Floating boat dock STRAY VOLTAGE

T

True.Electric

Member
Location
South Carolina
Occupation
Electrical Contractor
In my area, we have a lot of lakes with floating docks, and it's common practice to wire these docks with basic power for lighting, receptacles, and similar uses. We always bond the system according to NEC Article 555, running the bond back to the panel where power originates, and we also drive an 8-foot ground rod on shore next to the dock. The issue we’re encountering during the summer is stray voltage appearing on the dock—typically anywhere from 0 to 9 volts—carried through the equipment ground and bonding conductors. Anything above 2 volts can be felt when swimming and touching metal components of the dock, which raises serious safety concerns. I’m not an electrical engineer, but my best guess is that this voltage increases with the load on the power grid, often correlating with hotter weather. The best solution we’ve found so far is using a Victron isolation transformer, originally designed for marine applications. We install it at the house to feed the line going through the yard to the shore power panel and then out to the dock. We wire the transformer in a "floating" configuration, bonding ground and neutral (as if it were a boat floating in the water). The other option is and leaving them unbonded, intended for when the boat is trailered. I'm reaching out to see how others feel about this approach. I came up with it myself but haven’t found much documentation or examples online. While I've had dozens of people ask me to do the same for their docks, I don’t want to proceed further until I'm absolutely confident in the safety. Has anyone else run into this issue? Does anyone have a better solution, or a preferred isolation transformer they trust for this application?
 
There is no code complaint solution. In general is it not safe to provide power to docks, if people are going to be in the water, as you can't bond the water to prevent this issue as you can with a pool.

555.10 acknowledges this issue by requiring the a sign with the following wording at the dock.
“WARNING — POTENTIAL SHOCK HAZARD — ELECTRICAL CURRENTS MAY BE PRESENT IN THE WATER.”
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Often this voltage is from Neutral to Earth voltage (NEV) and is the result of the voltage drop on the utility primary and secondary neutrals. One solution maybe to get the utility to install an neutral isolator on the neutral on the line side of the service point, however it is often difficult to get a utility to do that.
 
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True.Electric said:
Meaning you dont consider the isolation transformer to be code compliant?
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There is no code compliant way to install one, as there will be a path via the primary EGC back to the main bonding jumper, and the required secondary system bonding jumper and/or supply side bonding jumper will provide a connection to the primary EGC.
 
True.Electric said:
In my area, we have a lot of lakes with floating docks, and it's common practice to wire these docks with basic power for lighting, receptacles, and similar uses. We always bond the system according to NEC Article 555,
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Article 555 has had a lot of rearranging but basically you install a Ground Fault Protection of Equipment (GFPE) breaker on the feeder to the dock, in the 2017 its 555.3 and 2020 and later I think its 555.35
True.Electric said:
running the bond back to the panel where power originates,
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Is that a insulated EGC with the feeder?
True.Electric said:
and we also drive an 8-foot ground rod on shore next to the dock.
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What is that bonded to?
True.Electric said:
The issue we’re encountering during the summer is stray voltage appearing on the dock—typically anywhere from 0 to 9 volts—carried through the equipment ground and bonding conductors.
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Is the voltage present with the feeder switched off?
True.Electric said:
The best solution we’ve found so far is using a Victron isolation transformer, originally designed for marine applications. We install it at the house to feed the line going through the yard to the shore power panel and then out to the dock. We wire the transformer in a "floating" configuration,
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Thats a interesting take, your creating a IN-S system in IEC terms, are these docks permanent structures powered year round?
Boat's typically use Residual Current Devices (RCD's) which are a faster acting GFPE breakers.
Do you install a GFCI or RCD as the secondary disconnect for the transformer like they show?
1754271002720.png

 
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tortuga said:
Article 555 has had a lot of rearranging but basically you install a GFPE breaker on the feeder to the dock, in the 2017 its 555.3 and 2020 and later I think its 555.35
Click to expand...
Many of the problems are related to elevated Neutral to Earth Voltage, and nothing in 555 addresses that issue.
 
tortuga said:
Article 555 has had a lot of rearranging but basically you install a Ground Fault Protection of Equipment (GFPE) breaker on the feeder to the dock, in the 2017 its 555.3 and 2020 and later I think its 555.35

Is that a insulated EGC with the feeder?

What is that bonded to?

Is the voltage present with the feeder switched off?

Thats a interesting take, your creating a IN-S system in IEC terms, are these docks permanent structures powered year round?
Boat's typically use Residual Current Devices (RCD's) which are a faster acting GFPE breakers.
Do you install a GFCI or RCD as the secondary disconnect for the transformer like they show?
View attachment 2578933

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Yes, GFCI protection is installed.
 
