to get mixed up. This is what causes short circuits.
Short circuits can emit sparks and cause fires. This is why we always recommend that you keep your appliances and electrical devices up-to-date.
However, any potential short circuits are derailed with a ground wire because this sends all those faulty electrical currents right back to the source, or the ground.
This is why you need to separate the ground and neutral. But more on that in a second.
The main point is that the ground wire generally isn’t used at all as part of the normal circuit operation. It’s a backup (also known as a secondary or earth ground) and is only there for safety in case the neutral fails. This way, the electricity doesn’t end up energizing the body of the appliance… or a human body.
In fact, neutral wires are also called grounded conductors because they technically act as a ground as well. Consider them the primary ground. Actually, on older homes (the kind with only 2-prong outlets), there often was no additional ground.
We have come a long way!
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Here’s What Improper Subpanel Bonding Looks Like
Your main panel and your subpanel both contain those four sets of wires.
So here’s the difference and how you can spot novice electrical work.
Below is the anatomy of the main service panel. Code Check’s image shows how the neutral wire (white) is connected to the bus bar on the right. Further down that bus bar is a screw or strap that “bonds” it to the metal bar that leads to the left side of the panel. And on the left side of the panel, you can see the ground wires (also known as the EGC/GEC’s). This effectively bonds (or connects) the neutral and ground together in the same service enclosure. But remember, this can only happen in the main/service panel – never the subpanel.
In Code Check’s subpanel wiring diagram, you can see that those neutral wires are not bonded. This means that the neutral wires are not connected to the ground wires. They both have their own individual paths in the subpanel.
Why do you separate the ground and neutral in a subpanel?
So, why do you separate the ground and neutral in a subpanel? Because when we bond them together, it gives your neutral wire (the one carrying electrical currents BACK to the source) multiple pathways.
That’s how the chassis of some equipment will become energized. The effect can be as seemingly harmless as “I touched the fridge, and it shocked me!” To far more serious matters.
The most common issues that come from not separating ground and neutral wires in a subpanel are:
As we mentioned before, ground wires help prevent short circuits, therefore preventing things like getting a shock and house fires.
A short circuit usually happens when there are loose or frayed wires. But when there are numerous electrical faults or massive amounts of electricity coursing through these bad wires, adding a third wire to the same pathway— the ground wire— creates more risk to these events.
Thus, the ground wire needs to have a limitless path (free of the neutral wire’s electrical currents) so that it can safely prevent short circuits.
Corrosion can happen because of improper subpanel bonding as well. This is because the excessive electrical currents (coming from the hot, neutral, and ground wires) can accelerate the rate at which metal pipes or buildings start to erode.
Check out this video for a visual!
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How To Separate Neutral and Ground in Subpanel
This is work for a qualified electrician. It’s often a simple repair, but needs to be left to the professionals.
The difference may be as simple as having a green screw or bonding strap connected in your service panel – and not having it connected in the subpanel. Sometimes we will see neutrals and ground wires terminated on the same bus bar in the subpanels as well, often when a novice adds additional circuits (such as in the garage). This is effectively bonding the ground and neutrals together. You can do this in the main panel, but never should do it in a subpanel.
Yep, separating your ground wire from the neutrals in your subpanel can be as simple as that. However, we always recommend having an experienced electrician take a look.
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Great information. Recently added a subpanel to my garage and was trying to remember what my shop teacher had told us in the residential wiring coarse I was in 52 years ago. I’m not sure he went into the detail you did…or perhaps I wasn’t paying attention.
I had a bit of concern about grounding and my confusion was that, for a subpanel I’m using a 100 amp main breaker panel I picked up that was already loaded with breakers. My garage is an all steel building and I don’t want to create any feedback in the framework of the structure.
So, my conclusion after watching the video and text you provided is that my grounds will run to a separate bus bar attached to a ground rod.
Let me know if my thinking is incorrect…Thanks so much for the info.
Thanks for the comment Ken. If your service (main) panel is already in the same building, then additional grounding rods shouldn’t be needed. However, if this is a separate structure you will need them. There are specific rules for this, particularly regarding impedance, and I would recommend contacting a qualified electrician for adding a subpanel instead of a DIY attempt.
If this is a new installation, you should have an EGC (ground wire) running back to the main panel, in addition to the ground rod (4 wires total), with the neutral and ground remaining separate back to the main panel. If, however, you have 3-wire service to the garage, neutral and ground *must* be bonded in the subpanel.
The idea that neutral and ground are never bonded outside of the main panel simply isn’t correct with respect to modern electrical codes–your ground rod doesn’t provide the low impedance path back to the source (i.e. the neutral from the power company) necessary to trip a breaker in the case of a short to ground.
Thanks for the input, Dave. This is also correct, particularly for a detached structure with a panel. Generally, for most single-family detached homes, not bonding beyond the main panel is the scenario roughly 95% of the time, so we say it that way for the sake of simplicity. There are exceptions however — such as the one you’ve mentioned. However, in all scenarios if you aren’t a qualified electrician, it is our opinion that you should have one perform this work.
I bought my house around 10 years ago, roughly 4 years ago they replaced the water main going down the center of the street.
