Shore Power Fault - Above my pay grade.....

jason flare · Nov 8, 2025

OK, I’m the friend.
The surveyor’s meter indicated that Hot and Ground were reversed.

Davidasailor26 · Nov 8, 2025

Rich Stidger said:
At the end of the Smart Plug cable when disconnected from the boat:
Hot to Neutral 122.6V
Hot to Ground 69.4V
Neutral to Ground 50.9V

I’m not an expert in 50A adapter wiring, but this sounds off to me. I would expect neutral and ground to be tied together at the source, so hot to neutral and ground should be 120, and neutral to ground should be 0. Maybe something’s gone awry in the adapter?

kloudie1 · Sunday at 3:43 AM

Not an expert either, but I'd suspect that the ground to neutral at that power pedestal has become disconnected.. It is being connected somewhere else down the line and the voltage reading on neutral to ground is a result of a poor connection where that neutral to ground is and the wire resistance between your pedestal and wherever the connection is happening.. The bigger readings at the end of the boat cable are most likely induced voltage from the open ground connection.. It is a pedestal side problem.

DaveJ · Sunday at 5:27 AM

Your answer is in your first voltage reading. Disconnected from the boat, the voltages are incorrect. There is an issue from the power source.
I think the neutral is not bonded correctly.
Cheers

High Current · Sunday at 6:47 AM

kloudie1 said:
Not an expert either, but I'd suspect that the ground to neutral at that power pedestal has become disconnected.. It is being connected somewhere else down the line and the voltage reading on neutral to ground is a result of a poor connection where that neutral to ground is and the wire resistance between your pedestal and wherever the connection is happening.. The bigger readings at the end of the boat cable are most likely induced voltage from the open ground connection.. It is a pedestal side problem.

It’s not the boat.

Ground is disconnected and somewhere there’s enough of a ground fault to power the indicator light, which I suspect is connected between ground and neutral. Note that the “ground fault” leakage could be capacitive and also quite small, depending on how many mA it takes to light the indicator, and therefore isn’t necessarily a concern. A typical multimeter has 10 megohms of resistance so 60V suggests a very small leak indeed (10 megohms)

dLj · Sunday at 10:49 AM

Rich Stidger said:
Thanks to all for your insight. I agree that a neutral to ground bad connection at the marina side is the most likely culprit. However I was hoping that someone could shed some light on the different measurements at the shore power tower, end of the shore cable, and on the boat itself. I would have expected the voltages to be nearly the same everywhere. No power is being drawn, and the only difference is longer wires.

Your open voltage measurement should show almost no difference. Maybe instead of 122.7 you may see like 122.1 or a bit less, something in that order if magnitude.

You will have a minor voltage drop due to the connections. If you see a significant voltage drop, then you have serious connection resistance.

The real voltage drop occurs as you increase loads.

dj

walt · Sunday at 12:42 PM

"So he made some voltage measurements, and the results are as follows:

At the pedestal, boat cable disconnected:
Hot to Neutral 122.7V
Hot to Ground 111.6V
Neutral to Ground 14.3V

At the end of the Smart Plug cable when disconnected from the boat:
Hot to Neutral 122.6V
Hot to Ground 69.4V
Neutral to Ground 50.9V"

--------------
The pedestal with the boat cable disconnected is a little off what you would expect with the neutral to ground of 14.3 VAC. If the wires are not carrying current this voltage would be zero but your line back the breaker box is likely shared with other slip loads.

When you connect an extension cord with no load (second measurement) - ie, no current flowing in the extension cord, the voltage at the end of the smart plug cable (disconnected from the boat) should be exactly the same as the pedestal voltage. No current, no IxR voltage drops.

Yet in this case, just the cable/ cord would seem to have 14.3 VAC N to G at the pedestal but 50.9 VAC N to G at the other end of the cord. This isnt right if there is no current flowing through the cord. Voltages at each end should be the same.

Almost sounds like the extension cord ground wire might have a break somewhere and you are just seeing an open circuit floating voltage on the ground at the other end.

When you connect to the boat, the ground voltage changes again which might indicate you have some sort of ground fault but somewhat hard to tell if the ground wire is just floating.

Just an option to consider..

dLj · Sunday at 3:43 PM

walt said:
When you connect an extension cord with no load (second measurement) - ie, no current flowing in the extension cord, the voltage at the end of the smart plug cable (disconnected from the boat) should be exactly the same as the pedestal voltage. No current, no IxR voltage drops.

If measuring DC current, V = I X R is correct, but for measuring AC current that formula is V = I X Z where Z is the impedance. Impedance and resistance are similar but not exactly the same. In this case, he has a connection between the 50A pedestal and the adaptor, a connection between the adaptor and cable 1, then a connection to cable 2. Each of those connections will affect the impedance - very slightly for sure, depending upon how clean they are. Then you have the multimeter which has an internal impedance, very small, but it exists. So that's 4 connections plus the multimeter.

