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tmiller1116 02-13-2014 04:55 PM

power voltage drops
I imagine that somehow this has been covered before... but here goes... I am not an electrician.. I understand enough to get by...

When I am hooked to 30amp120v shore, I can put a multi-meter on my power transfer switch connections and see 115vac. Start some item on the boat like a heater or microwave and it drops to 111vac. Start another thing and it drops to 108vac. plug in a lamp and it drops to 104... (all numbers are rough and vary but you get the idea). I can also plug in a device called a kill-a-watt and watch the same thing happen....

Testing also shows all AC outlets have an open ground.

When running the gennie, this problem does not occur. Even the open ground is gone.

At first I thought it was in the transfer switch, since this is the meeting point of the power sources... so got a new one and the problem still exists... double checked all my grounds.. everything is tight...

(side note: previous owner survey 3 years ago also showed an open ground... I thought it might be the electrical socket on the side of the boat, but he replaced that a few months ago.)

thought it might be the shore power cable, so brought over another one and the problem is still there...

I cannot put a load on the dockside plug in to see if the issue is there and if voltage drops at that point....

so what the heck do I do now?????

This boat has one piece of equipment that I am unfamiliar with and that is a 120/240 step up transformer or some such item.. I understand it was originally installed to run the 240vac oven, but that was removed and propane installed instead.. the transformer is still there and there is still some 240v lines up in the electrical box... the shore power runs through the transformer and up to the transfer switch... it has a ground that seems solid though...

rwidman 02-13-2014 05:32 PM

An open ground is very dangerous. :eek:

My suggestion is to get a qualified marine electrician to go over your entire system ASAP. This is not a good time to try to learn marine electrical systems.

Any other information you get on a web forum is suspect, both from the qualifications of the poster and the fact that he or she may not understand what you are saying or you may not know enough to describe your problem exactly. A qualified person needs to be able to see touch and test your system.

High Wire 02-13-2014 05:51 PM

From your description the voltage drop problem sounds like its on the dock side of the power cord. Theres lots of things that can cause it and the marina won't want you disassembling the the power post.
The missing ground could be a galvanic isolator on your boat (look for a small box with terminals on the boat side of the green wire inboard of the boats shore power connection) or a bad ground path between the boat end of the cable and the dock post. Easy to check with a volt meter at the boat end of the cord:
Hot to neutral should read ~120vac
Hot to ground should read ~120vac. If it reads zero volts, somewhere the ground connection is interrupted, maybe all the way back at the marina breaker panel.
Testing with a meter is the way to go. If your not sure of what you are doing, get a pro! Getting zapped on a damp dock can be fatal.

Tonic 02-13-2014 10:12 PM


Originally Posted by tmiller1116 (Post 213222)

(side note: previous owner survey 3 years ago also showed an open ground... I thought it might be the electrical socket on the side of the boat, but he replaced that a few months ago.)

It sounds like the PO finally tried to fix the 3 year old survey's open ground (maybe for the sale?) but, whatever he did, didn't solve the problem.
Assuming the dock supply is grounded (see High Wire's post), my guess is that you have a bad connection (or other wire problem) somewhere between the boat socket and the transfer switch. A loose or poor connection on either the hot or neutral would give you those abnormal voltage drops. You could be arcing across a small gap somewhere on those conductors. Because you are not putting any load on the ground, a similar gap on the grounding conductor would just show up as no connection.

Be careful! If you haven't managed to trace it yet, you would probably be better off paying a pro to investigate before it bites you.

FF 02-14-2014 06:09 AM

Most docks have power drops , usually from heavy usage , like a hot day with dozens of air cond cycling on or cold days when everyone heats with marina electric.

First easy trouble shoot would be an 30-15 adapter to plug into the power pole and plug in 1 or two 120V heaters ,.
This will show weather the V drop is from the marina wiring .

OR you could simply grab a slip at a different marina , plug in and measure the voltage drops.

My guess your marina power is crap.

boatpoker 02-14-2014 06:46 AM

The open ground probably goes away when you turn on the genny because the genny bonds neutral/ground automatically when its running.

If you do have an open ground onboard .... do not swim around this boat !
Get a pro in to check this, it could be lethal.

twistedtree 02-14-2014 07:03 AM

You said the shore power runs "through" the transformer? If that's the case, it's also likely the source of the open ground. The secondary side of the transformer is isolated from the shore side, and you will need to re bond the ground and neutral. If this hasn't been done, it will appear to be an open ground, though it's actually a floating neutral.

Go find Charles Induistries and download the installation manual for one of their isolation transformers. They include a number of wiring diagrams showing different arrangements, including the correct way to set up the ground and the ground-neutral bond.

Normally on a shore power connection the ground-neutral bond is provided by the shore power. With an isolation transformer, the shore-side ground ties to an internal safety shield in the xformer. Then the ships ground connects to the xformer's case, and somewhere on the boat you re-establish the ground-neutral bond.

An isolation transformer is the ultimate in safety for shore power, so getting it wired up correctly is much better than taking it out.

Also check to be sure the transformer is rated for the full ships load. If not, it might be what's causing the voltage drop.

Once you have your ground sorted out, then you can track down the voltage drop.

Jbear 02-14-2014 08:44 AM

According to the NEC AC return and ground are bonded at the last disconnect which should be the shore power pedestal at the marina. On your boat, shore power ground and the ac return should not be connected together. When you turn off shore power and use the gen set ground and genset AC return should be bonded at the gen set.