They went from cast iron to plastic
We started to experience, buzzing, and humming sounds, there were also times when if you flush the toilet or took a shower, excessive noise could be heard, this problem has continued to get worse. Most recently we had a new drop, put in coming off of the pole, the old wires that ran down the side of the house were cloth covered but yet the wires were insulated so per the town we replaced to drop off of the pole. It is encased now in PVC plastic a new meter box and outside service disconnect any new 200 amp service panel , we also have a subpanel in our basement and another subpanel in the garage which is detached one of the things that I stumbled upon is that even with the service disconnected from the pole I can still show 60 Hz running through the aluminum siding I found that one of the grounding rods that the professional electrician had installed “and by the way, this was inspected and passed”was only 2 1/2 foot long. I replaced it with one that was 8 foot long. So I now have two 8 ft grounding rods that are 6 feet apart, my question is inside of my 200 amp service panel. There does not appear to be a way to ground or bond the neutrals in the service disconnect on the outside of the house. There is a green screw that sits next to where the neutral wire from the pole comes into the service disconnect and then goes into the house. There’s also a place there for a grounding wire, but no wire is connected to ground from my main service panel in the basement, a large aluminum unshielded wire is bolted directly to the outside service disconnect box, which is, of course bolted to my aluminum siding but there is no ground going back to the pole, only three wires, which I am assuming are my two hot leads and my neutral, where the wires come out of the top of the PVC service, my white neutral wire connects to an unshielded aluminum wire that goes back to the pole. Is this a neutral or is this a ground? I have had my town come out and they told me everything is fine, but on certain days, the frequency and noise inside of this house will rattle the fillings out of your teeth. Plus I have a large German shepherd that can pick up on this very quickly and it varies from day today. Do amount of noise we get in the house, but most recently , we have noticed our air conditioner. Sounds like it’s straining. The old one only lasted seven years window. AC units tend to burn out after two or three years, when I do a continuity, check between ground and neutral in all three boxes, they all have continuity between all three any help you guys could give me on this would be greatly appreciated. Thank you.
Wow Mark, sounds like there’s a lot going on there. Many will be surprised to learn that public utilities used cast iron for water mains, but it’s true, they did! We usually only see them used on the low-pressure sewer side at the house level.
3 wires from the pole is normal for a typical residential service, as the ground wire itself doesn’t go back to the pole. So that bare wire going back to the pole is your neutral.
Your having the new rods installed was likely a good idea. Hate to hear that they were short, but in some cases, this can actually be acceptable depending on the impedance reading. Maybe not 2.5′ feet though. Deeper/longer rods are still better, and you were good to do that. Hopefully this alleviated any errant readings you were getting from your aluminum siding.
Replacing the cloth sheathed wiring was probably a good idea too because, even though it’s insulated, by this point that insulation was probably cracking or soon going to do it. Newer wiring also has insulation with a higher heat rating too, so you’re simply better off. Since you had a new drop for a 200 Amp service, it’s quite possible the cloth and meter can were undersized for that level of service, as well. Older services tended to be smaller than 200 amps, so we’re thinking maybe you upsized the service amperage rating?
Without seeing the setup at your home ourselves, it’s hard to confidently tell you what’s going on. The “frequency and noise” you describe is particularly perplexing, and regretfully we simply cannot assist from this vantage. We wish we could, but there are simply too many variables here. For instance, are there large electric transmission lines near your house? Is there a new manufacturing or research facility nearby? Do you have plaster and metal lath instead of drywall, and could there be an errant electrical connection to that metal lath somewhere, somehow? Are one or more of the nails holding your aluminum siding too long or overdriven, and maybe piercing a wire somewhere? Is there cloth wiring elsewhere in your house that may have bad insulation? Could it be a simple grounding issue? Do your neighbors have something similar going on? You can see how many different trajectories this could take.
You were right on the right track to have the city come out and take a look as a first step. If you feel confident it’s electrically related, you could always try the city inspectors again, and maybe ask if they will send a more senior inspector out — one with strong electrical knowledge. You could also have another qualified electrician come take a look, but maybe do good research and get someone other than the one who installed that short grounding rod, if you lack confidence in them. If these efforts are fruitless, and you’re feeling like this is getting beyond the competency of any of these groups (we’re all human, after all), then you may actually want to have a qualified forensic electrical engineer do further investigation and offer an opinion to make sure no hazardous conditions exist. We wish you the best with this, and please let us know what you eventually figure out!
Great information, I have a different configuration which I need help figuring out what is wrong and how to fix it.
I have a 400 AMP service line which demarks in a large service shutoff cabinet. In this service cabinet, There are 2 separate grounds, one going to the water pipe and a second going to a rod. Both grounds are bonded to the neutral in this cabinet.
From this cabinet, the neutral goes straight through a meter cabinet (sealed by local power company) and goes into a raceway cabinet. from the raceway cabinet, I have 4 load panels. Each one of the load panels is fed directly from the raceway cabinet. Each load panel has a main breaker panel. Right now, none of the load panels is bonded.
Now to the problem I am seeing: The Amperage coming from each of the load panels is going back on the individual neutral feeds from each panel, and the sum of the amperage from each panel neutral is going through the ground to the water pipe.
So I have 2 questions: 1. Should each of the load panels be bonded? 2. Should I remove 1 of the grounds from the service shutoff panel?
You certainly shouldn’t remove one of the grounds. Since they are bonded to neutral inside the panel, they are effectively all part of the same grounding system, not separate. Metal plumbing pipes are actually required to be bonded to the grounding system. Here in Florida, they actually also connect to the rebar inside the structure itself, such as inside the slab, which connects to the rebar inside the block walls. This is also known as an “Ufer” ground. Your having a 400-Amp system isn’t unheard of, but it is not common either. That’s typically found on very large, all electric homes. In some cases, it could even be 3-phase, which presents different circumstances. Your situation has a lot of nuance and regretfully is beyond what we can safely do with a mere reply to a blog post. We would strongly recommend hiring a qualified electrician to take a look if you are having any issues or concerns.