Running heat pump compressor 120vac generates 3 volts on the neutral to ground wire when measured?

Thread Starter

sdowney717

Joined Jul 18, 2012
705
I notice a small AC voltage when I measure between neutral and ground on my AC system in the boat, but only when the heat pump AC compressor is running. There is no 3 volts measured when just the AC water pump for the AC heat exchanger is running, only appears when compressor is running.
I have that Heat Pump circuit connected to a square D AFCI GFCI combo breaker, and it does NOT trip off... So is that normal? I know compressor motors generate a back EMF. The fact it does not trip a GFCI-AFCI breaker makes it seem legit. What do you think?

That small 3 volts appears across the entire AC system on the boat, such as on the outlets between neutral and ground when compressor is running.
When compressor is off and just the AC heat exchanger water pump runs, there is no 3 volts.

So I though it odd, and am wondering if other people can comment on this.
Boat is plugged into marina power at the dock. Marina has no ELCI breaker protection. Just those big 30 amp 120 VAC circular plugs.
AC is cooling as expected.

Compressor is your typical Start, Run capacitor design with an associated relay. And uses 120 vac power and draws 13 amps.
 
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BobTPH

Joined Jun 5, 2013
3,650
That is expected behavior. The neutral is carrying current, whereas the ground is not, so there is a voltage drop due to the resistance of the neutral wire.

Edited: not sure if the magnitude of the drop is as expected though.

Bob
 

LowQCab

Joined Nov 6, 2012
1,078
It's really extremely simple, your wiring is too small for the Load Current it is required to carry.
It's called Voltage-Drop, and it is totally normal.
Although, 3-Volts is probably too high, less than 1-Volt is more in the acceptable range.

Another angle on this is that,
You have 3-Volts of Voltage-Drop on the Neutral,
that means that You also have a 3-Volt loss on the Hot-Wire,
for a total of ~6-Volts,
6-Volts times ~13-Amps is 78-Watts of power being used just to heat-up your Wiring.

Over a ~100-foot long run, a Voltage-Drop this high might be expected and acceptable,
but your AC-Unit will be happier with heavier-Gauge Wire.
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Thread Starter

sdowney717

Joined Jul 18, 2012
705
The wire is a 12 gauge fine multistrand wire rated for 20 amps, has green ground. white neutral, black hot, typical rubber coated black exterior SO wire. Wire runs about 15 feet from the distribution box to the compressor. I replaced this wire maybe 5 years ago when replacing some wires. Previous owner had used solid copper Romex. The AC heat pump according to my digital meter draws 13 amps, so that is within the wire rating?

My LED AC digital meter does sag the volts from 119 to maybe 113 when running the compressor. I figured that is due to the marina wiring. I have your standard 10 gauge yellow shore cord, and I run 8 gauge wire from the shore plug sockets to the AC distribution panel.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
705
What I can do is verify the wire to start relay connections inside the heat pump, make sure there is no high resistances on the screw lugs.
And press the TEST button on the combo breaker with compressor on and see if it works.

The start relay uses screws, and capacitors are all the push terminals.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
705
It looks like your Wiring is fine.
It's that ~300-foot run from the Marina's Breaker-Panel to your Boat.
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Maybe so. Marina wiring is always somewhat sketchy from my experience, and I have had the boat at several marinas over the years, but none of them were upscale resort style ones.
How about this, I start the boat generator up, run the compressor and see if there is any voltage between neutral and ground?
Gen is 6500 watts. I have noticed the AC seems to work a little better on the gen than on shore power.
 

MisterBill2

Joined Jan 23, 2018
9,225
Definitely the first step is to tighten all of the screw connections. A loose connection can be a big problem. And of course there will be a voltage drop in any current carrying conductor.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
705
Update, I started boat generator, switched power over to gen, ran the AC, then measured for voltage between ground and neutral and there is ZERO, there is none. So my wiring must be ok, and marina wiring is too small for the run. I cant complain as the power is free, unmetered for us slip holders.

I dont get how marina wiring gets approved by inspectors when it is so obviously substandard just about everywhere I have been. That has been my experience, it is only borderline acceptable all over. Our marina also has no ELCI protection for ground faults. At 5 marinas in last 10 years, I have yet to see ELCI protection breakers installed. I bet though, a marina forced to comply with the latest and greatest standards would raise prices, so I really dont want them to do anything. I have enough GFCI-AFCI combo breakers on board to protect my boat circuits. My current marina was originally built in the late 1940's. A storm destroyed the place around 2000, so the entire marina pilings, wood and electrical was all replaced with new stuff. I heard boats floated away tied together like rafts down the river, but I was not there then. I left before the destruction happened.
 
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MisterBill2

Joined Jan 23, 2018
9,225
The wiring might be OK, but certainly it must also be very long. And with length comes resistance and the voltage drop that you are seeing. If they upgrade probably there will be meters and fees and thus I recommend not even mentioning it. A slightly lower voltage is not a big problem.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
705
The wiring might be OK, but certainly it must also be very long. And with length comes resistance and the voltage drop that you are seeing. If they upgrade probably there will be meters and fees and thus I recommend not even mentioning it. A slightly lower voltage is not a big problem.
Yes, no plans to say anything. I have no ELCI on the boat. But my AC panel has GFCI-AFCI combo breakers, it is a Square-D distribution panel. I have made certain my boats AC system has no leaks. I can plug the boat into a GFCI extension cord and it does not trip off the power, can't say that about many other boats. Some marinas have a bad boat plug in and it will trip off an entire dock's power.
What You Need to Know About ELCI-Protected Shore Power - Waggoner (waggonerguide.com)
 

Reloadron

Joined Jan 15, 2015
6,058
Boats, marinas and shore power can be a strange animal. Here is one I remember quite well not far from me.
Electrocuted at Put-in-Bay
As far as I know no conclusion or logical explanation was ever found. The boat power distribution all was gone over and checked to be fine. The marina power and distribution was also checked and found to be fine. Yet a 19 year old young man ended up dead and others including a family dog received a shock in the water immediately around the boat.

You could likely buy or fabricate a basic test circuit to test shore power before transferring boat to shore. A very long time ago I had a boat in North Carolina US. Since the water never freezes the boat remained in the slip year round. During winters we used a small electric heater to maintain a dry cabin but the slip power was limited by a 5 amp breaker really limiting what we could or could not run. Here in the US I would think the Coast Guard would monitor and test shore power safety but I guess not.

Ron
 
In some respects, I think you really need to create your own ground/neutral bond. You can do that with an isolation transformer, but it;s not being used to isolated. You would create a neutral/ground bond on the secondary side and the ground to shore power. if you had a boat ground plane, that would help too.

Look at some of the youtube videos from www.powervar.com They used to be called OneAC and their stuff shows up on ebay;

BTW. I used a OneAC power conditioner with a Triplite surge supressor on a Mac Centris 650 and the computer had only mechanical and dust issues. The SCSI hard drive was still OK after 17 years in use. It was used on the before and after replacement systems.

It protected some very expensive instrumentation. Their transformers are a little batter than just satndard isolation transformers.

I think the ground/Neutral bond for shore power is way to far away.
 

MisterBill2

Joined Jan 23, 2018
9,225
It is the voltage drop in the white wire that is giving you that 3 volt difference. Post #2 explains it quite well.It is not leakage or a short circuit and not a shock hazard.
 
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