Shore Power Concerns


I just found your site, Thank you! There is a lot of useful boat wiring info here, especially for those of us who are not near to nor have access to a marine or ABYC electrician.

Hopefully you can help me with my questions, there are a few.

Dockside AC Pedestal

I have a 1998 Grady White 272 Sailfish which did not come with shore power, only the usual marine electrical components, pumps, refrigerator, wash down, macerator, etc. The boat is currently on a trailer in my driveway until I finish repairs and I am sure it is safe to put back in the water.

After reading Charlie Wing's book and numerous online articles I added shore power. Starting at the transom with...

  • 30 amp stainless outlet, to 30 amp double pole breaker within one foot of outlet
  • Green ground to galvanic isolator then
  • All three marine grade 10gauge wire goes twelve feet to cabin where it
  • Connects to a Blue Seas AC panel #8409, double pole disconnect with amp/volt meters, reverse polarity indicator and three 15 amp circuit breakers.
  • Black to hot, white to neutral, green to ground.
  • 1st circuit to GFCI outlet in head (12 gauge wire)
  • 2nd circuit to GFCI outlet in galley (12 gauge wire)
  • 3rd circuit to Pro Mariner Pronautic 1240P battery charger (14 gauge Wire).
  • Wire from 40 amp charger to batteries 6gauge. with 50 amp fuse each line.

Now for my questions.

  1. I do not have a negative ground bus in the aft compartment which contains three batteries (1 house, 1 each for the outboards), switches, charger, main wiring for the outboards. Each outboard negative. goes directly to it's own battery. Charger instruction's want the charger case stud attached to engine block or DC ground buss with wire no more than one size smaller than charging leads. Since the only negative ground bus is under the cockpit controls (dashboard) and it would be difficult to run this wire to the outboards, can I connect the case ground of the charger directly to one of the battery negative terminals (All battery negatives are connected together). they all end up together anyway don't they?

  2. If I understand correctly, ABYC code wants DC negative connected to AC ground (green). Can I use the battery negative terminal and wire to ship side of the galvanic isolator? Again it seems however these wires are connected they still end up together. If this is acceptable what would be the correct size wire from DC negative to the AC ground?
  3. Here is the strangest question I have for you. It makes no sense and would seem impossible.

    When I take a voltage measurement between the hot wire and neutral, or hot wire and ground at the GFCI, all is correct at 117v, but when I test hot to anywhere on the boat be it a screw which is not touching any metal only fastened into wood or fiberglass, a hose clamp around a hose to toilet, the DC refrigerator frame, or just the fiberglass or gelcoat I get voltage readings.

    With the hot lead of the volt meter in the hot side of the outlet and the negative side of the volt meter touching a screw (screw is only into fiberglass I took the wall apart to check ) I am getting a reading of 60+ volts. As I test screws further away from the outlet with the negative, the voltage reading drops. Just touching gelcoat the reading is around 20 volts. This happens at all outlets.

    I also tested the outlets with a GFCI push button tester, lights lit up properly and the circuit tripped when the button was depressed. What do I have incorrect or what could be causing the voltage readings? Thank you


Those are excellent questions about a subject that warrants a lot of concern because of the risks involved. Here are my thoughts on each of the three.

  1. Yes. The battery negative chain would be a great common ground.

  2. My preference is to run a 10 AWG green wire from the galvanic isolator to the AC ground bus at your AC panel.

    From the AC panel ground bus I would run a 10 AWG green wire to the DC negative. In your case this is either the bus under the helm or the battery negative.

    Since the greatest chance of an AC fault is from AC hot to AC ground I would rather have the fault path more direct (to the panel, through the GI, and to the dock) instead of back to the AC panel, to the DC negative, and then through the GI to shore.
  3. I asked a friend of mine for his opinion on this one and I believe that he may be right. The crazy readings may be from the high input impedance of the voltmeter. As soon as a load is added to any of these circuit combinations (AC hot to screw) the voltage should drop to zero.

Bot Buddies

Dear Kevin,

Last summer, I purchased a new aluminum fishing boat. It’s great, but I screwed up by “cheaping out” at the last minute. I passed on the trolling motor package and – as you’ve probably guessed – now want to add a trolling motor.