I believe the code section you run into with a IN-S (or Impedance Grounded) dock system as done on a boat is 250.20(B) and 250.96.
True.Electric said:
Yes, GFCI protection is installed.
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And that GFCI breaker is the first disconnect at the transformer not a GFCI receptacle down the line?
 
tortuga said:
I believe the code section you run into with a IN-S (or Impedance Grounded) dock system as done on a boat is 250.20(B) and 250.96.

And that GFCI breaker is the first disconnect at the transformer not a GFCI receptacle down the line?
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Transformer feeds a subpanel on shore, subpanel then feeds the dock. GFCI protection comes from a breaker in the subpanel. Also, the dock is bonded with solid #8 that comes back to the ground rod, then the subpanel.

My transformer in on land NOT on the dock or boat as shown in the diagram you posted.
 
True.Electric said:
Transformer feeds a subpanel on shore, subpanel then feeds the dock. GFCI protection comes from a breaker in the subpanel. Also, the dock is bonded with solid #8 that comes back to the ground rod, then the subpanel.

My transformer in on land NOT on the dock or boat as shown in the diagram you posted.
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Though I haven't dealt with much of the code compliance side of this, but have wired a couple of these, I might first suggest pulling the EGC bond between primary and secondary side of transformer so all GECs on secondary side are isolated, just to see what voltage does.

I don't recall the exact build details but I think one was all Aluminum, other was steel/wood, both with the foam blocks and we dropped the steel pipes at the corners. I am real sure what I did would never pass code. What I prescribed was I think two of the steel poles got an ECG, the dock itself, and all that connected back into the circuit ECG, and GFI recep on the shore. I was certain some stray current was going to trip that GFI but never did. The other concern I had was people running the air pumps for air lifts, and many of those don't play well with GFI.

Basically I built a truly isolated electrical system. I could see some potential concerns but went with what I felt at the time was the most safe.
 
fastline said:
I was certain some stray current was going to trip that GFI but never did. The other concern I had was people running the air pumps for air lifts, and many of those don't play well with GFI.
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Keep in mind that there are 2 ways an electrical installation can present a shock hazard at the dock.

You can have current leaking from a fault in the electrical installation itself. This current leakage (if great enough) will trip a GFI or other residual current device. This is the sort of fault where the stray voltage goes away when you shut off power to the dock.

Or you can have current leaking from the bonded EGC itself. This won't trip a GFI, because the sensor is only looking at the circuit conductors (hot and neutral), not at the EGC. This current can come from the utility 'multi earth neutral' wire, or it can come from a 120V fault elsewhere on the property (or at a neighbor's property). If you shut off power to the dock, the stray voltage will remain.

About the only code compliant way to deal with this is to not run electrical to the dock. The isolation approach, where the grounded conductor connection was broken at a transformer, is probably the safest way to run power to a dock, but I don't believe it is code compliant. There is probably a failure mode that we (as a group) haven't considered. (For example, what happens to the system if there is a nearby lighting ground strike, when you have two _separate_ grounding electrode systems connected to a single transformer.)

IMHO this is the sort of approach that should be carefully evaluated (eg. a new UL requirement) and then allowed by code, but I'm not footing the bill :)

There are stray voltage sensor/alarm systems that are supposed to detect the voltage in the water before people go in to swim.
 
fastline said:
Though I haven't dealt with much of the code compliance side of this, but have wired a couple of these, I might first suggest pulling the EGC bond between primary and secondary side of transformer so all GECs on secondary side are isolated, just to see what voltage does.

I don't recall the exact build details but I think one was all Aluminum, other was steel/wood, both with the foam blocks and we dropped the steel pipes at the corners. I am real sure what I did would never pass code. What I prescribed was I think two of the steel poles got an ECG, the dock itself, and all that connected back into the circuit ECG, and GFI recep on the shore. I was certain some stray current was going to trip that GFI but never did. The other concern I had was people running the air pumps for air lifts, and many of those don't play well with GFI.