I was actually just at my boat a few days ago and had 200 feet of cable - that would be 4 - 50' lengths. So I had a connection at the pedestal, a connection from cable 1, from cable 2, from cable 3, from cable 4 and then my multimeter. That's 5 connections and the multimeter. I wish I had written down the voltages, but I wasn't expecting this thread. I clearly measured a small difference at the connection to my boat compared to the voltage at the pedestal. It was not a lot, so I didn't even think to write it down or worry about it...

Bear in mind, in 110 AC the voltage is oscillating between roughly +155 V and -155 V - the multimeter than measures RMS - Root Mean Square - voltage. For the multimeter to measure the RMS voltage, it must consume a tiny amount of current. All of those connections plus the multimeter will create a small but measurable amount of impedance.

dj

walt · Sunday at 4:52 PM

yep... both the pedestal power source and the copper wire in the power cord and the connections should be a huge amount lower resistance or impedance than the meter. With one end of the power cord plugged into the pedestal and the other end is not plugged into anything, the voltage at both ends of the extension cord should be very close to identical.

Lets say you have 120 VAC across hot to neutral.

With the idea of a broken ground connection somewhere in all that extension cord chain, the ground wire at the unloaded end of the cord would just be floating. Ie, not connected to anything. Capacitance from both hot and neutral would cause a voltage on the floating ground of about half way between the two H and N voltages.

You measured this with the extension cord disconnected

Hot to Ground 69.4V
Neutral to Ground 50.9V"

Not exactly half 120 VAC but there is a very high meter resistance which will affect this a little plus that 14 volts between neutral and ground at the pedestal which we cant really explain.

dLj · Sunday at 5:45 PM

Rich Stidger said:
Granted there are connections and each have a small impedance, but still, given that no current is flowing, I still fail to see why the voltages along the path are radically different.

In your described situation there is clearly a problem beginning at the pedestal.

In your original post you stated:

At the pedestal, boat cable disconnected:
Hot to Neutral 122.7V
Hot to Ground 111.6V
Neutral to Ground 14.3V

There is a problem at the pedestal. Hot to neutral and hot to ground should really be the same - 122.7 V in your case. Neutral to ground should be 0 V. Sometimes ground can float a bit, but with those readings - there is a problem at the pedestal.

jason flare said:
OK, I’m the friend.
The surveyor’s meter indicated that Hot and Ground were reversed.

Your friend has stated the above - I'm not understanding where you guys are in this?

dj

Whitespeed · Sunday at 8:03 PM

First time poster here and I hope I'm not thread jacking (I can start a new thread if that would be best), but this thread looks very relevant to an issue a friend is having with her boat.

She recently bought a 1981 Hunter 30 Cherubini to live on year round and I've been helping her with some electrical repairs and upgrades. Now, my education is in electronics, my career has focused on industrial control systems and my primary hobby has involved a lot of automotive wiring. But I don't have a whole lot of experience troubleshooting AC circuits.

This past week, the main 3 pole, 30A breaker on her boat tripped for no apparent reason. I couldn't find any evidence of any kind of short or overload. The breaker could be reset, but would trip even with an LED lamp that only draws maybe 10W. So I concluded that maybe breaker was shot. It looked original (44 years old), and she's been using it more in the past 2 months than it's likely been used in the past 10 years.

Now, the incoming ground wire connects directly to the ground buss. But the original 3 pole breaker had one of the poles connect to neutral on one side and ground on the other. The local marine shop only had a 2 pole, 30A breaker in stock and they suggested just connecting Neutral and Ground together. I was surprised by this, since everything I read indicated that Neutral and Ground should not be connected on a boat (I'm wondering now if they misunderstood me and thought I said the 3rd pole connected ground to the ground buss). So, as an experiment, I installed the new breaker and left Neutral and Ground disconnected from each other. I measured 115 between Live and Neutral and Live and Ground. But with it configured like this, turning on a load such as the LED lamp, resulted in the voltage between Live and Neutral dropping from 115V to ~100V. If it was a higher load, such as a 700W heater, it wouldn't even run and the reverse polarity indicator on the panel would glow.

I then connected Neutral to Ground, and everything works as expected. But I'm at a loss to understand why and all of my internet searches had not shed much light on this, until I found this forum thread. This thread indicates there is some sort of issue on the dock side wiring, and perhaps the original breaker was working properly after all.

Before I point fingers at the marina wiring and tell her to go after them, is there anything else I should test or check for? The marina hasn't been particularly helpful (I believe she called them when she first had issues and they said everything was fine with shore power), so I don't want her to go after them without good reason.