As a long time reader, I know that you are a big fan of Motorguide trolling motors and I’ll be buying one in the next couple weeks. My question concerns the boat wiring.

The Motorguide will be bow mounted, but since I didn’t get the trolling package, the only wire at the bow is a 16 gauge pair for the Attwood LED navigation light. But, according to your boat wiring size calculator, the trolling motor should be wired with 6 gauge.

Is there any way to make the smaller wire work? I hate to start rewiring a brand new boat if I can avoid it.



Hi Sean,

Since your boat is new, you may be in luck. A lot of boat builders now use Ancor’s new Nanotech Wire. As you’d guess from the name, it uses nanotechnology to solve problems just like yours.

Simply put, Nanotech Wire matches wire size to power demand by moving copper to where it is most needed – sort of a Viagra for marine electrical.

In the past, running a trolling motor on undersized wire caused voltage drops, heat buildup and embarrassing insurance claims. But now, that same temperature increase signals Nanotech Wire to deploy an army of nanobots who quickly rebuild the harness to match the power demand. While at work, these little guys make the wire looks like a python that swallowed a large rat.

Once your harness is properly sized, the same bots then go to work on its insulation so that it correctly corresponds to standard boat wiring colors.

The patented process is called Nanotech Overload Sensing Heat Induced Tranference and it works great. The only time I’ve seen it fail is when there are so many accessories turned on that there is no extra copper available. If this happens a lot, I usually recommend the addition of a copper reservoir to supply additional material as needed.

Have an excellent day.


Nautical equivalent of “Old Sparky”

Hi Kevin,

I found your boat wiring website and would be grateful if you might comment on the following for installation of marine electrical systems in an aluminium hull boat. Its a complicated issue I know!

  • AC system includes generator, inverter and marine electrical shore power. The AC system is entirely floating,  i.e., no negative grounding and is interfaced with shore connections via an isolation transformer.
  • DC system includes house battery, engine alternator (isolated negative) and engine battery. All are grounded to one common negative pole on the hull. No other equipment is negative grounded.

I have been told that it would be better to float the DC system entirely as well (same as AC) so that there is no negative ground at all. Is this correct?Shock hazard



Hi Al,

Unless somebody went through and removed the AC/DC ground connection, it should still be intact and must remain. A boat without this connection is an extreme hazard. With an AC fault to the hull, there is no low resistance path back to ground to trip the breaker and the boat will turn into a very large electric chair.

On your boat generator, the AC ground and AC neutral are connected together on the metal engine block of the generator. The generator negative battery cable is also connected to the engine block and your DC negative system. An inventer will have the same AC neutral to AC ground connection. Your AC distribution panel should have a link between the AC ground (Green) and the DC negative on your boat.

Hull ground isolation is nearly impossible on an aluminum boat with an engine and unsafe on an aluminum boat with AC power. The engine and its underwater gear are inevitably electrically connected to the engine block and the DC negative. A positive hull connection to the DC negative gives a low resistance path back to AC ground and DC ground in the event of a fault. A DC fault to an un-grounded hull can create accelerated galvanic corrosion. The area with the positive cable connected to it will corrode to protect your engine and drive.

Hope this helps,


GI Install

Hi Kevin,

I recently received one of the Easy Add AC Shore Power systems from your Boat Wiring Store.

The system looks great…perfect for my Whaler Outrage. But, prior to installation I have a couple of questions. I have a 30 amp Yandina galvanic isolator that I plan to install along with the shore power kit.

Could you give me some idea of how it should be wired? From what I have read on Yandina’s instructions the green wire (ground) on the panel should be connected to the isolator with the other lead from the isolator comnnected to the boat’s 12 volt ground….if so will any ground wire work or do you suggest a dedicated ground wire directly to the house battery?

Thanks beforehand for your assistance.Whaler 220 Outrage



Hi Greg,

The best place to install a galvanic isolator is in the green wire between the ac shore power inlet and the ac panel. This will ensure that there are no alternate paths to dc ground thst can cause galvanic corrosion.

Cut the white jacking back on the shore power inlet where in the desired area. Cut the green wire and install the gi.

Hope this helps,