Basically I built a truly isolated electrical system. I could see some potential concerns but went with what I felt at the time was the most safe.
Click to expand...
Yes, we have tested with the primary and secondary ECG separated. This is actually the manufacturers direction if this were a "floating boat". When isolated we saw our voltage go from 6.8v to .017v. GFCI protection was still in place and tripped with a tester.
 
how does one feel about the liability that can happe, seems like regardless if you do it safe- an issue could occur and you know have to defend your self. you may legally be okay but that is after your attorney fees—-

If it’s a resi doc you profit nothing for the hassle. maybe a large marina were you can make some money to outway the risk.

you can get sued leaving a wire exposed in a wall—— but the odds of a death are a lot smaller and more easily overlocked than a drowning—-

I passed on three jobs cause I told them I won’t run power unless we do a battery system on the dock with solar—-
Nope they want that endless power out there…
Plus water to cold around here to swim lol
 
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winnie said:
Or you can have current leaking from the bonded EGC itself. This won't trip a GFI, because the sensor is only looking at the circuit conductors (hot and neutral), not at the EGC. This current can come from the utility 'multi earth neutral' wire, or it can come from a 120V fault elsewhere on the property (or at a neighbor's property). If you shut off power to the dock, the stray voltage will remain.
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I call this what it is 'electrical trespass' utilities need easements to use your property and thats not one of the legal uses (unless it is in the easement) attorneys call this "inverse condemnation", when you get a group of land owners like dairy farmers the damages the utilities have to pay are usually so large that you'll have lawyers crawling all over each other to represent the owners.
If a oil pipeline leaked oil across your property everyone can plainly see the mess, same if a large gas or sewage pipe line was leaking across your property (hopefully you'd smell it or notice), just because electricity is invisible does not give them the right to leak across your property.
For the OP it may be more complicated since it is apparently:
True.Electric said:
the local power company(they own the lake)
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So there is probably a reason the POCO owns the lake and whatever that is may be elevating the lake voltage.
I'd install a NEC complaint system (250.20) as @AC\DC recommended, and have the land owners contact the utility and see if they will do some investigation on their end and possibly remove the source of the stray voltage if that goes nowhere contact an attorney that specializes in "inverse condemnation"
 
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winnie said:
Keep in mind that there are 2 ways an electrical installation can present a shock hazard at the dock.

You can have current leaking from a fault in the electrical installation itself. This current leakage (if great enough) will trip a GFI or other residual current device. This is the sort of fault where the stray voltage goes away when you shut off power to the dock.

Or you can have current leaking from the bonded EGC itself. This won't trip a GFI, because the sensor is only looking at the circuit conductors (hot and neutral), not at the EGC. This current can come from the utility 'multi earth neutral' wire, or it can come from a 120V fault elsewhere on the property (or at a neighbor's property). If you shut off power to the dock, the stray voltage will remain.

About the only code compliant way to deal with this is to not run electrical to the dock. The isolation approach, where the grounded conductor connection was broken at a transformer, is probably the safest way to run power to a dock, but I don't believe it is code compliant. There is probably a failure mode that we (as a group) haven't considered. (For example, what happens to the system if there is a nearby lighting ground strike, when you have two _separate_ grounding electrode systems connected to a single transformer.)

IMHO this is the sort of approach that should be carefully evaluated (eg. a new UL requirement) and then allowed by code, but I'm not footing the bill :)

There are stray voltage sensor/alarm systems that are supposed to detect the voltage in the water before people go in to swim.
Click to expand...
Yeah, like I said, I could see some 'potential' risks, but I felt my method had good merit to be the right decision. I should point out a few things. These were rich that would not take no for an answer so I simply verbally designed it for them and 'they' built the system. I rinsed my hands of it. They are friends of friends sort of deal, but I know that can all get rinsed the second someone is injured.

I realize some here are saying 'there is no safe way'. yet here we all are! Why? Because people won't take no for an answer. Again, I didn't reference the bible, I just designed in a prudent path.
 
Could’ve just said no and walk from the job, sounds like the gun they were holding to your head was full of dollar bills

Not knocking you, I’d almost probably have them shoot me five or six times if that was the case
 
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Well there is a safe way its done all the time,
I was just checking out a large pier or 'board walk' out East right on the Atlantic ocean.
It has a several primary services right onto the pier. There is a popular swimming area that has probably thousands of swimmers at a time that extends under the pier.
The service(s) power amusement rides on the pier. There are several transformers on the pier.
The local POCO takes the issue very very seriously and takes several detailed measures to make sure they dont electrify the water.
There is a whole mess of things they can do; lower primary voltage, protective relays, delta distribution with L-L primary connections etc etc, there just is nothing a EC can do other than detect it, because its not our problem.
fastline said:
These were rich that would not take no for an answer
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These types of people with deep pockets probably have good attorneys that can settle the matter of electrical trespass or tie the POCO up in court so long that they give in and just fix it, I have seen it with even a small residential pool where they switched out a 7200V L-N transformer to a two bushing 12.5kV L-L one, easy as that the voltage trespass was gone.
 
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