(again, I hope this isn't thread jacking. Apologies if it is)

dLj · Sunday at 9:21 PM

Actually, it just occurred to me, your question may well be related to the original question posed by the OP.

How old is the marina from the original post? The original 110V or 120V systems (however you may wish to call it) were two wire systems. This was changed in 1974 (according to a google search - but that sounds about right). That was roughly 50 years ago. If the electrical system predates that, it would be hard to change all the wiring in buried lines to the current 3 wire system.

Whitespeed said:
Now, the original 3 pole breaker had one of the poles connecting neutral and ground. The local marine shop only had a 2 pole, 30A breaker in stock and they suggested just connecting Neutral and Ground together. I was surprised by this, since everything I read indicated that Neutral and Ground should not be connected on a boat. So, as an experiment, I installed the new breaker and left Neutral and Ground disconnected. I measured 115 between Live and Neutral and Live and Ground. But with it configured like this, turning on a load such as the LED lamp, resulted in the voltage between Live and Neutral dropping from 115V to ~100V. If it was a higher load, such as a 700W heater, it wouldn't even run and the reverse polarity indicator on the panel would glow.

What you are describing above is pretty common - the ground and the neutral are joined at the source. The suggestion to join the neutral and ground together is the way this is done so the advise you got was correct. What I don't understand is how you could leave both ground and neutral disconnected and get anything at all for output... There must have been some sort of connection to one or the other.... Maybe one of the wires was touching the other side somehow?

Whitespeed said:
I then connected Neutral to Ground, and everything works as expected.

That is correct - your original breaker was shot so you've fixed that problem. You did this at the pedestal, correct?

Whitespeed said:
Before I point fingers at the marina wiring and tell her to go after them, is there anything else I should test or check for?

Everything is now working, correct? There is nothing to go after the marina for other than possibly getting a new breaker from them, I guess... If the breaker is in their pedestal, why aren't they getting you a breaker?

Above @walt said "With the idea of a broken ground connection somewhere in all that extension cord chain, the ground wire at the unloaded end of the cord would just be floating. Ie, not connected to anything. Capacitance from both hot and neutral would cause a voltage on the floating ground of about half way between the two H and N voltages."

While this may be true - the readings at the pedestal are still wrong meaning there is a problem there. If there is a problem in the extension cords, that's pretty easy to check, you can simply measure the resistance for each leg - should be 0 or very close. Then measure the resistance across each leg of each wire - that should definitely be infinity (I don't see how to put the symbol here) on all crosses. Now you may be right and one of the cables is bad, or a bit bad and that could account for the different readings seen.

dj

Whitespeed · Monday at 4:44 AM

Thanks for the quick replies!

dLj said:
How old is the marina from the original post? The original 110V or 120V systems (however you may wish to call it) were two wire systems. This was changed in 1974 (according to a google search - but that sounds about right). That was roughly 50 years ago. If the electrical system predates that, it would be hard to change all the wiring in buried lines to the current 3 wire system.

The marina dates back to early 70's. There have been some upgrades over the years. The pedestal she was originally connected to when this issue first occurred is supposedly 'new' (something about a fire at the original pedestal before she moved to this marina). But she also moved down the dock to a new location and pedestal this past week, and the same issue is occurring there.

dLj said:
What you are describing above is pretty common - the ground and the neutral are joined at the source. The suggestion to join the neutral and ground together is the way this is done so the advise you got was correct. What I don't understand is how you could leave both ground and neutral disconnected and get anything at all for output... There must have been some sort of connection to one or the other.... Maybe one of the wires was touching the other side somehow?

I realize now the way I worded 'disconnected' is somewhat misleading. What I meant was neutral and ground were disconnected from each other. Ground is connected directly to its buss bar and neutral is connected to a breaker. I've edited the post accordingly and it may be helpful to reread now. Sorry for the confusion.

dLj said:
That is correct - your original breaker was shot so you've fixed that problem. You did this at the pedestal, correct?

No, this was done in the boat.

dLj said:
Everything is now working, correct? There is nothing to go after the marina for other than possibly getting a new breaker from them, I guess... If the breaker is in their pedestal, why aren't they getting you a breaker?

It's working with ground and neutral connected to each other in her boat. I didn't touch the pedestal. I had been assuming everything was good on the dock end, but now I suspect that is not the case.

Rich Stidger said:
From your description I think that the 3-pole boat breaker switched hot, neutral, and ground, and on the output side of this breaker there was a jumper that connected neutral to ground.When you replaced that 3-pole breaker with a 2-pole breaker, it now switched the hot and neutral but not the ground. You connected the incoming ground wire to the ground wire that was the output from the original breaker.

Not quite. The incoming ground wire has always been connected directly to the ground buss in the boat panel. The 3rd pole on the breaker was connected to neutral on one side and ground on the other. Unfortunately, the label from the 3 pole breaker was long gone, but I suspect the 3rd pole was not just another 30A breaker. It may have had a much lower trip point to detect a bad neutral or reverse polarity.

