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gkesden 05-13-2019 12:26 AM

Mystery wiring
I've had my present boat, a 1981 trawler, for about 5-6 weekends and a very small few whole weeks and am still going through everything and ran into a mystery.

An exterior 120VAC outlet wasn't working and I traced what seemed to be the problem back to the black wire being disconnected at a wiring block in the engine room that was labeled 120VAC.

I figured water was probably leaking into the outlet so a PO disconnected it. I took the opportunity to upgrade it and the other outside outlet to GFCI with new enclosures. But, this one outlet wouldn't reset.

I went to the engine room and put a meter on the terminal block wires. That's when things got weird. Green, yellow, black 12 AWG wires.

-- 16.6V **AC** green to yellow
-- 1.5V **AC** yellow to black
-- 14.64 **AC** green to black

Also observed:
-- No meaningful DC component measured.
-- Turning off one of the 120VAC outlet breakers makes the voltage go away
-- No other breakers make a difference
-- The other outlets on that breaker work fine.
-- I called the PO and the PPO representing 15+ years of ownership. They know nothing about it and think it is really strange.

Not yet measured/observed
-- It seems that there could be at least one thing on that circuit I don't know about, so there could be more.
-- Behavior under load
-- Waveform
-- Current on circuit with nothing plugged into outlets.

This trip is actually a carpetting trip, so I don't have a current clamp for my meter or my oscilloscope on me. I'll bring those next time. So, for now I just wired the outlet to known-good power and it works fine.

I obviously need to chase that wire next trip. Until then, anyone have any ideas what on a boat,might show AC voltages like that? I was thinking maybe something like a doorbell transformer, but those are, I think, 24VAC.


tiltrider1 05-13-2019 12:38 AM

You need to trace the wires further. Don’t assume it’s a 120v supply just because it’s labled. Keep going until you know the true source of the wires.

gkesden 05-13-2019 12:54 AM

Hey Tiltrider1,


I'm not assuming that it is a 120VAC supply -- just observing that whatever is producing that voltage is controlled by a specific 120VAC breaker, specifically the one that has most of the outlets and nothing that I can identify other than outlets.

My goal is to trace the wire -- and, if appropriate, eliminate it and whatever abandoned device it is attached to.

My goal in trying to understand what it might be attached to is to possibly give me some sense as to where to look to find the other end so I can try ringing it from there or, alternately, some idea that might trigger the memory of either the PO or the PPO to help me do the same.

Basically, I'd like to be able to ask them, "Hey, did you ever have an X? Where was it?" And then be able to look there without needing to take apart everything from the engine room to there.

In talking with the PPO, we kicked around the idea that it could come off the solar system, or an old thermostat, or an old doorbell, or an old 1980s era portable TV power supply. None of those really matched, but were the right kind of ideas, I think. I'm not adverse to trying to chase down the PPPO, but I don't know him, or of him, or even if he is still around.

I'll chase this down one way or another -- it is just that, since I can't get it to ring, and it goes off into never-never land, it might mean a lot of disassembly.

Thanks again!


tiltrider1 05-13-2019 01:32 AM

Sounds like something else is in the circuit. Sometimes these things are real head scratchers.

gkesden 05-13-2019 01:53 AM

Hi Tiltrider1,

Thanks so much!

Did that. I know which breaker controls it. It is on the "main outlet" breaker, i.e. the breaker that has most of the 120VAC outlets in the boat and nothing that I can identify other than outlets.

So, somehow, something tied to that breaker is putting this weird voltage onto that set of wires. I don't know if it is converting the 120VAC into the weird voltage or if it is producing or conducting it from some other source, but needs 120VAC to do that.

Another idea the PPO had, which I asked the PO about without any luck, was if the boat ever had any type of low-voltage AC lighting, e.g. landscape-type lighting.

Thanks again!


kchace 05-13-2019 11:49 AM

Wires that are not connected to anything but run closely parallel to lines that have 120V AC can sometimes show voltage levels like you're seeing when measured with a typical meter.


Pcpete 05-13-2019 12:30 PM

Do you have an inverter that’s also on that 120v breaker? The wire colors are a bit off too. AC should be black or red for load, white for neutral and green for ground and be stranded wire. No solid wire on a boat. DC in newer boat have different colors and the ground color has a new standard being yellow for ground, black in older boats. Red is still positive, green is still negative. Still all stranded wire.
On the surface it sounds like you have the incorrect wires going to that breaker and plug.
I trace 120v with a Klein sensor. I just have to get the tip near an energized circuit for it to read. For 12v and circuits that are not energized I use a Fluke sound wire tracing tool. It works by running a light current through the wires and following the sound. It’s more sensitive than the 120v but do not use the 12 on energized 120. Yes, both sensors work through walls.

gkesden 05-15-2019 10:22 PM

Hi Ken. Hi PcPete,

Thanks so much for your help!

Ken: You nailed it! I ended up licking my finger, putting the wires onto it (best high resistence load I had "on hand") and watching the voltage disappear.

After seeing your post and thinking about it, it made a lot of sense. I just didnt expect a voltage that high across a cheap meter. I figured its internal resistence would be too low for that and the chase too short for that. If it would have been just a few volts, I'd have thought differently -- but I thought wrong!

I eventually chased it back to the vestigial remains of a long ago abandoned 2nd AC panel.

I still dont know why my tracer wouldnt ring it. But, my guess is a low battery on the transmitter. I'll bring spares next trip.

Thanks again!

gsholz 05-15-2019 10:51 PM

Sometimes you just see "ghost" voltages when you measure with a high-impedance multi-meter. Some meters have a "low Z" low impedance mode that will eliminate these ghost voltages.

gkesden 05-18-2019 05:19 PM

Hey gsholz,

Thanks so much!

That was it, exactly.

I didn't think my meter was high enough impedance for me to see ghost voltages in the teens. I would have believed on or two or a small few volts.

But, I was wrong. When I finally chased it down, the other ends dead ended in an area that long, long ago had an electrical panel.

I suspect Ken got it right in suggesting that the root cause was induction from parallel AC wiring. There was a good bundle along with it for much of the ride.

kchace 05-18-2019 09:21 PM

Yeah I've personally seen it before. It's the sort of thing I would never have believed otherwise.

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