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