Rich Stidger said:
Now at no load you measure 115V between hot and neutral and between hot and ground. Sounds good so far.
But when you apply a small load on the boat, the hot to neutral voltage drops to 100V. I think this voltage drop, which increases with bigger loads, is indicative of a high resistance in the neutral wire. Then when you connected the neutral to the ground in the boat, the loads powered up and the voltage returned to 115V.
This occurred because the ground wire is now carrying the return current from the loads instead of the neutral wire carrying the return current.

That sounds plausible and a high impedance in the neutral is what I suspect as well. The question is where.

Rich Stidger said:
The way to determine if the problem is on the boat side is to disconnect from the marina pedestal and using an ohmmeter measure the resistance between the neutral pin of the shore cable end and the neutral position on one of the boat's duplex outlets. While you are at it, I would measure the resistance of the hot and ground circuits as well.

That's very helpful and something I can probably have her do. She's deathly afraid of electricity (rightly so), so there aren't many tests I can have her do. Also, the boat is almost an hour away from me, so it's not easy for me to just pop over.

Nigel

artboas · Monday at 5:05 AM

Rich Stidger said:
@Whitespeed First, welcome aboard.
From your description I think that the 3-pole boat breaker switched hot, neutral, and ground, and on the output side of this breaker there was a jumper that connected neutral to ground.
When you replaced that 3-pole breaker with a 2-pole breaker, it now switched the hot and neutral but not the ground. You connected the incoming ground wire to the ground wire that was the output from the original breaker.
Now at no load you measure 115V between hot and neutral and between hot and ground. Sounds good so far.
But when you apply a small load on the boat, the hot to neutral voltage drops to 100V. I think this voltage drop, which increases with bigger loads, is indicative of a high resistance in the neutral wire. Then when you connected the neutral to the ground in the boat, the loads powered up and the voltage returned to 115V.
This occurred because the ground wire is now carrying the return current from the loads instead of the neutral wire carrying the return current.
If everything I have described is accurate, the fault that you have is a high resistance in the neutral wire. That high resistance could be on the boat, in the power cords, or in the marina side.
The way to determine if the problem is on the boat side is to disconnect from the marina pedestal and using an ohmmeter measure the resistance between the neutral pin of the shore cable end and the neutral position on one of the boat's duplex outlets. While you are at it, I would measure the resistance of the hot and ground circuits as well.
As for why the original breaker began tripping- maybe it just failed from old age.

is
I’ve been reading this thread, and my tongue is bleeding because I’ve been biting it so hard.

So, I’ll now lift my cone of silence

Firstly, the ground and neutral are not bonded at the pedestal. This is an incorrect assumption.
The ground and neutral are ONLY bonded at a point of voltage generation such as an inverter output, or on shore at the distribution transformer or main panel.
If the bonding is done correctly, then there is zero volts potential between neutral and ground.
So, you go back to the dock outlet. If the PD between N & G is anything but zero, you can stop there.
Meters have a high input impedance. When you measure the dock voltage with no load, then the PD between N & G must be zero.
Not to get “fancy.”, but there is virtually no reactive current (Impedance) at 60 Hz.
Reactive current is the inverse function of capacitance & frequency, or non-inverted to measure inductive current.
At 60 Hz, the imaginary (non-real) current is BA. (Bugger all)

The electrician who opined that there was a hot to ground reversal was not someone I would hire again.

My interpretation is that the N G bonding at source isn’t a clean connection. The other possibility, (although unlikely) is a floating ground at the pedestal.
If this was the case, you’d be reading wonky N to G voltages, but only if there was no load. This is because of the capacitance in the shore power cords. Once again , the high input impedance of the DVM would definitely show a PD in this scenario.

This all starts at the dock outlet, and go linearly from there.

walt · Monday at 5:13 AM

(two different discussions here - this is for what whitespeed brought up)
I think Rich's idea of the high impedance neutral is a good place to start looking at.

Just another idea for debug... Recreate the conditions where you see the 100V (I guess at the breaker on the boat) with the ground and neutral disconnected and the 10W load. Then see if you can measure those voltage with extension cord connected up starting at the pedestal. If you see the 100 VAC at the pedestal between H/N, the marina has a problem. But if that pedestal under that load is 115VAC, the problem is "down stream" and you could continue to measure the voltages in the "chain" to isolate where the problem is or possibly check for continuity.

FYI, we can assume that the marina pedestal does not have gfci or elci because you would never have been able to connect ground and neutral at the boat without the gfci tripping.

If the boat is in fresh water. best to get this fixed (ie ground and neutral should not be directly connected at the boat) as its possibly a safety hazard to someone in the water near the boat.