I agree, get a marine electrician to help sort this out.


FF 02-15-2014 06:45 AM

the gen set ground and genset AC return should be bonded at the gen set.

Same for another source , the inverter.

rwidman 02-15-2014 09:18 AM


Originally Posted by FF (Post 213589)
the gen set ground and genset AC return should be bonded at the gen set.

Same for another source , the inverter.

When they are the power source, not when on shorepower.

Jbear 02-15-2014 10:44 AM

FF and WesK,
Agreed. Someone said it earlier, something as important and potentially dangerous needs to be dealt with by a professional IF you do not posses the electrical knowledge yourself. On many issues following the advice on the forums on what/how to fix something is fine- this isn't one of those. Do a little web searching and you will find many horror stories of people that have died on the boat or in the water because of bad grounding, etc.

Just my opinion of course. ��

tmiller1116 02-25-2014 11:17 AM

partial update!

as for the voltage drop issues, we have found out the cause... and it is so simple we should have looked at it first. When the dock was built out however many years ago, they ran the electrical from the main service box to our end tie. The wire in question serves 5 slips.... it is 10 ga wire and runs almost 400 feet!!! Right now we are the only ones drawing any power from this line and if I start more than one item (be it heater, microwave, lamp, etc) that is all she wrote... fire off any more and it drops to almost useless levels.

SO, right now my options are to 1) live with it
2)move to a less desirable location with dual 30A outlets to tie into my 50A outlet and hope that spot has no similar issues
3) change marinas to one I know has true 50A service (newer) but not my preferred choice of locations.
4) go live on the hook....

sorry, Mama doesn't go for option 4 while we are still in the work force.


rwidman 02-25-2014 02:31 PM

Option #5 is to request that the marina take care of the problem. I assume you are paying for shorepower so the marina should be supplying effective shorepower.

psneeld 02-25-2014 08:50 PM

or set yourself up for "cruising"...while tied to the dock...enough batteries and inverters and you can live like everything is normal with the shore power just supplying minimal power for a battery charger. Add solar or wind or both and run a genny a bit if need be (offset the fuel burned by the genny by bashing the marina for letting you plug into a hazardous situation and ask for a price reduction). Most gennies need more running time anyhow...(also ask for a price reduction on genny parts till they fix their dock).

FF 02-26-2014 07:04 AM

The simplest and perhaps cheapest solution that will work every place that has rotten power is ,

Automatic Load Shedding.

salmonchanted 03-02-2014 04:04 PM

Neutral to Ground Connection by Mike Holt for EC&M magazine

For clarification on this topic I encourage all to visit Mike Holts web posting. Important clarification can be gleaned of the sole point for neutral bonding at the Service Disconnecting Means [250.24(B)], and separately derived systems in 250.30(A)(1)] in accordance with 250.142 (The isolation Xfmr discussed above).
"Separately Derived Systems – The neutral-to-case bond for a separately derived system shall not be made at more than one location because doing so results in a parallel path for neutral return current".
This is important stuff and definitly the way to go for isolating your onboard electrical system.

Important generator neutral bonding and isolation from shore power neutral is addressed also here.
Generator – If the grounded (neutral) conductor in a transfer switch is not opened, then the grounded (neutral) from the generator will be solidly connected to the utility’s service grounded (neutral) conductor. Under this condition, the generator is not a separately derived system, and a neutral-to-case bond shall not be made at the generator or at the generator disconnect [250.20(D) FPN 1].

If a neutral-to-case bond is made at both the generator and generator disconnect, then objectionable neutral current will flow through metal raceways and grounding and bonding

The reading can be a bit dry, for those not active participants of the electrical industry please do seek professional guidance relating to this most important safety issue.

To FF, we do use an auto loadshed program predicated on the 27 and 81uf elements to initiate 86 feeder lock-out starting with everyone elses feeders and ending with our own!! yuck-yuck

caltexflanc 03-02-2014 04:41 PM

Most Iso transformers can be tapped and switched to boost voltage. One thing that was attractive and that I was very grateful for on my boat was the PO's installation of Charles Iso Boost transformers. Not an economy solution, but made things very easy, a no brainer vs a manual system which has to be actively monitored and managed.

Marine: Isolation and Boosting Transformers

Charles now also makes an add on auto boost unit for standard 50A transformers.
Marine: Smartboost AC Transformer Booster

bluebyu 03-02-2014 05:55 PM

Boosting a low voltage caused by the voltage drop of the long run will yield lower amps.

caltexflanc 03-02-2014 07:41 PM


Originally Posted by bluebyu (Post 217159)
Boosting a low voltage caused by the voltage drop of the long run will yield lower amps.

Well of course it does. Many motors and compressors and various other items are quite sensitive to volts. As far as amps, you have to manage loads accordingly. Low voltage is much more damaging than not having enough amps to go around.

twistedtree 03-02-2014 07:46 PM


Originally Posted by bluebyu (Post 217159)
Boosting a low voltage caused by the voltage drop of the long run will yield lower amps.

True, but at reduced voltage most loads will draw more current, so one way or another you need to deal with lower available power when the voltage is reduced. A boost transformer just handles the situation with a device built for the purpose rather than stressing everyhing else.

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